Using and Porting the GNU Compiler Collection (GCC)

Table of Contents

• 1 Compile C, C++, Objective C, or Fortran
• 2 GCC Command Options
  • 2.1 Option Summary
  • 2.2 Options Controlling the Kind of Output
  • 2.3 Compiling C++ Programs
  • 2.4 Options Controlling C Dialect
  • 2.5 Options Controlling C++ Dialect
  • 2.6 Options to Request or Suppress Warnings
  • 2.7 Options for Debugging Your Program or GCC
  • 2.8 Options That Control Optimization
  • 2.9 Options Controlling the Preprocessor
  • 2.10 Passing Options to the Assembler
  • 2.11 Options for Linking
  • 2.12 Options for Directory Search
  • 2.13 Specifying Target Machine and Compiler Version
  • 2.14 Hardware Models and Configurations
    • 2.14.1 M680x0 Options
    • 2.14.2 VAX Options
    • 2.14.3 SPARC Options
    • 2.14.4 Convex Options
    • 2.14.5 AMD29K Options
    • 2.14.6 ARM Options
    • 2.14.7 Thumb Options
    • 2.14.8 MN10200 Options
    • 2.14.9 MN10300 Options
    • 2.14.10 M32R/D Options
    • 2.14.11 M88K Options
    • 2.14.12 IBM RS/6000 and PowerPC Options
    • 2.14.13 IBM RT Options
    • 2.14.14 MIPS Options
    • 2.14.15 Intel 386 Options
    • 2.14.16 HPPA Options
    • 2.14.17 Intel 960 Options
    • 2.14.18 DEC Alpha Options
    • 2.14.19 Clipper Options
    • 2.14.20 H8/300 Options
    • 2.14.21 SH Options
    • 2.14.22 Options for System V
    • 2.14.23 TMS320C3x/C4x Options
    • 2.14.24 V850 Options
    • 2.14.25 ARC Options
    • 2.14.26 NS32K Options
  • 2.15 Options for Code Generation Conventions
  • 2.16 Environment Variables Affecting GCC
  • 2.17 Running Protoize
• 3 Installing GNU CC
  • 3.1 Files Created by configure
  • 3.2 Configurations Supported by GNU CC
  • 3.3 Compilation in a Separate Directory
  • 3.4 Building and Installing a Cross-Compiler
    • 3.4.1 Steps of Cross-Compilation
    • 3.4.2 Configuring a Cross-Compiler
    • 3.4.3 Tools and Libraries for a Cross-Compiler
    • 3.4.4 `libgcc.a' and Cross-Compilers
    • 3.4.5 Cross-Compilers and Header Files
    • 3.4.6 Actually Building the Cross-Compiler
  • 3.5 Installing GNU CC on the Sun
  • 3.6 Installing GNU CC on VMS
  • 3.7 collect2
  • 3.8 Standard Header File Directories
• 4 Extensions to the C Language Family
  • 4.1 Statements and Declarations in Expressions
  • 4.2 Locally Declared Labels
  • 4.3 Labels as Values
  • 4.4 Nested Functions
  • 4.5 Constructing Function Calls
  • 4.6 Naming an Expression's Type
  • 4.7 Referring to a Type with typeof
  • 4.8 Generalized Lvalues
  • 4.9 Conditionals with Omitted Operands
  • 4.10 Double-Word Integers
  • 4.11 Complex Numbers
  • 4.12 Hex Floats
  • 4.13 Arrays of Length Zero
  • 4.14 Arrays of Variable Length
  • 4.15 Macros with Variable Numbers of Arguments
  • 4.16 Non-Lvalue Arrays May Have Subscripts
  • 4.17 Arithmetic on void- and Function-Pointers
  • 4.18 Non-Constant Initializers
  • 4.19 Constructor Expressions
  • 4.20 Labeled Elements in Initializers
  • 4.21 Case Ranges
  • 4.22 Cast to a Union Type
  • 4.23 Declaring Attributes of Functions
  • 4.24 Prototypes and Old-Style Function Definitions
  • 4.25 C++ Style Comments
  • 4.26 Dollar Signs in Identifier Names
  • 4.27 The Character ESC in Constants
  • 4.28 Inquiring on Alignment of Types or Variables
  • 4.29 Specifying Attributes of Variables
  • 4.30 Specifying Attributes of Types
  • 4.31 An Inline Function is As Fast As a Macro
  • 4.32 Assembler Instructions with C Expression Operands
    • 4.32.1 i386 floating point asm operands
  • 4.33 Controlling Names Used in Assembler Code
  • 4.34 Variables in Specified Registers
    • 4.34.1 Defining Global Register Variables
    • 4.34.2 Specifying Registers for Local Variables
  • 4.35 Alternate Keywords
  • 4.36 Incomplete enum Types
  • 4.37 Function Names as Strings
  • 4.38 Getting the Return or Frame Address of a Function
  • 4.39 Other built-in functions provided by GNU CC
  • 4.40 Deprecated Features
• 5 Extensions to the C++ Language
  • 5.1 Named Return Values in C++
  • 5.2 Minimum and Maximum Operators in C++
  • 5.3 goto and Destructors in GNU C++
  • 5.4 Declarations and Definitions in One Header
  • 5.5 Where's the Template?
  • 5.6 Extracting the function pointer from a bound pointer to member function
  • 5.7 Type Abstraction using Signatures
• 6 gcov: a Test Coverage Program
  • 6.1 Introduction to gcov
  • 6.2 Invoking gcov
  • 6.3 Using gcov with GCC Optimization
  • 6.4 Brief description of gcov data files
• 7 Known Causes of Trouble with GCC
  • 7.1 Actual Bugs We Haven't Fixed Yet
  • 7.2 Installation Problems
  • 7.3 Cross-Compiler Problems
  • 7.4 Interoperation
  • 7.5 Problems Compiling Certain Programs
  • 7.6 Incompatibilities of GCC
  • 7.7 Fixed Header Files
  • 7.8 Standard Libraries
  • 7.9 Disappointments and Misunderstandings
  • 7.10 Common Misunderstandings with GNU C++
    • 7.10.1 Declare and Define Static Members
    • 7.10.2 Temporaries May Vanish Before You Expect
    • 7.10.3 Implicit Copy-Assignment for Virtual Bases
  • 7.11 Caveats of using protoize
  • 7.12 Certain Changes We Don't Want to Make
  • 7.13 Warning Messages and Error Messages
• 8 Reporting Bugs
  • 8.1 Have You Found a Bug?
  • 8.2 Where to Report Bugs
  • 8.3 How to Report Bugs
  • 8.4 Sending Patches for GCC
• 9 How To Get Help with GCC
• 10 Contributing to GCC Development
• 11 Using GCC on VMS
  • 11.1 Include Files and VMS
  • 11.2 Global Declarations and VMS
  • 11.3 Other VMS Issues
• 12 GCC and Portability
• 13 Interfacing to GCC Output
• 14 Passes and Files of the Compiler
• 15 RTL Representation
  • 15.1 RTL Object Types
  • 15.2 RTL Classes and Formats
  • 15.3 Access to Operands
  • 15.4 Flags in an RTL Expression
  • 15.5 Machine Modes
  • 15.6 Constant Expression Types
  • 15.7 Registers and Memory
  • 15.8 RTL Expressions for Arithmetic
  • 15.9 Comparison Operations
  • 15.10 Bit Fields
  • 15.11 Conversions
  • 15.12 Declarations
  • 15.13 Side Effect Expressions
  • 15.14 Embedded Side-Effects on Addresses
  • 15.15 Assembler Instructions as Expressions
  • 15.16 Insns
  • 15.17 RTL Representation of Function-Call Insns
  • 15.18 Structure Sharing Assumptions
  • 15.19 Reading RTL
• 16 Machine Descriptions
  • 16.1 Everything about Instruction Patterns
  • 16.2 Example of define_insn
  • 16.3 RTL Template
  • 16.4 Output Templates and Operand Substitution
  • 16.5 C Statements for Assembler Output
  • 16.6 Operand Constraints
    • 16.6.1 Simple Constraints
    • 16.6.2 Multiple Alternative Constraints
    • 16.6.3 Register Class Preferences
    • 16.6.4 Constraint Modifier Characters
    • 16.6.5 Constraints for Particular Machines
    • 16.6.6 Not Using Constraints
  • 16.7 Standard Pattern Names For Generation
  • 16.8 When the Order of Patterns Matters
  • 16.9 Interdependence of Patterns
  • 16.10 Defining Jump Instruction Patterns
  • 16.11 Canonicalization of Instructions
  • 16.12 Machine-Specific Peephole Optimizers
  • 16.13 Defining RTL Sequences for Code Generation
  • 16.14 Defining How to Split Instructions
  • 16.15 Instruction Attributes
    • 16.15.1 Defining Attributes and their Values
    • 16.15.2 Attribute Expressions
    • 16.15.3 Assigning Attribute Values to Insns
    • 16.15.4 Example of Attribute Specifications
    • 16.15.5 Computing the Length of an Insn
    • 16.15.6 Constant Attributes
    • 16.15.7 Delay Slot Scheduling
    • 16.15.8 Specifying Function Units
• 17 Target Description Macros
  • 17.1 Controlling the Compilation Driver, `gcc'
  • 17.2 Run-time Target Specification
  • 17.3 Storage Layout
  • 17.4 Layout of Source Language Data Types
  • 17.5 Register Usage
    • 17.5.1 Basic Characteristics of Registers
    • 17.5.2 Order of Allocation of Registers
    • 17.5.3 How Values Fit in Registers
    • 17.5.4 Handling Leaf Functions
    • 17.5.5 Registers That Form a Stack
    • 17.5.6 Obsolete Macros for Controlling Register Usage
  • 17.6 Register Classes
  • 17.7 Stack Layout and Calling Conventions
    • 17.7.1 Basic Stack Layout
    • 17.7.2 Specifying How Stack Checking is Done
    • 17.7.3 Registers That Address the Stack Frame
    • 17.7.4 Eliminating Frame Pointer and Arg Pointer
    • 17.7.5 Passing Function Arguments on the Stack
    • 17.7.6 Passing Arguments in Registers
    • 17.7.7 How Scalar Function Values Are Returned
    • 17.7.8 How Large Values Are Returned
    • 17.7.9 Caller-Saves Register Allocation
    • 17.7.10 Function Entry and Exit
    • 17.7.11 Generating Code for Profiling
  • 17.8 Implementing the Varargs Macros
  • 17.9 Trampolines for Nested Functions
  • 17.10 Implicit Calls to Library Routines
  • 17.11 Addressing Modes
  • 17.12 Condition Code Status
  • 17.13 Describing Relative Costs of Operations
  • 17.14 Dividing the Output into Sections (Texts, Data, ...)
  • 17.15 Position Independent Code
  • 17.16 Defining the Output Assembler Language
    • 17.16.1 The Overall Framework of an Assembler File
    • 17.16.2 Output of Data
    • 17.16.3 Output of Uninitialized Variables
    • 17.16.4 Output and Generation of Labels
    • 17.16.5 How Initialization Functions Are Handled
    • 17.16.6 Macros Controlling Initialization Routines
    • 17.16.7 Output of Assembler Instructions
    • 17.16.8 Output of Dispatch Tables
    • 17.16.9 Assembler Commands for Exception Regions
    • 17.16.10 Assembler Commands for Alignment
  • 17.17 Controlling Debugging Information Format
    • 17.17.1 Macros Affecting All Debugging Formats
    • 17.17.2 Specific Options for DBX Output
    • 17.17.3 Open-Ended Hooks for DBX Format
    • 17.17.4 File Names in DBX Format
    • 17.17.5 Macros for SDB and DWARF Output
  • 17.18 Cross Compilation and Floating Point
  • 17.19 Miscellaneous Parameters
• 18 The Configuration File
• 19 Makefile Fragments
  • 19.1 The Target Makefile Fragment
  • 19.2 The Host Makefile Fragment
• Funding Free Software
• Linux and the GNU Project
• GNU GENERAL PUBLIC LICENSE
  • Preamble
  • TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
  • How to Apply These Terms to Your New Programs
• Contributors to GCC
• Index

1 Compile C, C++, Objective C, or Fortran

The C, C++, and Objective C, and Fortran versions of the compiler are integrated; this is why we use the name "GNU Compiler Collection". GCC can compile programs written in C, C++, Objective C, or Fortran. The Fortran compiler is described in a separate manual.

"GCC" is a common shorthand term for the GNU Compiler Collection. This is both the most general name for the compiler, and the name used when the emphasis is on compiling C programs (as the abbreviation formerly stood for "GNU C Compiler").

When referring to C++ compilation, it is usual to call the compiler "G++". Since there is only one compiler, it is also accurate to call it "GCC" no matter what the language context; however, the term "G++" is more useful when the emphasis is on compiling C++ programs.

We use the name "GCC" to refer to the compilation system as a whole, and more specifically to the language-independent part of the compiler. For example, we refer to the optimization options as affecting the behavior of "GCC" or sometimes just "the compiler".

Front ends for other languages, such as Ada 9X, Fortran, Modula-3, and Pascal, are under development. These front-ends, like that for C++, are built in subdirectories of GCC and link to it. The result is an integrated compiler that can compile programs written in C, C++, Objective C, or any of the languages for which you have installed front ends.

In this manual, we only discuss the options for the C, Objective-C, and C++ compilers and those of the GCC core. Consult the documentation of the other front ends for the options to use when compiling programs written in other languages.

G++ is a compiler, not merely a preprocessor. G++ builds object code directly from your C++ program source. There is no intermediate C version of the program. (By contrast, for example, some other implementations use a program that generates a C program from your C++ source.) Avoiding an intermediate C representation of the program means that you get better object code, and better debugging information. The GNU debugger, GDB, works with this information in the object code to give you comprehensive C++ source-level editing capabilities (see section `C and C++' in Debugging with GDB).

2 GCC Command Options

When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall options" allow you to stop this process at an intermediate stage. For example, the `-c' option says not to run the linker. Then the output consists of object files output by the assembler.

Other options are passed on to one stage of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them.

Most of the command line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages.

See section 2.3 Compiling C++ Programs, for a summary of special options for compiling C++ programs.

The gcc program accepts options and file names as operands. Many options have multiletter names; therefore multiple single-letter options may not be grouped: `-dr' is very different from `-d -r'.

You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter when you use several options of the same kind; for example, if you specify `-L' more than once, the directories are searched in the order specified.

Many options have long names starting with `-f' or with `-W'---for example, `-fforce-mem', `-fstrength-reduce', `-Wformat' and so on. Most of these have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. This manual documents only one of these two forms, whichever one is not the default.

2.1 Option Summary

Here is a summary of all the options, grouped by type. Explanations are in the following sections.

Overall Options

See section 2.2 Options Controlling the Kind of Output.

-c  -S  -E  -o file  -pipe  -v  --help  -x language

C Language Options

See section 2.4 Options Controlling C Dialect.

-ansi -flang-isoc9x -fallow-single-precision  -fcond-mismatch  -fno-asm
-fno-builtin  -ffreestanding  -fhosted  -fsigned-bitfields  -fsigned-char
-funsigned-bitfields  -funsigned-char  -fwritable-strings
-traditional  -traditional-cpp  -trigraphs

C++ Language Options

See section 2.5 Options Controlling C++ Dialect.

-fno-access-control  -fcheck-new  -fconserve-space  -fdollars-in-identifiers
-fno-elide-constructors  -fexternal-templates  -ffor-scope  
-fno-for-scope  -fno-gnu-keywords  -fguiding-decls  -fhandle-signatures
-fhonor-std -fhuge-objects  -fno-implicit-templates  -finit-priority
-fno-implement-inlines -fname-mangling-version-n  -fno-default-inline  
-foperator-names  -fno-optional-diags  -fpermissive -frepo  -fstrict-prototype
-fsquangle  -ftemplate-depth-n  -fthis-is-variable  -fvtable-thunks
-nostdinc++  -Wctor-dtor-privacy -Wno-deprecated -Weffc++  
-Wno-non-template-friend 
-Wnon-virtual-dtor  -Wold-style-cast  -Woverloaded-virtual  
-Wno-pmf-conversions  -Wreorder  -Wsign-promo  -Wsynth

Warning Options

See section 2.6 Options to Request or Suppress Warnings.

-fsyntax-only  -pedantic  -pedantic-errors
-w  -W  -Wall  -Waggregate-return  -Wbad-function-cast
-Wcast-align  -Wcast-qual  -Wchar-subscripts  -Wcomment
-Wconversion  -Werror  -Wformat
-Wid-clash-len  -Wimplicit -Wimplicit-int 
-Wimplicit-function-declaration  -Wimport
-Werror-implicit-function-declaration  -Winline
-Wlarger-than-len  -Wlong-long
-Wmain  -Wmissing-declarations  -Wmissing-noreturn
-Wmissing-prototypes  -Wmultichar  -Wnested-externs  -Wno-import  
-Wparentheses -Wpointer-arith  -Wredundant-decls
-Wreturn-type -Wshadow  -Wsign-compare  -Wstrict-prototypes  
-Wswitch  -Wtraditional  
-Wtrigraphs -Wundef  -Wuninitialized  -Wunused  -Wwrite-strings
-Wunknown-pragmas

Debugging Options

See section 2.7 Options for Debugging Your Program or GCC.

-a  -ax  -dletters  -fdump-unnumbered -fpretend-float
-fprofile-arcs  -ftest-coverage
-g  -glevel  -gcoff  -gdwarf  -gdwarf-1  -gdwarf-1+  -gdwarf-2
-ggdb  -gstabs  -gstabs+  -gxcoff  -gxcoff+
-p  -pg  -print-file-name=library  -print-libgcc-file-name
-print-prog-name=program  -print-search-dirs  -save-temps

Optimization Options

See section 2.8 Options That Control Optimization.

-fbranch-probabilities  -foptimize-register-moves
-fcaller-saves  -fcse-follow-jumps  -fcse-skip-blocks
-fdelayed-branch   -fexpensive-optimizations
-ffast-math  -ffloat-store  -fforce-addr  -fforce-mem
-fdata-sections -ffunction-sections  -fgcse 
-finline-functions -finline-limit-n -fkeep-inline-functions
-fno-default-inline -fno-defer-pop  -fno-function-cse
-fno-inline  -fno-peephole  -fomit-frame-pointer -fregmove
-frerun-cse-after-loop  -frerun-loop-opt -fschedule-insns
-fschedule-insns2  -fstrength-reduce  -fthread-jumps
-funroll-all-loops  -funroll-loops
-fmove-all-movables  -freduce-all-givs -fstrict-aliasing
-O  -O0  -O1  -O2  -O3 -Os

Preprocessor Options

See section 2.9 Options Controlling the Preprocessor.

-Aquestion(answer)  -C  -dD  -dM  -dN
-Dmacro[=defn]  -E  -H
-idirafter dir
-include file  -imacros file
-iprefix file  -iwithprefix dir
-iwithprefixbefore dir  -isystem dir -isystem-c++ dir
-M  -MD  -MM  -MMD  -MG  -nostdinc  -P  -trigraphs
-undef  -Umacro  -Wp,option

Assembler Option

See section 2.10 Passing Options to the Assembler.

-Wa,option

Linker Options

See section 2.11 Options for Linking.

object-file-name  -llibrary
-nostartfiles  -nodefaultlibs  -nostdlib
-s  -static  -shared  -symbolic
-Wl,option  -Xlinker option
-u symbol

Directory Options

See section 2.12 Options for Directory Search.

-Bprefix  -Idir  -I-  -Ldir  -specs=file

Target Options

See section 2.13 Specifying Target Machine and Compiler Version.

-b machine  -V version

Machine Dependent Options

See section 2.14 Hardware Models and Configurations.

M680x0 Options
-m68000  -m68020  -m68020-40  -m68020-60  -m68030  -m68040
-m68060  -mcpu32 -m5200  -m68881  -mbitfield  -mc68000  -mc68020  
-mfpa -mnobitfield  -mrtd  -mshort  -msoft-float  
-malign-int

VAX Options
-mg  -mgnu  -munix

SPARC Options
-mcpu=cpu type
-mtune=cpu type
-mcmodel=code model
-malign-jumps=num  -malign-loops=num
-malign-functions=num
-m32  -m64
-mapp-regs  -mbroken-saverestore  -mcypress  -mepilogue
-mflat  -mfpu  -mhard-float  -mhard-quad-float
-mimpure-text  -mlive-g0  -mno-app-regs  -mno-epilogue
-mno-flat  -mno-fpu  -mno-impure-text
-mno-stack-bias  -mno-unaligned-doubles
-msoft-float  -msoft-quad-float  -msparclite  -mstack-bias
-msupersparc  -munaligned-doubles  -mv8

Convex Options
-mc1  -mc2  -mc32  -mc34  -mc38
-margcount  -mnoargcount
-mlong32  -mlong64
-mvolatile-cache  -mvolatile-nocache

AMD29K Options
-m29000  -m29050  -mbw  -mnbw  -mdw  -mndw
-mlarge  -mnormal  -msmall
-mkernel-registers  -mno-reuse-arg-regs
-mno-stack-check  -mno-storem-bug
-mreuse-arg-regs  -msoft-float  -mstack-check
-mstorem-bug  -muser-registers

ARM Options
-mapcs-frame -mno-apcs-frame
-mapcs-26 -mapcs-32
-mapcs-stack-check -mno-apcs-stack-check
-mapcs-float -mno-apcs-float
-mapcs-reentrant -mno-apcs-reentrant
-msched-prolog -mno-sched-prolog
-mlittle-endian -mbig-endian -mwords-little-endian
-mshort-load-bytes -mno-short-load-bytes -mshort-load-words -mno-short-load-words
-msoft-float -mhard-float -mfpe
-mthumb-interwork -mno-thumb-interwork
-mcpu= -march= -mfpe= 
-mstructure-size-boundary=
-mbsd -mxopen -mno-symrename
-mabort-on-noreturn
-mno-sched-prolog

Thumb Options
-mtpcs-frame -mno-tpcs-frame
-mtpcs-leaf-frame -mno-tpcs-leaf-frame
-mlittle-endian  -mbig-endian
-mthumb-interwork -mno-thumb-interwork
-mstructure-size-boundary=

MN10200 Options
-mrelax

MN10300 Options
-mmult-bug
-mno-mult-bug
-mrelax

M32R/D Options
-mcode-model=model type  -msdata=sdata type
-G num

M88K Options
-m88000  -m88100  -m88110  -mbig-pic
-mcheck-zero-division  -mhandle-large-shift
-midentify-revision  -mno-check-zero-division
-mno-ocs-debug-info  -mno-ocs-frame-position
-mno-optimize-arg-area  -mno-serialize-volatile
-mno-underscores  -mocs-debug-info
-mocs-frame-position  -moptimize-arg-area
-mserialize-volatile  -mshort-data-num  -msvr3
-msvr4  -mtrap-large-shift  -muse-div-instruction
-mversion-03.00  -mwarn-passed-structs

RS/6000 and PowerPC Options
-mcpu=cpu type
-mtune=cpu type
-mpower  -mno-power  -mpower2  -mno-power2
-mpowerpc  -mno-powerpc
-mpowerpc-gpopt  -mno-powerpc-gpopt
-mpowerpc-gfxopt  -mno-powerpc-gfxopt
-mnew-mnemonics  -mno-new-mnemonics
-mfull-toc   -mminimal-toc  -mno-fop-in-toc  -mno-sum-in-toc
-maix64  -maix32  -mxl-call  -mno-xl-call  -mthreads  -mpe
-msoft-float  -mhard-float  -mmultiple  -mno-multiple
-mstring  -mno-string  -mupdate  -mno-update
-mfused-madd  -mno-fused-madd  -mbit-align  -mno-bit-align
-mstrict-align  -mno-strict-align  -mrelocatable
-mno-relocatable  -mrelocatable-lib  -mno-relocatable-lib
-mtoc  -mno-toc -mlittle  -mlittle-endian  -mbig  -mbig-endian
-mcall-aix  -mcall-sysv  -mprototype  -mno-prototype
-msim  -mmvme  -mads  -myellowknife  -memb -msdata
-msdata=opt  -G num

RT Options
-mcall-lib-mul  -mfp-arg-in-fpregs  -mfp-arg-in-gregs
-mfull-fp-blocks  -mhc-struct-return  -min-line-mul
-mminimum-fp-blocks  -mnohc-struct-return

MIPS Options
-mabicalls  -mcpu=cpu type  -membedded-data
-membedded-pic  -mfp32  -mfp64  -mgas  -mgp32  -mgp64
-mgpopt  -mhalf-pic  -mhard-float  -mint64  -mips1
-mips2  -mips3 -mips4 -mlong64  -mlong32 -mlong-calls  -mmemcpy
-mmips-as  -mmips-tfile  -mno-abicalls
-mno-embedded-data  -mno-embedded-pic
-mno-gpopt  -mno-long-calls
-mno-memcpy  -mno-mips-tfile  -mno-rnames  -mno-stats
-mrnames  -msoft-float
-m4650  -msingle-float  -mmad
-mstats  -EL  -EB  -G num  -nocpp
-mabi=32 -mabi=n32 -mabi=64 -mabi=eabi

i386 Options
-mcpu=cpu type
-march=cpu type
-mieee-fp  -mno-fancy-math-387
-mno-fp-ret-in-387  -msoft-float  -msvr3-shlib
-mno-wide-multiply  -mrtd  -malign-double
-mreg-alloc=list  -mregparm=num
-malign-jumps=num  -malign-loops=num
-malign-functions=num -mpreferred-stack-boundary=num

HPPA Options
-march=architecture type
-mbig-switch  -mdisable-fpregs  -mdisable-indexing  
-mfast-indirect-calls -mgas  -mjump-in-delay  
-mlong-load-store  -mno-big-switch  -mno-disable-fpregs
-mno-disable-indexing  -mno-fast-indirect-calls  -mno-gas
-mno-jump-in-delay  -mno-long-load-store  
-mno-portable-runtime  -mno-soft-float  -mno-space  
-mno-space-regs  -msoft-float  -mpa-risc-1-0  
-mpa-risc-1-1  -mpa-risc-2-0 -mportable-runtime
-mschedule=cpu type  -mspace  -mspace-regs

Intel 960 Options
-mcpu type  -masm-compat  -mclean-linkage
-mcode-align  -mcomplex-addr  -mleaf-procedures
-mic-compat  -mic2.0-compat  -mic3.0-compat
-mintel-asm  -mno-clean-linkage  -mno-code-align
-mno-complex-addr  -mno-leaf-procedures
-mno-old-align  -mno-strict-align  -mno-tail-call
-mnumerics  -mold-align  -msoft-float  -mstrict-align
-mtail-call

DEC Alpha Options
-mfp-regs  -mno-fp-regs -mno-soft-float  -msoft-float
-malpha-as -mgas
-mieee  -mieee-with-inexact  -mieee-conformant
-mfp-trap-mode=mode  -mfp-rounding-mode=mode
-mtrap-precision=mode  -mbuild-constants
-mcpu=cpu type
-mbwx -mno-bwx -mcix -mno-cix -mmax -mno-max
-mmemory-latency=time

Clipper Options
-mc300  -mc400

H8/300 Options
-mrelax  -mh -ms -mint32  -malign-300

SH Options
-m1  -m2  -m3  -m3e  -mb  -ml  -mdalign -mrelax

System V Options
-Qy  -Qn  -YP,paths  -Ym,dir

ARC Options
-EB  -EL
-mmangle-cpu  -mcpu=cpu  -mtext=text section
-mdata=data section  -mrodata=readonly data section

TMS320C3x/C4x Options
-mcpu=cpu -mbig -msmall -mregparm -mmemparm
-mfast-fix -mmpyi -mbk -mti -mdp-isr-reload
-mrpts=count  -mrptb -mdb -mloop-unsigned
-mparallel-insns -mparallel-mpy -mpreserve-float

V850 Options
-mlong-calls -mno-long-calls -mep -mno-ep
-mprolog-function -mno-prolog-function -mspace
-mtda=n -msda=n -mzda=n
-mv850 -mbig-switch

NS32K Options
-m32032 -m32332 -m32532 -m32081 -m32381 -mmult-add -mnomult-add
-msoft-float -mrtd -mnortd -mregparam -mnoregparam -msb -mnosb
-mbitfield -mnobitfield -mhimem -mnohimem

Code Generation Options

See section 2.15 Options for Code Generation Conventions.

-fcall-saved-reg  -fcall-used-reg
-fexceptions -ffixed-reg  -finhibit-size-directive
-fcheck-memory-usage  -fprefix-function-name
-fno-common  -fno-ident  -fno-gnu-linker
-fpcc-struct-return  -fpic  -fPIC
-freg-struct-return  -fshared-data  -fshort-enums
-fshort-double  -fvolatile  -fvolatile-global -fvolatile-static
-fverbose-asm -fpack-struct  -fstack-check
-fargument-alias  -fargument-noalias
-fargument-noalias-global
-fleading-underscore

2.2 Options Controlling the Kind of Output

Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order. The first three stages apply to an individual source file, and end by producing an object file; linking combines all the object files (those newly compiled, and those specified as input) into an executable file.

For any given input file, the file name suffix determines what kind of compilation is done:

file.c

C source code which must be preprocessed.

file.i

C source code which should not be preprocessed.

file.ii

C++ source code which should not be preprocessed.

file.m

Objective-C source code. Note that you must link with the library `libobjc.a' to make an Objective-C program work.

file.h

C header file (not to be compiled or linked).

file.cc

file.cxx

file.cpp

file.C

C++ source code which must be preprocessed. Note that in `.cxx', the last two letters must both be literally `x'. Likewise, `.C' refers to a literal capital C.

file.s

Assembler code.

file.S

Assembler code which must be preprocessed.

other

An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.

You can specify the input language explicitly with the `-x' option:

-x language

Specify explicitly the language for the following input files (rather than letting the compiler choose a default based on the file name suffix). This option applies to all following input files until the next `-x' option. Possible values for language are:

c  objective-c  c++
c-header  cpp-output  c++-cpp-output
assembler  assembler-with-cpp

-x none

Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as they are if `-x' has not been used at all).

If you only want some of the stages of compilation, you can use `-x' (or filename suffixes) to tell gcc where to start, and one of the options `-c', `-S', or `-E' to say where gcc is to stop. Note that some combinations (for example, `-x cpp-output -E' instruct gcc to do nothing at all.

-c

Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file. By default, the object file name for a source file is made by replacing the suffix `.c', `.i', `.s', etc., with `.o'. Unrecognized input files, not requiring compilation or assembly, are ignored.

-S

Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file for each non-assembler input file specified. By default, the assembler file name for a source file is made by replacing the suffix `.c', `.i', etc., with `.s'. Input files that don't require compilation are ignored.

-E

Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output. Input files which don't require preprocessing are ignored.

-o file

Place output in file file. This applies regardless to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code. Since only one output file can be specified, it does not make sense to use `-o' when compiling more than one input file, unless you are producing an executable file as output. If `-o' is not specified, the default is to put an executable file in `a.out', the object file for `source.suffix' in `source.o', its assembler file in `source.s', and all preprocessed C source on standard output.

-v

Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper.

-pipe

Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble.

--help

Print (on the standard output) a description of the command line options understood by gcc. If the -v option is also specified then --help will also be passed on to the various processes invoked by gcc, so that they can display the command line options they accept. If the -W option is also specified then command line options which have no documentation associated with them will also be displayed.

2.3 Compiling C++ Programs

C++ source files conventionally use one of the suffixes `.C', `.cc', `.cpp', `.c++', `.cp', or `.cxx'; preprocessed C++ files use the suffix `.ii'. GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc).

However, C++ programs often require class libraries as well as a compiler that understands the C++ language--and under some circumstances, you might want to compile programs from standard input, or otherwise without a suffix that flags them as C++ programs. g++ is a program that calls GCC with the default language set to C++, and automatically specifies linking against the C++ library. On many systems, the script g++ is also installed with the name c++.

When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs. See section 2.4 Options Controlling C Dialect, for explanations of options for languages related to C. See section 2.5 Options Controlling C++ Dialect, for explanations of options that are meaningful only for C++ programs.

2.4 Options Controlling C Dialect

The following options control the dialect of C (or languages derived from C, such as C++ and Objective C) that the compiler accepts:

-ansi

In C mode, support all ANSI standard C programs. In C++ mode, remove GNU extensions that conflict with ANSI C++. This turns off certain features of GCC that are incompatible with ANSI C (when compiling C code), or of ANSI standard C++ (when compiling C++ code), such as the asm and typeof keywords, and predefined macros such as unix and vax that identify the type of system you are using. It also enables the undesirable and rarely used ANSI trigraph feature. For the C compiler, it disables recognition of C++ style `//' comments as well as the inline keyword. For the C++ compiler, `-foperator-names' is enabled as well. The alternate keywords __asm__, __extension__, __inline__ and __typeof__ continue to work despite `-ansi'. You would not want to use them in an ANSI C program, of course, but it is useful to put them in header files that might be included in compilations done with `-ansi'. Alternate predefined macros such as __unix__ and __vax__ are also available, with or without `-ansi'. The `-ansi' option does not cause non-ANSI programs to be rejected gratuitously. For that, `-pedantic' is required in addition to `-ansi'. See section 2.6 Options to Request or Suppress Warnings. The macro __STRICT_ANSI__ is predefined when the `-ansi' option is used. Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ANSI standard doesn't call for; this is to avoid interfering with any programs that might use these names for other things. The functions alloca, abort, exit, and _exit are not builtin functions when `-ansi' is used.

-flang-isoc9x

Enable support for features found in the C9X standard. In particular, enable support for the C9X restrict keyword. Even when this option is not specified, you can still use some C9X features in so far as they do not conflict with previous C standards. For example, you may use __restrict__ even when -flang-isoc9x is not specified.

-fno-asm

Do not recognize asm, inline or typeof as a keyword, so that code can use these words as identifiers. You can use the keywords __asm__, __inline__ and __typeof__ instead. `-ansi' implies `-fno-asm'. In C++, this switch only affects the typeof keyword, since asm and inline are standard keywords. You may want to use the `-fno-gnu-keywords' flag instead, as it also disables the other, C++-specific, extension keywords such as headof.

-fno-builtin

Don't recognize builtin functions that do not begin with `__builtin_' as prefix. Currently, the functions affected include abort, abs, alloca, cos, exit, fabs, ffs, labs, memcmp, memcpy, sin, sqrt, strcmp, strcpy, and strlen. GCC normally generates special code to handle certain builtin functions more efficiently; for instance, calls to alloca may become single instructions that adjust the stack directly, and calls to memcpy may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library. The `-ansi' option prevents alloca and ffs from being builtin functions, since these functions do not have an ANSI standard meaning.

-fhosted

Assert that compilation takes place in a hosted environment. This implies `-fbuiltin'. A hosted environment is one in which the entire standard library is available, and in which main has a return type of int. Examples are nearly everything except a kernel. This is equivalent to `-fno-freestanding'.

-ffreestanding

Assert that compilation takes place in a freestanding environment. This implies `-fno-builtin'. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at main. The most obvious example is an OS kernel. This is equivalent to `-fno-hosted'.

-trigraphs

Support ANSI C trigraphs. You don't want to know about this brain-damage. The `-ansi' option implies `-trigraphs'.

-traditional

Attempt to support some aspects of traditional C compilers. Specifically:

• All extern declarations take effect globally even if they are written inside of a function definition. This includes implicit declarations of functions.
• The newer keywords typeof, inline, signed, const and volatile are not recognized. (You can still use the alternative keywords such as __typeof__, __inline__, and so on.)
• Comparisons between pointers and integers are always allowed.
• Integer types unsigned short and unsigned char promote to unsigned int.
• Out-of-range floating point literals are not an error.
• Certain constructs which ANSI regards as a single invalid preprocessing number, such as `0xe-0xd', are treated as expressions instead.
• String "constants" are not necessarily constant; they are stored in writable space, and identical looking constants are allocated separately. (This is the same as the effect of `-fwritable-strings'.)
• All automatic variables not declared register are preserved by longjmp. Ordinarily, GNU C follows ANSI C: automatic variables not declared volatile may be clobbered.
• The character escape sequences `\x' and `\a' evaluate as the literal characters `x' and `a' respectively. Without `-traditional', `\x' is a prefix for the hexadecimal representation of a character, and `\a' produces a bell.

You may wish to use `-fno-builtin' as well as `-traditional' if your program uses names that are normally GNU C builtin functions for other purposes of its own. You cannot use `-traditional' if you include any header files that rely on ANSI C features. Some vendors are starting to ship systems with ANSI C header files and you cannot use `-traditional' on such systems to compile files that include any system headers. The `-traditional' option also enables `-traditional-cpp', which is described next.

-traditional-cpp

Attempt to support some aspects of traditional C preprocessors. Specifically:

• Comments convert to nothing at all, rather than to a space. This allows traditional token concatenation.
• In a preprocessing directive, the `#' symbol must appear as the first character of a line.
• Macro arguments are recognized within string constants in a macro definition (and their values are stringified, though without additional quote marks, when they appear in such a context). The preprocessor always considers a string constant to end at a newline.
• The predefined macro __STDC__ is not defined when you use `-traditional', but __GNUC__ is (since the GNU extensions which __GNUC__ indicates are not affected by `-traditional'). If you need to write header files that work differently depending on whether `-traditional' is in use, by testing both of these predefined macros you can distinguish four situations: GNU C, traditional GNU C, other ANSI C compilers, and other old C compilers. The predefined macro __STDC_VERSION__ is also not defined when you use `-traditional'. See section `Standard Predefined Macros' in The C Preprocessor, for more discussion of these and other predefined macros.
• The preprocessor considers a string constant to end at a newline (unless the newline is escaped with `\'). (Without `-traditional', string constants can contain the newline character as typed.)

-fcond-mismatch

Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void.

-funsigned-char

Let the type char be unsigned, like unsigned char. Each kind of machine has a default for what char should be. It is either like unsigned char by default or like signed char by default. Ideally, a portable program should always use signed char or unsigned char when it depends on the signedness of an object. But many programs have been written to use plain char and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default. The type char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two.

-fsigned-char

Let the type char be signed, like signed char. Note that this is equivalent to `-fno-unsigned-char', which is the negative form of `-funsigned-char'. Likewise, the option `-fno-signed-char' is equivalent to `-funsigned-char'. You may wish to use `-fno-builtin' as well as `-traditional' if your program uses names that are normally GNU C builtin functions for other purposes of its own. You cannot use `-traditional' if you include any header files that rely on ANSI C features. Some vendors are starting to ship systems with ANSI C header files and you cannot use `-traditional' on such systems to compile files that include any system headers.

-fsigned-bitfields

-funsigned-bitfields

-fno-signed-bitfields

-fno-unsigned-bitfields

These options control whether a bitfield is signed or unsigned, when the declaration does not use either signed or unsigned. By default, such a bitfield is signed, because this is consistent: the basic integer types such as int are signed types. However, when `-traditional' is used, bitfields are all unsigned no matter what.

-fwritable-strings

Store string constants in the writable data segment and don't uniquize them. This is for compatibility with old programs which assume they can write into string constants. The option `-traditional' also has this effect. Writing into string constants is a very bad idea; "constants" should be constant.

-fallow-single-precision

Do not promote single precision math operations to double precision, even when compiling with `-traditional'. Traditional K&R C promotes all floating point operations to double precision, regardless of the sizes of the operands. On the architecture for which you are compiling, single precision may be faster than double precision. If you must use `-traditional', but want to use single precision operations when the operands are single precision, use this option. This option has no effect when compiling with ANSI or GNU C conventions (the default).

2.5 Options Controlling C++ Dialect

This section describes the command-line options that are only meaningful for C++ programs; but you can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file firstClass.C like this:

g++ -g -frepo -O -c firstClass.C

In this example, only `-frepo' is an option meant only for C++ programs; you can use the other options with any language supported by GCC.

Here is a list of options that are only for compiling C++ programs:

-fno-access-control

Turn off all access checking. This switch is mainly useful for working around bugs in the access control code.

-fcheck-new

Check that the pointer returned by operator new is non-null before attempting to modify the storage allocated. The current Working Paper requires that operator new never return a null pointer, so this check is normally unnecessary. An alternative to using this option is to specify that your operator new does not throw any exceptions; if you declare it `throw()', g++ will check the return value. See also `new (nothrow)'.

-fconserve-space

Put uninitialized or runtime-initialized global variables into the common segment, as C does. This saves space in the executable at the cost of not diagnosing duplicate definitions. If you compile with this flag and your program mysteriously crashes after main() has completed, you may have an object that is being destroyed twice because two definitions were merged. This option is no longer useful on most targets, now that support has been added for putting variables into BSS without making them common.

-fdollars-in-identifiers

Accept `$' in identifiers. You can also explicitly prohibit use of `$' with the option `-fno-dollars-in-identifiers'. (GNU C allows `$' by default on most target systems, but there are a few exceptions.) Traditional C allowed the character `$' to form part of identifiers. However, ANSI C and C++ forbid `$' in identifiers.

-fno-elide-constructors

The C++ standard allows an implementation to omit creating a temporary which is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces g++ to call the copy constructor in all cases.

-fexternal-templates

Cause template instantiations to obey `#pragma interface' and `implementation'; template instances are emitted or not according to the location of the template definition. See section 5.5 Where's the Template?, for more information. This option is deprecated.

-falt-external-templates

Similar to -fexternal-templates, but template instances are emitted or not according to the place where they are first instantiated. See section 5.5 Where's the Template?, for more information. This option is deprecated.

-ffor-scope

-fno-for-scope

If -ffor-scope is specified, the scope of variables declared in a for-init-statement is limited to the `for' loop itself, as specified by the draft C++ standard. If -fno-for-scope is specified, the scope of variables declared in a for-init-statement extends to the end of the enclosing scope, as was the case in old versions of gcc, and other (traditional) implementations of C++. The default if neither flag is given to follow the standard, but to allow and give a warning for old-style code that would otherwise be invalid, or have different behavior.

-fno-gnu-keywords

Do not recognize classof, headof, signature, sigof or typeof as a keyword, so that code can use these words as identifiers. You can use the keywords __classof__, __headof__, __signature__, __sigof__, and __typeof__ instead. `-ansi' implies `-fno-gnu-keywords'.

-fguiding-decls

Treat a function declaration with the same type as a potential function template instantiation as though it declares that instantiation, not a normal function. If a definition is given for the function later in the translation unit (or another translation unit if the target supports weak symbols), that definition will be used; otherwise the template will be instantiated. This behavior reflects the C++ language prior to September 1996, when guiding declarations were removed. This option implies `-fname-mangling-version-0', and will not work with other name mangling versions. Like all options that change the ABI, all C++ code, including libgcc.a must be built with the same setting of this option.

-fhandle-signatures

Recognize the signature and sigof keywords for specifying abstract types. The default (`-fno-handle-signatures') is not to recognize them. See section 5.7 Type Abstraction using Signatures.

-fhonor-std

Treat the namespace std as a namespace, instead of ignoring it. For compatibility with earlier versions of g++, the compiler will, by default, ignore namespace-declarations, using-declarations, using-directives, and namespace-names, if they involve std.

-fhuge-objects

Support virtual function calls for objects that exceed the size representable by a `short int'. Users should not use this flag by default; if you need to use it, the compiler will tell you so. This flag is not useful when compiling with -fvtable-thunks. Like all options that change the ABI, all C++ code, including libgcc must be built with the same setting of this option.

-fno-implicit-templates

Never emit code for non-inline templates which are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. See section 5.5 Where's the Template?, for more information.

-fno-implicit-inline-templates

Don't emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization will need the same set of explicit instantiations.

-finit-priority

Support `__attribute__ ((init_priority (n)))' for controlling the order of initialization of file-scope objects. On ELF targets, this requires GNU ld 2.10 or later.

-fno-implement-inlines

To save space, do not emit out-of-line copies of inline functions controlled by `#pragma implementation'. This will cause linker errors if these functions are not inlined everywhere they are called.

-fname-mangling-version-n

Control the way in which names are mangled. Version 0 is compatible with versions of g++ before 2.8. Version 1 is the default. Version 1 will allow correct mangling of function templates. For example, version 0 mangling does not mangle foo<int, double> and foo<int, char> given this declaration:

template <class T, class U> void foo(T t);

Like all options that change the ABI, all C++ code, including libgcc must be built with the same setting of this option.

-foperator-names

Recognize the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms for the symbols they refer to. `-ansi' implies `-foperator-names'.

-fno-optional-diags

Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such diagnostic issued by g++ is the one for a name having multiple meanings within a class.

-fpermissive

Downgrade messages about nonconformant code from errors to warnings. By default, g++ effectively sets `-pedantic-errors' without `-pedantic'; this option reverses that. This behavior and this option are superceded by `-pedantic', which works as it does for GNU C.

-frepo

Enable automatic template instantiation. This option also implies `-fno-implicit-templates'. See section 5.5 Where's the Template?, for more information.

-fno-rtti

Disable generation of the information used by C++ runtime type identification features (`dynamic_cast' and `typeid'). If you don't use those parts of the language (or exception handling, which uses `dynamic_cast' internally), you can save some space by using this flag.

-fstrict-prototype

Within an `extern "C"' linkage specification, treat a function declaration with no arguments, such as `int foo ();', as declaring the function to take no arguments. Normally, such a declaration means that the function foo can take any combination of arguments, as in C. `-pedantic' implies `-fstrict-prototype' unless overridden with `-fno-strict-prototype'. Specifying this option will also suppress implicit declarations of functions. This flag no longer affects declarations with C++ linkage.

-fsquangle

-fno-squangle

`-fsquangle' will enable a compressed form of name mangling for identifiers. In particular, it helps to shorten very long names by recognizing types and class names which occur more than once, replacing them with special short ID codes. This option also requires any C++ libraries being used to be compiled with this option as well. The compiler has this disabled (the equivalent of `-fno-squangle') by default. Like all options that change the ABI, all C++ code, including libgcc.a must be built with the same setting of this option.

-ftemplate-depth-n

Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17.

-fthis-is-variable

Permit assignment to this. The incorporation of user-defined free store management into C++ has made assignment to `this' an anachronism. Therefore, by default it is invalid to assign to this within a class member function; that is, GNU C++ treats `this' in a member function of class X as a non-lvalue of type `X *'. However, for backwards compatibility, you can make it valid with `-fthis-is-variable'.

-fvtable-thunks

Use `thunks' to implement the virtual function dispatch table (`vtable'). The traditional (cfront-style) approach to implementing vtables was to store a pointer to the function and two offsets for adjusting the `this' pointer at the call site. Newer implementations store a single pointer to a `thunk' function which does any necessary adjustment and then calls the target function. This option also enables a heuristic for controlling emission of vtables; if a class has any non-inline virtual functions, the vtable will be emitted in the translation unit containing the first one of those. Like all options that change the ABI, all C++ code, including libgcc.a must be built with the same setting of this option.

-nostdinc++

Do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.)

In addition, these optimization, warning, and code generation options have meanings only for C++ programs:

-fno-default-inline

Do not assume `inline' for functions defined inside a class scope. See section 2.8 Options That Control Optimization. Note that these functions will have linkage like inline functions; they just won't be inlined by default.

-Wctor-dtor-privacy (C++ only)

Warn when a class seems unusable, because all the constructors or destructors in a class are private and the class has no friends or public static member functions.

-Wnon-virtual-dtor (C++ only)

Warn when a class declares a non-virtual destructor that should probably be virtual, because it looks like the class will be used polymorphically.

-Wreorder (C++ only)

Warn when the order of member initializers given in the code does not match the order in which they must be executed. For instance:

struct A {
  int i;
  int j;
  A(): j (0), i (1) { }
};

Here the compiler will warn that the member initializers for `i' and `j' will be rearranged to match the declaration order of the members.

The following `-W...' options are not affected by `-Wall'.

-Weffc++ (C++ only)

Warn about violations of various style guidelines from Scott Meyers' Effective C++ books. If you use this option, you should be aware that the standard library headers do not obey all of these guidelines; you can use `grep -v' to filter out those warnings.

-Wno-deprecated (C++ only)

Do not warn about usage of deprecated features. See section 4.40 Deprecated Features.

-Wno-non-template-friend (C++ only)

Disable warnings when non-templatized friend functions are declared within a template. With the advent of explicit template specification support in g++, if the name of the friend is an unqualified-id (ie, `friend foo(int)'), the C++ language specification demands that the friend declare or define an ordinary, nontemplate function. (Section 14.5.3). Before g++ implemented explicit specification, unqualified-ids could be interpreted as a particular specialization of a templatized function. Because this non-conforming behavior is no longer the default behavior for g++, `-Wnon-template-friend' allows the compiler to check existing code for potential trouble spots, and is on by default. This new compiler behavior can also be turned off with the flag `-fguiding-decls', which activates the older, non-specification compiler code, or with `-Wno-non-template-friend' which keeps the conformant compiler code but disables the helpful warning.

-Wold-style-cast (C++ only)

Warn if an old-style (C-style) cast is used within a C++ program. The new-style casts (`static_cast', `reinterpret_cast', and `const_cast') are less vulnerable to unintended effects.

-Woverloaded-virtual (C++ only)

Warn when a derived class function declaration may be an error in defining a virtual function. In a derived class, the definitions of virtual functions must match the type signature of a virtual function declared in the base class. With this option, the compiler warns when you define a function with the same name as a virtual function, but with a type signature that does not match any declarations from the base class.

-Wno-pmf-conversions (C++ only)

Disable the diagnostic for converting a bound pointer to member function to a plain pointer.

-Wsign-promo (C++ only)

Warn when overload resolution chooses a promotion from unsigned or enumeral type to a signed type over a conversion to an unsigned type of the same size. Previous versions of g++ would try to preserve unsignedness, but the standard mandates the current behavior.

-Wsynth (C++ only)

Warn when g++'s synthesis behavior does not match that of cfront. For instance:

struct A {
  operator int ();
  A& operator = (int);
};

main ()
{
  A a,b;
  a = b;
}

In this example, g++ will synthesize a default `A& operator = (const A&);', while cfront will use the user-defined `operator ='.

2.6 Options to Request or Suppress Warnings

Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there may have been an error.

You can request many specific warnings with options beginning `-W', for example `-Wimplicit' to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning `-Wno-' to turn off warnings; for example, `-Wno-implicit'. This manual lists only one of the two forms, whichever is not the default.

These options control the amount and kinds of warnings produced by GCC:

-fsyntax-only

Check the code for syntax errors, but don't do anything beyond that.

-pedantic

Issue all the warnings demanded by strict ANSI C and ISO C++; reject all programs that use forbidden extensions. Valid ANSI C and ISO C++ programs should compile properly with or without this option (though a rare few will require `-ansi'). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are rejected. `-pedantic' does not cause warning messages for use of the alternate keywords whose names begin and end with `__'. Pedantic warnings are also disabled in the expression that follows __extension__. However, only system header files should use these escape routes; application programs should avoid them. See section 4.35 Alternate Keywords. This option is not intended to be useful; it exists only to satisfy pedants who would otherwise claim that GCC fails to support the ANSI standard. Some users try to use `-pedantic' to check programs for strict ANSI C conformance. They soon find that it does not do quite what they want: it finds some non-ANSI practices, but not all--only those for which ANSI C requires a diagnostic. A feature to report any failure to conform to ANSI C might be useful in some instances, but would require considerable additional work and would be quite different from `-pedantic'. We don't have plans to support such a feature in the near future.

-pedantic-errors

Like `-pedantic', except that errors are produced rather than warnings.

-w

Inhibit all warning messages.

-Wno-import

Inhibit warning messages about the use of `#import'.

-Wchar-subscripts

Warn if an array subscript has type char. This is a common cause of error, as programmers often forget that this type is signed on some machines.

-Wcomment

Warn whenever a comment-start sequence `/*' appears in a `/*' comment, or whenever a Backslash-Newline appears in a `//' comment.

-Wformat

Check calls to printf and scanf, etc., to make sure that the arguments supplied have types appropriate to the format string specified.

-Wimplicit-int

Warn when a declaration does not specify a type.

-Wimplicit-function-declaration

-Werror-implicit-function-declaration

Give a warning (or error) whenever a function is used before being declared.

-Wimplicit

Same as `-Wimplicit-int' and `-Wimplicit-function-'
`declaration'.

-Wmain

Warn if the type of `main' is suspicious. `main' should be a function with external linkage, returning int, taking either zero arguments, two, or three arguments of appropriate types.

-Wmultichar

Warn if a multicharacter constant (`'FOOF'') is used. Usually they indicate a typo in the user's code, as they have implementation-defined values, and should not be used in portable code.

-Wparentheses

Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about. Also warn about constructions where there may be confusion to which if statement an else branch belongs. Here is an example of such a case:

{
  if (a)
    if (b)
      foo ();
  else
    bar ();
}

In C, every else branch belongs to the innermost possible if statement, which in this example is if (b). This is often not what the programmer expected, as illustrated in the above example by indentation the programmer chose. When there is the potential for this confusion, GNU C will issue a warning when this flag is specified. To eliminate the warning, add explicit braces around the innermost if statement so there is no way the else could belong to the enclosing if. The resulting code would look like this:

{
  if (a)
    {
      if (b)
        foo ();
      else
        bar ();
    }
}

-Wreturn-type

Warn whenever a function is defined with a return-type that defaults to int. Also warn about any return statement with no return-value in a function whose return-type is not void.

-Wswitch

Warn whenever a switch statement has an index of enumeral type and lacks a case for one or more of the named codes of that enumeration. (The presence of a default label prevents this warning.) case labels outside the enumeration range also provoke warnings when this option is used.

-Wtrigraphs

Warn if any trigraphs are encountered (assuming they are enabled).

-Wunused

Warn whenever a variable is unused aside from its declaration, whenever a function is declared static but never defined, whenever a label is declared but not used, and whenever a statement computes a result that is explicitly not used. In order to get a warning about an unused function parameter, you must specify both `-W' and `-Wunused'. To suppress this warning for an expression, simply cast it to void. For unused variables, parameters and labels, use the `unused' attribute (see section 4.29 Specifying Attributes of Variables).

-Wuninitialized

An automatic variable is used without first being initialized. These warnings are possible only in optimizing compilation, because they require data flow information that is computed only when optimizing. If you don't specify `-O', you simply won't get these warnings. These warnings occur only for variables that are candidates for register allocation. Therefore, they do not occur for a variable that is declared volatile, or whose address is taken, or whose size is other than 1, 2, 4 or 8 bytes. Also, they do not occur for structures, unions or arrays, even when they are in registers. Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed. These warnings are made optional because GCC is not smart enough to see all the reasons why the code might be correct despite appearing to have an error. Here is one example of how this can happen:

{
  int x;
  switch (y)
    {
    case 1: x = 1;
      break;
    case 2: x = 4;
      break;
    case 3: x = 5;
    }
  foo (x);
}

If the value of y is always 1, 2 or 3, then x is always initialized, but GCC doesn't know this. Here is another common case:

{
  int save_y;
  if (change_y) save_y = y, y = new_y;
  ...
  if (change_y) y = save_y;
}

This has no bug because save_y is used only if it is set. Some spurious warnings can be avoided if you declare all the functions you use that never return as noreturn. See section 4.23 Declaring Attributes of Functions.

-Wunknown-pragmas

Warn when a #pragma directive is encountered which is not understood by GCC. If this command line option is used, warnings will even be issued for unknown pragmas in system header files. This is not the case if the warnings were only enabled by the `-Wall' command line option.

-Wall

All of the above `-W' options combined. This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros.

The following `-W...' options are not implied by `-Wall'. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning.

-W

Print extra warning messages for these events:

• A nonvolatile automatic variable might be changed by a call to longjmp. These warnings as well are possible only in optimizing compilation. The compiler sees only the calls to setjmp. It cannot know where longjmp will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem because longjmp cannot in fact be called at the place which would cause a problem.
• A function can return either with or without a value. (Falling off the end of the function body is considered returning without a value.) For example, this function would evoke such a warning:

  foo (a)
  {
    if (a > 0)
      return a;
  }
  

• An expression-statement or the left-hand side of a comma expression contains no side effects. To suppress the warning, cast the unused expression to void. For example, an expression such as `x[i,j]' will cause a warning, but `x[(void)i,j]' will not.
• An unsigned value is compared against zero with `<' or `<='.
• A comparison like `x<=y<=z' appears; this is equivalent to `(x<=y ? 1 : 0) <= z', which is a different interpretation from that of ordinary mathematical notation.
• Storage-class specifiers like static are not the first things in a declaration. According to the C Standard, this usage is obsolescent.
• If `-Wall' or `-Wunused' is also specified, warn about unused arguments.
• A comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. (But don't warn if `-Wno-sign-compare' is also specified.)
• An aggregate has a partly bracketed initializer. For example, the following code would evoke such a warning, because braces are missing around the initializer for x.h:

  struct s { int f, g; };
  struct t { struct s h; int i; };
  struct t x = { 1, 2, 3 };
  

• An aggregate has an initializer which does not initialize all members. For example, the following code would cause such a warning, because x.h would be implicitly initialized to zero:

  struct s { int f, g, h; };
  struct s x = { 3, 4 };
  

-Wtraditional

Warn about certain constructs that behave differently in traditional and ANSI C.

• Macro arguments occurring within string constants in the macro body. These would substitute the argument in traditional C, but are part of the constant in ANSI C.
• A function declared external in one block and then used after the end of the block.
• A switch statement has an operand of type long.
• A non-static function declaration follows a static one. This construct is not accepted by some traditional C compilers.

-Wundef

Warn if an undefined identifier is evaluated in an `#if' directive.

-Wshadow

Warn whenever a local variable shadows another local variable.

-Wid-clash-len

Warn whenever two distinct identifiers match in the first len characters. This may help you prepare a program that will compile with certain obsolete, brain-damaged compilers.

-Wlarger-than-len

Warn whenever an object of larger than len bytes is defined.

-Wpointer-arith

Warn about anything that depends on the "size of" a function type or of void. GNU C assigns these types a size of 1, for convenience in calculations with void * pointers and pointers to functions.

-Wbad-function-cast

Warn whenever a function call is cast to a non-matching type. For example, warn if int malloc() is cast to anything *.

-Wcast-qual

Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if a const char * is cast to an ordinary char *.

-Wcast-align

Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a char * is cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries.

-Wwrite-strings

Give string constants the type const char[length] so that copying the address of one into a non-const char * pointer will get a warning. These warnings will help you find at compile time code that can try to write into a string constant, but only if you have been very careful about using const in declarations and prototypes. Otherwise, it will just be a nuisance; this is why we did not make `-Wall' request these warnings.

-Wconversion

Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed point argument except when the same as the default promotion. Also, warn if a negative integer constant expression is implicitly converted to an unsigned type. For example, warn about the assignment x = -1 if x is unsigned. But do not warn about explicit casts like (unsigned) -1.

-Wsign-compare

Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. This warning is also enabled by `-W'; to get the other warnings of `-W' without this warning, use `-W -Wno-sign-compare'.

-Waggregate-return

Warn if any functions that return structures or unions are defined or called. (In languages where you can return an array, this also elicits a warning.)

-Wstrict-prototypes

Warn if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if preceded by a declaration which specifies the argument types.)

-Wmissing-prototypes

Warn if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. The aim is to detect global functions that fail to be declared in header files.

-Wmissing-declarations

Warn if a global function is defined without a previous declaration. Do so even if the definition itself provides a prototype. Use this option to detect global functions that are not declared in header files.

-Wmissing-noreturn

Warn about functions which might be candidates for attribute noreturn. Note these are only possible candidates, not absolute ones. Care should be taken to manually verify functions actually do not ever return before adding the noreturn attribute, otherwise subtle code generation bugs could be introduced.

-Wredundant-decls

Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing.

-Wnested-externs

Warn if an extern declaration is encountered within an function.

-Winline

Warn if a function can not be inlined, and either it was declared as inline, or else the `-finline-functions' option was given.

-Wlong-long

Warn if `long long' type is used. This is default. To inhibit the warning messages, use `-Wno-long-long'. Flags `-Wlong-long' and `-Wno-long-long' are taken into account only when `-pedantic' flag is used.

-Werror

Make all warnings into errors.

2.7 Options for Debugging Your Program or GCC

GCC has various special options that are used for debugging either your program or GCC:

-g

Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information. On most systems that use stabs format, `-g' enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but will probably make other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use `-gstabs+', `-gstabs', `-gxcoff+', `-gxcoff', `-gdwarf-1+', or `-gdwarf-1' (see below). Unlike most other C compilers, GCC allows you to use `-g' with `-O'. The shortcuts taken by optimized code may occasionally produce surprising results: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values were already at hand; some statements may execute in different places because they were moved out of loops. Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs. The following options are useful when GCC is generated with the capability for more than one debugging format.

-ggdb

Produce debugging information for use by GDB. This means to use the most expressive format available (DWARF 2, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible.

-gstabs

Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output which is not understood by DBX or SDB. On System V Release 4 systems this option requires the GNU assembler.

-gstabs+

Produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program.

-gcoff

Produce debugging information in COFF format (if that is supported). This is the format used by SDB on most System V systems prior to System V Release 4.

-gxcoff

Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems.

-gxcoff+

Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error.

-gdwarf

Produce debugging information in DWARF version 1 format (if that is supported). This is the format used by SDB on most System V Release 4 systems.

-gdwarf+

Produce debugging information in DWARF version 1 format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program.

-gdwarf-2

Produce debugging information in DWARF version 2 format (if that is supported). This is the format used by DBX on IRIX 6.

-glevel

-ggdblevel

-gstabslevel

-gcofflevel

-gxcofflevel

-gdwarflevel

-gdwarf-2level

Request debugging information and also use level to specify how much information. The default level is 2. Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, but no information about local variables and no line numbers. Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use `-g3'.

-p

Generate extra code to write profile information suitable for the analysis program prof. You must use this option when compiling the source files you want data about, and you must also use it when linking.

-pg

Generate extra code to write profile information suitable for the analysis program gprof. You must use this option when compiling the source files you want data about, and you must also use it when linking.

-a

Generate extra code to write profile information for basic blocks, which will record the number of times each basic block is executed, the basic block start address, and the function name containing the basic block. If `-g' is used, the line number and filename of the start of the basic block will also be recorded. If not overridden by the machine description, the default action is to append to the text file `bb.out'. This data could be analyzed by a program like tcov. Note, however, that the format of the data is not what tcov expects. Eventually GNU gprof should be extended to process this data.

-Q

Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes.

-ax

Generate extra code to profile basic blocks. Your executable will produce output that is a superset of that produced when `-a' is used. Additional output is the source and target address of the basic blocks where a jump takes place, the number of times a jump is executed, and (optionally) the complete sequence of basic blocks being executed. The output is appended to file `bb.out'. You can examine different profiling aspects without recompilation. Your executable will read a list of function names from file `bb.in'. Profiling starts when a function on the list is entered and stops when that invocation is exited. To exclude a function from profiling, prefix its name with `-'. If a function name is not unique, you can disambiguate it by writing it in the form `/path/filename.d:functionname'. Your executable will write the available paths and filenames in file `bb.out'. Several function names have a special meaning:

__bb_jumps__

Write source, target and frequency of jumps to file `bb.out'.

__bb_hidecall__

Exclude function calls from frequency count.

__bb_showret__

Include function returns in frequency count.

__bb_trace__

Write the sequence of basic blocks executed to file `bbtrace.gz'. The file will be compressed using the program `gzip', which must exist in your PATH. On systems without the `popen' function, the file will be named `bbtrace' and will not be compressed. Profiling for even a few seconds on these systems will produce a very large file. Note: __bb_hidecall__ and __bb_showret__ will not affect the sequence written to `bbtrace.gz'.

Here's a short example using different profiling parameters in file `bb.in'. Assume function foo consists of basic blocks 1 and 2 and is called twice from block 3 of function main. After the calls, block 3 transfers control to block 4 of main. With __bb_trace__ and main contained in file `bb.in', the following sequence of blocks is written to file `bbtrace.gz': 0 3 1 2 1 2 4. The return from block 2 to block 3 is not shown, because the return is to a point inside the block and not to the top. The block address 0 always indicates, that control is transferred to the trace from somewhere outside the observed functions. With `-foo' added to `bb.in', the blocks of function foo are removed from the trace, so only 0 3 4 remains. With __bb_jumps__ and main contained in file `bb.in', jump frequencies will be written to file `bb.out'. The frequencies are obtained by constructing a trace of blocks and incrementing a counter for every neighbouring pair of blocks in the trace. The trace 0 3 1 2 1 2 4 displays the following frequencies:

Jump from block 0x0 to block 0x3 executed 1 time(s)
Jump from block 0x3 to block 0x1 executed 1 time(s)
Jump from block 0x1 to block 0x2 executed 2 time(s)
Jump from block 0x2 to block 0x1 executed 1 time(s)
Jump from block 0x2 to block 0x4 executed 1 time(s)

With __bb_hidecall__, control transfer due to call instructions is removed from the trace, that is the trace is cut into three parts: 0 3 4, 0 1 2 and 0 1 2. With __bb_showret__, control transfer due to return instructions is added to the trace. The trace becomes: 0 3 1 2 3 1 2 3 4. Note, that this trace is not the same, as the sequence written to `bbtrace.gz'. It is solely used for counting jump frequencies.

-fprofile-arcs

Instrument arcs during compilation. For each function of your program, GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code. Since not every arc in the program must be instrumented, programs compiled with this option run faster than programs compiled with `-a', which adds instrumentation code to every basic block in the program. The tradeoff: since gcov does not have execution counts for all branches, it must start with the execution counts for the instrumented branches, and then iterate over the program flow graph until the entire graph has been solved. Hence, gcov runs a little more slowly than a program which uses information from `-a'. `-fprofile-arcs' also makes it possible to estimate branch probabilities, and to calculate basic block execution counts. In general, basic block execution counts do not give enough information to estimate all branch probabilities. When the compiled program exits, it saves the arc execution counts to a file called `sourcename.da'. Use the compiler option `-fbranch-probabilities' (see section 2.8 Options That Control Optimization) when recompiling, to optimize using estimated branch probabilities.

-ftest-coverage

Create data files for the gcov code-coverage utility (see section 6 gcov: a Test Coverage Program). The data file names begin with the name of your source file:

sourcename.bb

A mapping from basic blocks to line numbers, which gcov uses to associate basic block execution counts with line numbers.

sourcename.bbg

A list of all arcs in the program flow graph. This allows gcov to reconstruct the program flow graph, so that it can compute all basic block and arc execution counts from the information in the sourcename.da file (this last file is the output from `-fprofile-arcs').

-Q

Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes.

-dletters

Says to make debugging dumps during compilation at times specified by letters. This is used for debugging the compiler. The file names for most of the dumps are made by appending a word to the source file name (e.g. `foo.c.rtl' or `foo.c.jump'). Here are the possible letters for use in letters, and their meanings:

`b'

Dump after computing branch probabilities, to `file.bp'.

`c'

Dump after instruction combination, to the file `file.combine'.

`d'

Dump after delayed branch scheduling, to `file.dbr'.

`D'

Dump all macro definitions, at the end of preprocessing, in addition to normal output.

`r'

Dump after RTL generation, to `file.rtl'.

`j'

Dump after first jump optimization, to `file.jump'.

`F'

Dump after purging ADDRESSOF, to `file.addressof'.

`f'

Dump after flow analysis, to `file.flow'.

`g'

Dump after global register allocation, to `file.greg'.

`G'

Dump after GCSE, to `file.gcse'.

`j'

Dump after first jump optimization, to `file.jump'.

`J'

Dump after last jump optimization, to `file.jump2'.

`k'

Dump after conversion from registers to stack, to `file.stack'.

`l'

Dump after local register allocation, to `file.lreg'.

`L'

Dump after loop optimization, to `file.loop'.

`M'

Dump after performing the machine dependent reorganisation pass, to `file.mach'.

`N'

Dump after the register move pass, to `file.regmove'.

`r'

Dump after RTL generation, to `file.rtl'.

`R'

Dump after the second instruction scheduling pass, to `file.sched2'.

`s'

Dump after CSE (including the jump optimization that sometimes follows CSE), to `file.cse'.

`S'

Dump after the first instruction scheduling pass, to `file.sched'.

`t'

Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to `file.cse2'.

`a'

Produce all the dumps listed above.

`m'

Print statistics on memory usage, at the end of the run, to standard error.

`p'

Annotate the assembler output with a comment indicating which pattern and alternative was used. The length of each instruction is also printed.

`x'

Just generate RTL for a function instead of compiling it. Usually used with `r'.

`y'

Dump debugging information during parsing, to standard error.

`A'

Annotate the assembler output with miscellaneous debugging information.

-fdump-unnumbered

When doing debugging dumps (see -d option above), suppress instruction numbers and line number note output. This makes it more feasible to use diff on debugging dumps for compiler invokations with different options, in particular with and without -g.

-fpretend-float

When running a cross-compiler, pretend that the target machine uses the same floating point format as the host machine. This causes incorrect output of the actual floating constants, but the actual instruction sequence will probably be the same as GCC would make when running on the target machine.

-save-temps

Store the usual "temporary" intermediate files permanently; place them in the current directory and name them based on the source file. Thus, compiling `foo.c' with `-c -save-temps' would produce files `foo.i' and `foo.s', as well as `foo.o'.

-print-file-name=library

Print the full absolute name of the library file library that would be used when linking--and don't do anything else. With this option, GCC does not compile or link anything; it just prints the file name.

-print-prog-name=program

Like `-print-file-name', but searches for a program such as `cpp'.

-print-libgcc-file-name

Same as `-print-file-name=libgcc.a'. This is useful when you use `-nostdlib' or `-nodefaultlibs' but you do want to link with `libgcc.a'. You can do

gcc -nostdlib files... `gcc -print-libgcc-file-name`

-print-search-dirs

Print the name of the configured installation directory and a list of program and library directories gcc will search--and don't do anything else. This is useful when gcc prints the error message `installation problem, cannot exec cpp: No such file or directory'. To resolve this you either need to put `cpp' and the other compiler components where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don't forget the trailing '/'. See section 2.16 Environment Variables Affecting GCC.

2.8 Options That Control Optimization

These options control various sorts of optimizations:

-O

-O1

Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function. Without `-O', the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you would expect from the source code. Without `-O', the compiler only allocates variables declared register in registers. The resulting compiled code is a little worse than produced by PCC without `-O'. With `-O', the compiler tries to reduce code size and execution time. When you specify `-O', the compiler turns on `-fthread-jumps' and `-fdefer-pop' on all machines. The compiler turns on `-fdelayed-branch' on machines that have delay slots, and `-fomit-frame-pointer' on machines that can support debugging even without a frame pointer. On some machines the compiler also turns on other flags.

-O2

Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. The compiler does not perform loop unrolling or function inlining when you specify `-O2'. As compared to `-O', this option increases both compilation time and the performance of the generated code. `-O2' turns on all optional optimizations except for loop unrolling, function inlining, and strict aliasing optimizations. It also turns on the `-fforce-mem' option on all machines and frame pointer elimination on machines where doing so does not interfere with debugging.

-O3

Optimize yet more. `-O3' turns on all optimizations specified by `-O2' and also turns on the `inline-functions' option.

-O0

Do not optimize.

-Os

Optimize for size. `-Os' enables all `-O2' optimizations that do not typically increase code size. It also performs further optimizations designed to reduce code size. If you use multiple `-O' options, with or without level numbers, the last such option is the one that is effective.

Options of the form `-fflag' specify machine-independent flags. Most flags have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. In the table below, only one of the forms is listed--the one which is not the default. You can figure out the other form by either removing `no-' or adding it.

-ffloat-store

Do not store floating point variables in registers, and inhibit other options that might change whether a floating point value is taken from a register or memory. This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a double is supposed to have. Similarly for the x86 architecture. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use `-ffloat-store' for such programs, after modifying them to store all pertinent intermediate computations into variables.

-fno-default-inline

Do not make member functions inline by default merely because they are defined inside the class scope (C++ only). Otherwise, when you specify `-O', member functions defined inside class scope are compiled inline by default; i.e., you don't need to add `inline' in front of the member function name.

-fno-defer-pop

Always pop the arguments to each function call as soon as that function returns. For machines which must pop arguments after a function call, the compiler normally lets arguments accumulate on the stack for several function calls and pops them all at once.

-fforce-mem

Force memory operands to be copied into registers before doing arithmetic on them. This produces better code by making all memory references potential common subexpressions. When they are not common subexpressions, instruction combination should eliminate the separate register-load. The `-O2' option turns on this option.

-fforce-addr

Force memory address constants to be copied into registers before doing arithmetic on them. This may produce better code just as `-fforce-mem' may.

-fomit-frame-pointer

Don't keep the frame pointer in a register for functions that don't need one. This avoids the instructions to save, set up and restore frame pointers; it also makes an extra register available in many functions. It also makes debugging impossible on some machines. On some machines, such as the Vax, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't exist. The machine-description macro FRAME_POINTER_REQUIRED controls whether a target machine supports this flag. See section 17.5 Register Usage.

-fno-inline

Don't pay attention to the inline keyword. Normally this option is used to keep the compiler from expanding any functions inline. Note that if you are not optimizing, no functions can be expanded inline.

-finline-functions

Integrate all simple functions into their callers. The compiler heuristically decides which functions are simple enough to be worth integrating in this way. If all calls to a given function are integrated, and the function is declared static, then the function is normally not output as assembler code in its own right.

-finline-limit-n

By default, gcc limits the size of functions that can be inlined. This flag allows the control of this limit for functions that are explicitly marked as inline (ie marked with the inline keyword or defined within the class definition in c++). n is the size of functions that can be inlined in number of pseudo instructions (not counting parameter handling). The default value of n is 10000. Increasing this value can result in more inlined code at the cost of compilation time and memory consumption. Decreasing usually makes the compilation faster and less code will be inlined (which presumably means slower programs). This option is particularly useful for programs that use inlining heavily such as those based on recursive templates with c++. Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way, it represents a count of assembly instructions and as such its exact meaning might change from one release to an another.

-fkeep-inline-functions

Even if all calls to a given function are integrated, and the function is declared static, nevertheless output a separate run-time callable version of the function. This switch does not affect extern inline functions.

-fkeep-static-consts

Emit variables declared static const when optimization isn't turned on, even if the variables aren't referenced. GCC enables this option by default. If you want to force the compiler to check if the variable was referenced, regardless of whether or not optimization is turned on, use the `-fno-keep-static-consts' option.

-fno-function-cse

Do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly. This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used.

-ffast-math

This option allows GCC to violate some ANSI or IEEE rules and/or specifications in the interest of optimizing code for speed. For example, it allows the compiler to assume arguments to the sqrt function are non-negative numbers and that no floating-point values are NaNs. This option should never be turned on by any `-O' option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ANSI rules/specifications for math functions.

The following options control specific optimizations. The `-O2' option turns on all of these optimizations except `-funroll-loops' `-funroll-all-loops', and `-fstrict-aliasing'. On most machines, the `-O' option turns on the `-fthread-jumps' and `-fdelayed-branch' options, but specific machines may handle it differently.

You can use the following flags in the rare cases when "fine-tuning" of optimizations to be performed is desired.

-fstrength-reduce

Perform the optimizations of loop strength reduction and elimination of iteration variables.

-fthread-jumps

Perform optimizations where we check to see if a jump branches to a location where another comparison subsumed by the first is found. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false.

-fcse-follow-jumps

In common subexpression elimination, scan through jump instructions when the target of the jump is not reached by any other path. For example, when CSE encounters an if statement with an else clause, CSE will follow the jump when the condition tested is false.

-fcse-skip-blocks

This is similar to `-fcse-follow-jumps', but causes CSE to follow jumps which conditionally skip over blocks. When CSE encounters a simple if statement with no else clause, `-fcse-skip-blocks' causes CSE to follow the jump around the body of the if.

-frerun-cse-after-loop

Re-run common subexpression elimination after loop optimizations has been performed.

-frerun-loop-opt

Run the loop optimizer twice.

-fgcse

Perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation.

-fexpensive-optimizations

Perform a number of minor optimizations that are relatively expensive.

-foptimize-register-moves

-fregmove

Attempt to reassign register numbers in move instructions and as operands of other simple instructions in order to maximize the amount of register tying. This is especially helpful on machines with two-operand instructions. GCC enables this optimization by default with `-O2' or higher. Note -fregmove and -foptimize-register-moves are the same optimization.

-fdelayed-branch

If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions.

-fschedule-insns

If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to required data being unavailable. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating point instruction is required.

-fschedule-insns2

Similar to `-fschedule-insns', but requests an additional pass of instruction scheduling after register allocation has been done. This is especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle.

-ffunction-sections

-fdata-sections

Place each function or data item into its own section in the output file if the target supports arbitrary sections. The name of the function or the name of the data item determines the section's name in the output file. Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. HPPA processors running HP-UX and Sparc processors running Solaris 2 have linkers with such optimizations. Other systems using the ELF object format as well as AIX may have these optimizations in the future. Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker will create larger object and executable files and will also be slower. You will not be able to use gprof on all systems if you specify this option and you may have problems with debugging if you specify both this option and `-g'.

-fcaller-saves

Enable values to be allocated in registers that will be clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls. Such allocation is done only when it seems to result in better code than would otherwise be produced. This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead. For all machines, optimization level 2 and higher enables this flag by default.

-funroll-loops

Perform the optimization of loop unrolling. This is only done for loops whose number of iterations can be determined at compile time or run time. `-funroll-loops' implies both `-fstrength-reduce' and `-frerun-cse-after-loop'.

-funroll-all-loops

Perform the optimization of loop unrolling. This is done for all loops and usually makes programs run more slowly. `-funroll-all-loops' implies `-fstrength-reduce' as well as `-frerun-cse-after-loop'.

-fmove-all-movables

Forces all invariant computations in loops to be moved outside the loop.

-freduce-all-givs

Forces all general-induction variables in loops to be strength-reduced. Note: When compiling programs written in Fortran, `-fmove-all-movables' and `-freduce-all-givs' are enabled by default when you use the optimizer. These options may generate better or worse code; results are highly dependent on the structure of loops within the source code. These two options are intended to be removed someday, once they have helped determine the efficacy of various approaches to improving loop optimizations. Please let us (gcc@gcc.gnu.org and fortran@gnu.org) know how use of these options affects the performance of your production code. We're very interested in code that runs slower when these options are enabled.

-fno-peephole

Disable any machine-specific peephole optimizations.

-fbranch-probabilities

After running a program compiled with `-fprofile-arcs' (see section 2.7 Options for Debugging Your Program or GCC), you can compile it a second time using `-fbranch-probabilities', to improve optimizations based on guessing the path a branch might take. With `-fbranch-probabilities', GCC puts a `REG_EXEC_COUNT' note on the first instruction of each basic block, and a `REG_BR_PROB' note on each `JUMP_INSN' and `CALL_INSN'. These can be used to improve optimization. Currently, they are only used in one place: in `reorg.c', instead of guessing which path a branch is mostly to take, the `REG_BR_PROB' values are used to exactly determine which path is taken more often.

-fstrict-aliasing

Allows the compiler to assume the strictest aliasing rules applicable to the language being compiled. For C (and C++), this activates optimizations based on the type of expressions. In particular, an object of one type is assumed never to reside at the same address as an object of a different type, unless the types are almost the same. For example, an unsigned int can alias an int, but not a void* or a double. A character type may alias any other type. Pay special attention to code like this:

union a_union { 
  int i;
  double d;
};

int f() {
  a_union t;
  t.d = 3.0;
  return t.i;
}

The practice of reading from a different union member than the one most recently written to (called "type-punning") is common. Even with `-fstrict-aliasing', type-punning is allowed, provided the memory is accessed through the union type. So, the code above will work as expected. However, this code might not:

int f() { 
  a_union t;
  int* ip;
  t.d = 3.0;
  ip = &t.i;
  return *ip;
}

Every language that wishes to perform language-specific alias analysis should define a function that computes, given an tree node, an alias set for the node. Nodes in different alias sets are not allowed to alias. For an example, see the C front-end function c_get_alias_set.

2.9 Options Controlling the Preprocessor

These options control the C preprocessor, which is run on each C source file before actual compilation.

If you use the `-E' option, nothing is done except preprocessing. Some of these options make sense only together with `-E' because they cause the preprocessor output to be unsuitable for actual compilation.

-include file

Process file as input before processing the regular input file. In effect, the contents of file are compiled first. Any `-D' and `-U' options on the command line are always processed before `-include file', regardless of the order in which they are written. All the `-include' and `-imacros' options are processed in the order in which they are written.

-imacros file

Process file as input, discarding the resulting output, before processing the regular input file. Because the output generated from file is discarded, the only effect of `-imacros file' is to make the macros defined in file available for use in the main input. Any `-D' and `-U' options on the command line are always processed before `-imacros file', regardless of the order in which they are written. All the `-include' and `-imacros' options are processed in the order in which they are written.

-idirafter dir

Add the directory dir to the second include path. The directories on the second include path are searched when a header file is not found in any of the directories in the main include path (the one that `-I' adds to).

-iprefix prefix

Specify prefix as the prefix for subsequent `-iwithprefix' options.

-iwithprefix dir

Add a directory to the second include path. The directory's name is made by concatenating prefix and dir, where prefix was specified previously with `-iprefix'. If you have not specified a prefix yet, the directory containing the installed passes of the compiler is used as the default.

-iwithprefixbefore dir

Add a directory to the main include path. The directory's name is made by concatenating prefix and dir, as in the case of `-iwithprefix'.

-isystem dir

Add a directory to the beginning of the second include path, marking it as a system directory, so that it gets the same special treatment as is applied to the standard system directories.

-nostdinc

Do not search the standard system directories for header files. Only the directories you have specified with `-I' options (and the current directory, if appropriate) are searched. See section 2.12 Options for Directory Search, for information on `-I'. By using both `-nostdinc' and `-I-', you can limit the include-file search path to only those directories you specify explicitly.

-undef

Do not predefine any nonstandard macros. (Including architecture flags).

-E

Run only the C preprocessor. Preprocess all the C source files specified and output the results to standard output or to the specified output file.

-C

Tell the preprocessor not to discard comments. Used with the `-E' option.

-P

Tell the preprocessor not to generate `#line' directives. Used with the `-E' option.

-M

Tell the preprocessor to output a rule suitable for make describing the dependencies of each object file. For each source file, the preprocessor outputs one make-rule whose target is the object file name for that source file and whose dependencies are all the #include header files it uses. This rule may be a single line or may be continued with `\'-newline if it is long. The list of rules is printed on standard output instead of the preprocessed C program. `-M' implies `-E'. Another way to specify output of a make rule is by setting the environment variable DEPENDENCIES_OUTPUT (see section 2.16 Environment Variables Affecting GCC).

-MM

Like `-M' but the output mentions only the user header files included with `#include "file"'. System header files included with `#include <file>' are omitted.

-MD

Like `-M' but the dependency information is written to a file made by replacing ".c" with ".d" at the end of the input file names. This is in addition to compiling the file as specified---`-MD' does not inhibit ordinary compilation the way `-M' does. In Mach, you can use the utility md to merge multiple dependency files into a single dependency file suitable for using with the `make' command.

-MMD

Like `-MD' except mention only user header files, not system header files.

-MG

Treat missing header files as generated files and assume they live in the same directory as the source file. If you specify `-MG', you must also specify either `-M' or `-MM'. `-MG' is not supported with `-MD' or `-MMD'.

-H

Print the name of each header file used, in addition to other normal activities.

-Aquestion(answer)

Assert the answer answer for question, in case it is tested with a preprocessing conditional such as `#if #question(answer)'. `-A-' disables the standard assertions that normally describe the target machine.

-Dmacro

Define macro macro with the string `1' as its definition.

-Dmacro=defn

Define macro macro as defn. All instances of `-D' on the command line are processed before any `-U' options.

-Umacro

Undefine macro macro. `-U' options are evaluated after all `-D' options, but before any `-include' and `-imacros' options.

-dM

Tell the preprocessor to output only a list of the macro definitions that are in effect at the end of preprocessing. Used with the `-E' option.

-dD

Tell the preprocessing to pass all macro definitions into the output, in their proper sequence in the rest of the output.

-dN

Like `-dD' except that the macro arguments and contents are omitted. Only `#define name' is included in the output.

-trigraphs

Support ANSI C trigraphs. The `-ansi' option also has this effect.

-Wp,option

Pass option as an option to the preprocessor. If option contains commas, it is split into multiple options at the commas.

2.10 Passing Options to the Assembler

You can pass options to the assembler.

-Wa,option

Pass option as an option to the assembler. If option contains commas, it is split into multiple options at the commas.

2.11 Options for Linking

These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step.

object-file-name

A file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents.) If linking is done, these object files are used as input to the linker.

-c

-S

-E

If any of these options is used, then the linker is not run, and object file names should not be used as arguments. See section 2.2 Options Controlling the Kind of Output.

-llibrary

Search the library named library when linking. It makes a difference where in the command you write this option; the linker searches processes libraries and object files in the order they are specified. Thus, `foo.o -lz bar.o' searches library `z' after file `foo.o' but before `bar.o'. If `bar.o' refers to functions in `z', those functions may not be loaded. The linker searches a standard list of directories for the library, which is actually a file named `liblibrary.a'. The linker then uses this file as if it had been specified precisely by name. The directories searched include several standard system directories plus any that you specify with `-L'. Normally the files found this way are library files--archive files whose members are object files. The linker handles an archive file by scanning through it for members which define symbols that have so far been referenced but not defined. But if the file that is found is an ordinary object file, it is linked in the usual fashion. The only difference between using an `-l' option and specifying a file name is that `-l' surrounds library with `lib' and `.a' and searches several directories.

-lobjc

You need this special case of the `-l' option in order to link an Objective C program.

-nostartfiles

Do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib or -nodefaultlibs is used.

-nodefaultlibs

Do not use the standard system libraries when linking. Only the libraries you specify will be passed to the linker. The standard startup files are used normally, unless -nostartfiles is used. The compiler may generate calls to memcmp, memset, and memcpy for System V (and ANSI C) environments or to bcopy and bzero for BSD environments. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.

-nostdlib

Do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify will be passed to the linker. The compiler may generate calls to memcmp, memset, and memcpy for System V (and ANSI C) environments or to bcopy and bzero for BSD environments. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified. One of the standard libraries bypassed by `-nostdlib' and `-nodefaultlibs' is `libgcc.a', a library of internal subroutines that GCC uses to overcome shortcomings of particular machines, or special needs for some languages. (See section 13 Interfacing to GCC Output, for more discussion of `libgcc.a'.) In most cases, you need `libgcc.a' even when you want to avoid other standard libraries. In other words, when you specify `-nostdlib' or `-nodefaultlibs' you should usually specify `-lgcc' as well. This ensures that you have no unresolved references to internal GCC library subroutines. (For example, `__main', used to ensure C++ constructors will be called; see section 3.7 collect2.)

-s

Remove all symbol table and relocation information from the executable.

-static

On systems that support dynamic linking, this prevents linking with the shared libraries. On other systems, this option has no effect.

-shared

Produce a shared object which can then be linked with other objects to form an executable. Not all systems support this option. You must also specify `-fpic' or `-fPIC' on some systems when you specify this option.

-symbolic

Bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option `-Xlinker -z -Xlinker defs'). Only a few systems support this option.

-Xlinker option

Pass option as an option to the linker. You can use this to supply system-specific linker options which GCC does not know how to recognize. If you want to pass an option that takes an argument, you must use `-Xlinker' twice, once for the option and once for the argument. For example, to pass `-assert definitions', you must write `-Xlinker -assert -Xlinker definitions'. It does not work to write `-Xlinker "-assert definitions"', because this passes the entire string as a single argument, which is not what the linker expects.

-Wl,option

Pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas.

-u symbol

Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use `-u' multiple times with different symbols to force loading of additional library modules.

2.12 Options for Directory Search

These options specify directories to search for header files, for libraries and for parts of the compiler:

-Idir

Add the directory dir to the head of the list of directories to be searched for header files. This can be used to override a system header file, substituting your own version, since these directories are searched before the system header file directories. If you use more than one `-I' option, the directories are scanned in left-to-right order; the standard system directories come after.

-I-

Any directories you specify with `-I' options before the `-I-' option are searched only for the case of `#include "file"'; they are not searched for `#include <file>'. If additional directories are specified with `-I' options after the `-I-', these directories are searched for all `#include' directives. (Ordinarily all `-I' directories are used this way.) In addition, the `-I-' option inhibits the use of the current directory (where the current input file came from) as the first search directory for `#include "file"'. There is no way to override this effect of `-I-'. With `-I.' you can specify searching the directory which was current when the compiler was invoked. That is not exactly the same as what the preprocessor does by default, but it is often satisfactory. `-I-' does not inhibit the use of the standard system directories for header files. Thus, `-I-' and `-nostdinc' are independent.

-Ldir

Add directory dir to the list of directories to be searched for `-l'.

-Bprefix

This option specifies where to find the executables, libraries, include files, and data files of the compiler itself. The compiler driver program runs one or more of the subprograms `cpp', `cc1', `as' and `ld'. It tries prefix as a prefix for each program it tries to run, both with and without `machine/version/' (see section 2.13 Specifying Target Machine and Compiler Version). For each subprogram to be run, the compiler driver first tries the `-B' prefix, if any. If that name is not found, or if `-B' was not specified, the driver tries two standard prefixes, which are `/usr/lib/gcc/' and `/usr/local/lib/gcc-lib/'. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your `PATH' environment variable. `-B' prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into `-L' options for the linker. They also apply to includes files in the preprocessor, because the compiler translates these options into `-isystem' options for the preprocessor. In this case, the compiler appends `include' to the prefix. The run-time support file `libgcc.a' can also be searched for using the `-B' prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means. Another way to specify a prefix much like the `-B' prefix is to use the environment variable GCC_EXEC_PREFIX. See section 2.16 Environment Variables Affecting GCC.

-specs=file

Process file after the compiler reads in the standard `specs' file, in order to override the defaults that the `gcc' driver program uses when determining what switches to pass to `cc1', `cc1plus', `as', `ld', etc. More than one `-specs='file can be specified on the command line, and they are processed in order, from left to right.

2.13 Specifying Target Machine and Compiler Version

By default, GCC compiles code for the same type of machine that you are using. However, it can also be installed as a cross-compiler, to compile for some other type of machine. In fact, several different configurations of GCC, for different target machines, can be installed side by side. Then you specify which one to use with the `-b' option.

In addition, older and newer versions of GCC can be installed side by side. One of them (probably the newest) will be the default, but you may sometimes wish to use another.

-b machine

The argument machine specifies the target machine for compilation. This is useful when you have installed GCC as a cross-compiler. The value to use for machine is the same as was specified as the machine type when configuring GCC as a cross-compiler. For example, if a cross-compiler was configured with `configure i386v', meaning to compile for an 80386 running System V, then you would specify `-b i386v' to run that cross compiler. When you do not specify `-b', it normally means to compile for the same type of machine that you are using.

-V version

The argument version specifies which version of GCC to run. This is useful when multiple versions are installed. For example, version might be `2.0', meaning to run GCC version 2.0. The default version, when you do not specify `-V', is the last version of GCC that you installed.

The `-b' and `-V' options actually work by controlling part of the file name used for the executable files and libraries used for compilation. A given version of GCC, for a given target machine, is normally kept in the directory `/usr/local/lib/gcc-lib/machine/version'.

Thus, sites can customize the effect of `-b' or `-V' either by changing the names of these directories or adding alternate names (or symbolic links). If in directory `/usr/local/lib/gcc-lib/' the file `80386' is a link to the file `i386v', then `-b 80386' becomes an alias for `-b i386v'.

In one respect, the `-b' or `-V' do not completely change to a different compiler: the top-level driver program gcc that you originally invoked continues to run and invoke the other executables (preprocessor, compiler per se, assembler and linker) that do the real work. However, since no real work is done in the driver program, it usually does not matter that the driver program in use is not the one for the specified target and version.

The only way that the driver program depends on the target machine is in the parsing and handling of special machine-specific options. However, this is controlled by a file which is found, along with the other executables, in the directory for the specified version and target machine. As a result, a single installed driver program adapts to any specified target machine and compiler version.

The driver program executable does control one significant thing, however: the default version and target machine. Therefore, you can install different instances of the driver program, compiled for different targets or versions, under different names.

For example, if the driver for version 2.0 is installed as ogcc and that for version 2.1 is installed as gcc, then the command gcc will use version 2.1 by default, while ogcc will use 2.0 by default. However, you can choose either version with either command with the `-V' option.

2.14 Hardware Models and Configurations

Earlier we discussed the standard option `-b' which chooses among different installed compilers for completely different target machines, such as Vax vs. 68000 vs. 80386.

In addition, each of these target machine types can have its own special options, starting with `-m', to choose among various hardware models or configurations--for example, 68010 vs 68020, floating coprocessor or none. A single installed version of the compiler can compile for any model or configuration, according to the options specified.

Some configurations of the compiler also support additional special options, usually for compatibility with other compilers on the same platform.

These options are defined by the macro TARGET_SWITCHES in the machine description. The default for the options is also defined by that macro, which enables you to change the defaults.

2.14.1 M680x0 Options

These are the `-m' options defined for the 68000 series. The default values for these options depends on which style of 68000 was selected when the compiler was configured; the defaults for the most common choices are given below.

-m68000

-mc68000

Generate output for a 68000. This is the default when the compiler is configured for 68000-based systems. Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.

-m68020

-mc68020

Generate output for a 68020. This is the default when the compiler is configured for 68020-based systems.

-m68881

Generate output containing 68881 instructions for floating point. This is the default for most 68020 systems unless `-nfp' was specified when the compiler was configured.

-m68030

Generate output for a 68030. This is the default when the compiler is configured for 68030-based systems.

-m68040

Generate output for a 68040. This is the default when the compiler is configured for 68040-based systems. This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Use this option if your 68040 does not have code to emulate those instructions.

-m68060

Generate output for a 68060. This is the default when the compiler is configured for 68060-based systems. This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use this option if your 68060 does not have code to emulate those instructions.

-mcpu32

Generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems. Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360.

-m5200

Generate output for a 520X "coldfire" family cpu. This is the default when the compiler is configured for 520X-based systems. Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5202.

-m68020-40

Generate output for a 68040, without using any of the new instructions. This results in code which can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040.

-m68020-60

Generate output for a 68060, without using any of the new instructions. This results in code which can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68060.

-mfpa

Generate output containing Sun FPA instructions for floating point.

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all m68k targets. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets `m68k-*-aout' and `m68k-*-coff' do provide software floating point support.

-mshort

Consider type int to be 16 bits wide, like short int.

-mnobitfield

Do not use the bit-field instructions. The `-m68000', `-mcpu32' and `-m5200' options imply `-mnobitfield'.

-mbitfield

Do use the bit-field instructions. The `-m68020' option implies `-mbitfield'. This is the default if you use a configuration designed for a 68020.

-mrtd

Use a different function-calling convention, in which functions that take a fixed number of arguments return with the rtd instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there. This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code will be generated for calls to those functions. In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.) The rtd instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.

-malign-int

-mno-align-int

Control whether GCC aligns int, long, long long, float, double, and long double variables on a 32-bit boundary (`-malign-int') or a 16-bit boundary (`-mno-align-int'). Aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses at the expense of more memory. Warning: if you use the `-malign-int' switch, GCC will align structures containing the above types differently than most published application binary interface specifications for the m68k.

2.14.2 VAX Options

These `-m' options are defined for the Vax:

-munix

Do not output certain jump instructions (aobleq and so on) that the Unix assembler for the Vax cannot handle across long ranges.

-mgnu

Do output those jump instructions, on the assumption that you will assemble with the GNU assembler.

-mg

Output code for g-format floating point numbers instead of d-format.

2.14.3 SPARC Options

These `-m' switches are supported on the SPARC:

-mno-app-regs

-mapp-regs

Specify `-mapp-regs' to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. This is the default. To be fully SVR4 ABI compliant at the cost of some performance loss, specify `-mno-app-regs'. You should compile libraries and system software with this option.

-mfpu

-mhard-float

Generate output containing floating point instructions. This is the default.

-mno-fpu

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets `sparc-*-aout' and `sparclite-*-*' do provide software floating point support. `-msoft-float' changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile `libgcc.a', the library that comes with GCC, with `-msoft-float' in order for this to work.

-mhard-quad-float

Generate output containing quad-word (long double) floating point instructions.

-msoft-quad-float

Generate output containing library calls for quad-word (long double) floating point instructions. The functions called are those specified in the SPARC ABI. This is the default. As of this writing, there are no sparc implementations that have hardware support for the quad-word floating point instructions. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the `-msoft-quad-float' option is the default.

-mno-epilogue

-mepilogue

With `-mepilogue' (the default), the compiler always emits code for function exit at the end of each function. Any function exit in the middle of the function (such as a return statement in C) will generate a jump to the exit code at the end of the function. With `-mno-epilogue', the compiler tries to emit exit code inline at every function exit.

-mno-flat

-mflat

With `-mflat', the compiler does not generate save/restore instructions and will use a "flat" or single register window calling convention. This model uses %i7 as the frame pointer and is compatible with the normal register window model. Code from either may be intermixed. The local registers and the input registers (0-5) are still treated as "call saved" registers and will be saved on the stack as necessary. With `-mno-flat' (the default), the compiler emits save/restore instructions (except for leaf functions) and is the normal mode of operation.

-mno-unaligned-doubles

-munaligned-doubles

Assume that doubles have 8 byte alignment. This is the default. With `-munaligned-doubles', GCC assumes that doubles have 8 byte alignment only if they are contained in another type, or if they have an absolute address. Otherwise, it assumes they have 4 byte alignment. Specifying this option avoids some rare compatibility problems with code generated by other compilers. It is not the default because it results in a performance loss, especially for floating point code.

-mv8

-msparclite

These two options select variations on the SPARC architecture. By default (unless specifically configured for the Fujitsu SPARClite), GCC generates code for the v7 variant of the SPARC architecture. `-mv8' will give you SPARC v8 code. The only difference from v7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC v8 but not in SPARC v7. `-msparclite' will give you SPARClite code. This adds the integer multiply, integer divide step and scan (ffs) instructions which exist in SPARClite but not in SPARC v7. These options are deprecated and will be deleted in a future GCC release. They have been replaced with `-mcpu=xxx'.

-mcypress

-msupersparc

These two options select the processor for which the code is optimised. With `-mcypress' (the default), the compiler optimizes code for the Cypress CY7C602 chip, as used in the SparcStation/SparcServer 3xx series. This is also appropriate for the older SparcStation 1, 2, IPX etc. With `-msupersparc' the compiler optimizes code for the SuperSparc cpu, as used in the SparcStation 10, 1000 and 2000 series. This flag also enables use of the full SPARC v8 instruction set. These options are deprecated and will be deleted in a future GCC release. They have been replaced with `-mcpu=xxx'.

-mcpu=cpu_type

Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are `v7', `cypress', `v8', `supersparc', `sparclite', `hypersparc', `sparclite86x', `f930', `f934', `sparclet', `tsc701', `v9', and `ultrasparc'. Default instruction scheduling parameters are used for values that select an architecture and not an implementation. These are `v7', `v8', `sparclite', `sparclet', `v9'. Here is a list of each supported architecture and their supported implementations.

    v7:             cypress
    v8:             supersparc, hypersparc
    sparclite:      f930, f934, sparclite86x
    sparclet:       tsc701
    v9:             ultrasparc

-mtune=cpu_type

Set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option `-mcpu='cpu_type would. The same values for `-mcpu='cpu_type are used for `-mtune='
cpu_type, though the only useful values are those that select a particular cpu implementation: `cypress', `supersparc', `hypersparc', `f930', `f934', `sparclite86x', `tsc701', `ultrasparc'.

-malign-loops=num

Align loops to a 2 raised to a num byte boundary. If `-malign-loops' is not specified, the default is 2.

-malign-jumps=num

Align instructions that are only jumped to to a 2 raised to a num byte boundary. If `-malign-jumps' is not specified, the default is 2.

-malign-functions=num

Align the start of functions to a 2 raised to num byte boundary. If `-malign-functions' is not specified, the default is 2 if compiling for 32 bit sparc, and 5 if compiling for 64 bit sparc.

These `-m' switches are supported in addition to the above on the SPARCLET processor.

-mlittle-endian

Generate code for a processor running in little-endian mode.

-mlive-g0

Treat register %g0 as a normal register. GCC will continue to clobber it as necessary but will not assume it always reads as 0.

-mbroken-saverestore

Generate code that does not use non-trivial forms of the save and restore instructions. Early versions of the SPARCLET processor do not correctly handle save and restore instructions used with arguments. They correctly handle them used without arguments. A save instruction used without arguments increments the current window pointer but does not allocate a new stack frame. It is assumed that the window overflow trap handler will properly handle this case as will interrupt handlers.

These `-m' switches are supported in addition to the above on SPARC V9 processors in 64 bit environments.

-mlittle-endian

Generate code for a processor running in little-endian mode.

-m32

-m64

Generate code for a 32 bit or 64 bit environment. The 32 bit environment sets int, long and pointer to 32 bits. The 64 bit environment sets int to 32 bits and long and pointer to 64 bits.

-mcmodel=medlow

Generate code for the Medium/Low code model: the program must be linked in the low 32 bits of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked.

-mcmodel=medmid

Generate code for the Medium/Middle code model: the program must be linked in the low 44 bits of the address space, the text segment must be less than 2G bytes, and data segment must be within 2G of the text segment. Pointers are 64 bits.

-mcmodel=medany

Generate code for the Medium/Anywhere code model: the program may be linked anywhere in the address space, the text segment must be less than 2G bytes, and data segment must be within 2G of the text segment. Pointers are 64 bits.

-mcmodel=embmedany

Generate code for the Medium/Anywhere code model for embedded systems: assume a 32 bit text and a 32 bit data segment, both starting anywhere (determined at link time). Register %g4 points to the base of the data segment. Pointers still 64 bits. Programs are statically linked, PIC is not supported.

-mstack-bias

-mno-stack-bias

With `-mstack-bias', GCC assumes that the stack pointer, and frame pointer if present, are offset by -2047 which must be added back when making stack frame references. Otherwise, assume no such offset is present.

2.14.4 Convex Options

These `-m' options are defined for Convex:

-mc1

Generate output for C1. The code will run on any Convex machine. The preprocessor symbol __convex__c1__ is defined.

-mc2

Generate output for C2. Uses instructions not available on C1. Scheduling and other optimizations are chosen for max performance on C2. The preprocessor symbol __convex_c2__ is defined.

-mc32

Generate output for C32xx. Uses instructions not available on C1. Scheduling and other optimizations are chosen for max performance on C32. The preprocessor symbol __convex_c32__ is defined.

-mc34

Generate output for C34xx. Uses instructions not available on C1. Scheduling and other optimizations are chosen for max performance on C34. The preprocessor symbol __convex_c34__ is defined.

-mc38

Generate output for C38xx. Uses instructions not available on C1. Scheduling and other optimizations are chosen for max performance on C38. The preprocessor symbol __convex_c38__ is defined.

-margcount

Generate code which puts an argument count in the word preceding each argument list. This is compatible with regular CC, and a few programs may need the argument count word. GDB and other source-level debuggers do not need it; this info is in the symbol table.

-mnoargcount

Omit the argument count word. This is the default.

-mvolatile-cache

Allow volatile references to be cached. This is the default.

-mvolatile-nocache

Volatile references bypass the data cache, going all the way to memory. This is only needed for multi-processor code that does not use standard synchronization instructions. Making non-volatile references to volatile locations will not necessarily work.

-mlong32

Type long is 32 bits, the same as type int. This is the default.

-mlong64

Type long is 64 bits, the same as type long long. This option is useless, because no library support exists for it.

2.14.5 AMD29K Options

These `-m' options are defined for the AMD Am29000:

-mdw

Generate code that assumes the DW bit is set, i.e., that byte and halfword operations are directly supported by the hardware. This is the default.

-mndw

Generate code that assumes the DW bit is not set.

-mbw

Generate code that assumes the system supports byte and halfword write operations. This is the default.

-mnbw

Generate code that assumes the systems does not support byte and halfword write operations. `-mnbw' implies `-mndw'.

-msmall

Use a small memory model that assumes that all function addresses are either within a single 256 KB segment or at an absolute address of less than 256k. This allows the call instruction to be used instead of a const, consth, calli sequence.

-mnormal

Use the normal memory model: Generate call instructions only when calling functions in the same file and calli instructions otherwise. This works if each file occupies less than 256 KB but allows the entire executable to be larger than 256 KB. This is the default.

-mlarge

Always use calli instructions. Specify this option if you expect a single file to compile into more than 256 KB of code.

-m29050

Generate code for the Am29050.

-m29000

Generate code for the Am29000. This is the default.

-mkernel-registers

Generate references to registers gr64-gr95 instead of to registers gr96-gr127. This option can be used when compiling kernel code that wants a set of global registers disjoint from that used by user-mode code. Note that when this option is used, register names in `-f' flags must use the normal, user-mode, names.

-muser-registers

Use the normal set of global registers, gr96-gr127. This is the default.

-mstack-check

-mno-stack-check

Insert (or do not insert) a call to __msp_check after each stack adjustment. This is often used for kernel code.

-mstorem-bug

-mno-storem-bug

`-mstorem-bug' handles 29k processors which cannot handle the separation of a mtsrim insn and a storem instruction (most 29000 chips to date, but not the 29050).

-mno-reuse-arg-regs

-mreuse-arg-regs

`-mno-reuse-arg-regs' tells the compiler to only use incoming argument registers for copying out arguments. This helps detect calling a function with fewer arguments than it was declared with.

-mno-impure-text

-mimpure-text

`-mimpure-text', used in addition to `-shared', tells the compiler to not pass `-assert pure-text' to the linker when linking a shared object.

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

-mno-multm

Do not generate multm or multmu instructions. This is useful for some embedded systems which do not have trap handlers for these instructions.

2.14.6 ARM Options

These `-m' options are defined for Advanced RISC Machines (ARM) architectures:

-mapcs-frame

Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is not strictly necessary for correct execution of the code. Specifying `-fomit-frame-pointer' with this option will cause the stack frames not to be generated for leaf functions. The default is `-mno-apcs-frame'.

-mapcs

This is a synonym for `-mapcs-frame'.

-mapcs-26

Generate code for a processor running with a 26-bit program counter, and conforming to the function calling standards for the APCS 26-bit option. This option replaces the `-m2' and `-m3' options of previous releases of the compiler.

-mapcs-32

Generate code for a processor running with a 32-bit program counter, and conforming to the function calling standards for the APCS 32-bit option. This option replaces the `-m6' option of previous releases of the compiler.

-mapcs-stack-check

Generate code to check the amount of stack space available upon entry to every function (that actually uses some stack space). If there is insufficient space available then either the function `__rt_stkovf_split_small' or `__rt_stkovf_split_big' will be called, depending upon the amount of stack space required. The run time system is required to provide these functions. The default is `-mno-apcs-stack-check', since this produces smaller code.

-mapcs-float

Pass floating point arguments using the float point registers. This is one of the variants of the APCS. This option is reccommended if the target hardware has a floating point unit or if a lot of floating point arithmetic is going to be performed by the code. The default is `-mno-apcs-float', since integer only code is slightly increased in size if `-mapcs-float' is used.

-mapcs-reentrant

Generate reentrant, position independent code. This is the equivalent to specifying the `-fpic' option. The default is `-mno-apcs-reentrant'.

-mthumb-interwork

Generate code which supports calling between the ARM and THUMB instruction sets. Without this option the two instruction sets cannot be reliably used inside one program. The default is `-mno-thumb-interwork', since slightly larger code is generated when `-mthumb-interwork' is specified.

-mno-sched-prolog

Prevent the reordering of instructions in the function prolog, or the merging of those instruction with the instructions in the function's body. This means that all functions will start with a recognisable set of instructions (or in fact one of a chioce from a small set of different function prologues), and this information can be used to locate the start if functions inside an executable piece of code. The default is `-msched-prolog'.

-mhard-float

Generate output containing floating point instructions. This is the default.

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all ARM targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. `-msoft-float' changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile `libgcc.a', the library that comes with GCC, with `-msoft-float' in order for this to work.

-mlittle-endian

Generate code for a processor running in little-endian mode. This is the default for all standard configurations.

-mbig-endian

Generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor.

-mwords-little-endian

This option only applies when generating code for big-endian processors. Generate code for a little-endian word order but a big-endian byte order. That is, a byte order of the form `32107654'. Note: this option should only be used if you require compatibility with code for big-endian ARM processors generated by versions of the compiler prior to 2.8.

-mshort-load-bytes

Do not try to load half-words (eg `short's) by loading a word from an unaligned address. For some targets the MMU is configured to trap unaligned loads; use this option to generate code that is safe in these environments.

-mno-short-load-bytes

Use unaligned word loads to load half-words (eg `short's). This option produces more efficient code, but the MMU is sometimes configured to trap these instructions.

-mshort-load-words

This is a synonym for the `-mno-short-load-bytes'.

-mno-short-load-words

This is a synonym for the `-mshort-load-bytes'.

-mbsd

This option only applies to RISC iX. Emulate the native BSD-mode compiler. This is the default if `-ansi' is not specified.

-mxopen

This option only applies to RISC iX. Emulate the native X/Open-mode compiler.

-mno-symrename

This option only applies to RISC iX. Do not run the assembler post-processor, `symrename', after code has been assembled. Normally it is necessary to modify some of the standard symbols in preparation for linking with the RISC iX C library; this option suppresses this pass. The post-processor is never run when the compiler is built for cross-compilation.

-mcpu=<name>

-mtune=<name>

This specifies the name of the target ARM processor. GCC uses this name to determine what kind of instructions it can use when generating assembly code. Permissable names are: arm2, arm250, arm3, arm6, arm60, arm600, arm610, arm620, arm7, arm7m, arm7d, arm7dm, arm7di, arm7dmi, arm70, arm700, arm700i, arm710, arm710c, arm7100, arm7500, arm7500fe, arm7tdmi, arm8, strongarm, strongarm110, strongarm1100, arm8, arm810, arm9, arm9tdmi. `-mtune=' is a synonym for `-mcpue=' to support older versions of GCC.

-march=<name>

This specifies the name of the target ARM architecture. GCC uses this name to determine what kind of instructions it can use when generating assembly code. This option can be used in conjunction with or instead of the `-mcpu=' option. Permissable names are: armv2, armv2a, armv3, armv3m, armv4, armv4t

-mfpe=<number>

-mfp=<number>

This specifes the version of the floating point emulation available on the target. Permissable values are 2 and 3. `-mfp=' is a synonym for `-mfpe=' to support older versions of GCC.

-mstructure-size-boundary=<n>

The size of all structures and unions will be rounded up to a multiple of the number of bits set by this option. Permissable values are 8 and 32. The default value varies for different toolchains. For the COFF targeted toolchain the default value is 8. Specifying the larger number can produced faster, more efficient code, but can also increase the size of the program. The two values are potentially incompatible. Code compiled with one value cannot necessarily expect to work with code or libraries compiled with the other value, if they exchange information using structures or unions. Programmers are encouraged to use the 32 value as future versions of the toolchain may default to this value.

-mabort-on-noreturn

Generate a call to the function abort at the end of a noreturn function. It will be executed if the function tries to return.

2.14.7 Thumb Options

-mthumb-interwork

Generate code which supports calling between the THUMB and ARM instruction sets. Without this option the two instruction sets cannot be reliably used inside one program. The default is `-mno-thumb-interwork', since slightly smaller code is generated with this option.

-mtpcs-frame

Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that does not call any other functions). The default is `-mno-apcs-frame'.

-mtpcs-leaf-frame

Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf function is one that does not call any other functions). The default is `-mno-apcs-leaf-frame'.

-mlittle-endian

Generate code for a processor running in little-endian mode. This is the default for all standard configurations.

-mbig-endian

Generate code for a processor running in big-endian mode.

-mstructure-size-boundary=<n>

The size of all structures and unions will be rounded up to a multiple of the number of bits set by this option. Permissable values are 8 and 32. The default value varies for different toolchains. For the COFF targeted toolchain the default value is 8. Specifying the larger number can produced faster, more efficient code, but can also increase the size of the program. The two values are potentially incompatible. Code compiled with one value cannot necessarily expect to work with code or libraries compiled with the other value, if they exchange information using structures or unions. Programmers are encouraged to use the 32 value as future versions of the toolchain may default to this value.

2.14.8 MN10200 Options

These `-m' options are defined for Matsushita MN10200 architectures:

-mrelax

Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only has an effect when used on the command line for the final link step. This option makes symbolic debugging impossible.

2.14.9 MN10300 Options

These `-m' options are defined for Matsushita MN10300 architectures:

-mmult-bug

Generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.

-mno-mult-bug

Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.

-mrelax

Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only has an effect when used on the command line for the final link step. This option makes symbolic debugging impossible.

2.14.10 M32R/D Options

These `-m' options are defined for Mitsubishi M32R/D architectures:

-mcode-model=small

Assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with the ld24 instruction), and assume all subroutines are reachable with the bl instruction. This is the default. The addressability of a particular object can be set with the model attribute.

-mcode-model=medium

Assume objects may be anywhere in the 32 bit address space (the compiler will generate seth/add3 instructions to load their addresses), and assume all subroutines are reachable with the bl instruction.

-mcode-model=large

Assume objects may be anywhere in the 32 bit address space (the compiler will generate seth/add3 instructions to load their addresses), and assume subroutines may not be reachable with the bl instruction (the compiler will generate the much slower seth/add3/jl instruction sequence).

-msdata=none

Disable use of the small data area. Variables will be put into one of `.data', `bss', or `.rodata' (unless the section attribute has been specified). This is the default. The small data area consists of sections `.sdata' and `.sbss'. Objects may be explicitly put in the small data area with the section attribute using one of these sections.

-msdata=sdata

Put small global and static data in the small data area, but do not generate special code to reference them.

-msdata=use

Put small global and static data in the small data area, and generate special instructions to reference them.

-G num

Put global and static objects less than or equal to num bytes into the small data or bss sections instead of the normal data or bss sections. The default value of num is 8. The `-msdata' option must be set to one of `sdata' or `use' for this option to have any effect. All modules should be compiled with the same `-G num' value. Compiling with different values of num may or may not work; if it doesn't the linker will give an error message - incorrect code will not be generated.

2.14.11 M88K Options

These `-m' options are defined for Motorola 88k architectures:

-m88000

Generate code that works well on both the m88100 and the m88110.

-m88100

Generate code that works best for the m88100, but that also runs on the m88110.

-m88110

Generate code that works best for the m88110, and may not run on the m88100.

-mbig-pic

Obsolete option to be removed from the next revision. Use `-fPIC'.

-midentify-revision

Include an ident directive in the assembler output recording the source file name, compiler name and version, timestamp, and compilation flags used.

-mno-underscores

In assembler output, emit symbol names without adding an underscore character at the beginning of each name. The default is to use an underscore as prefix on each name.

-mocs-debug-info

-mno-ocs-debug-info

Include (or omit) additional debugging information (about registers used in each stack frame) as specified in the 88open Object Compatibility Standard, "OCS". This extra information allows debugging of code that has had the frame pointer eliminated. The default for DG/UX, SVr4, and Delta 88 SVr3.2 is to include this information; other 88k configurations omit this information by default.

-mocs-frame-position

When emitting COFF debugging information for automatic variables and parameters stored on the stack, use the offset from the canonical frame address, which is the stack pointer (register 31) on entry to the function. The DG/UX, SVr4, Delta88 SVr3.2, and BCS configurations use `-mocs-frame-position'; other 88k configurations have the default `-mno-ocs-frame-position'.

-mno-ocs-frame-position

When emitting COFF debugging information for automatic variables and parameters stored on the stack, use the offset from the frame pointer register (register 30). When this option is in effect, the frame pointer is not eliminated when debugging information is selected by the -g switch.

-moptimize-arg-area

-mno-optimize-arg-area

Control how function arguments are stored in stack frames. `-moptimize-arg-area' saves space by optimizing them, but this conflicts with the 88open specifications. The opposite alternative, `-mno-optimize-arg-area', agrees with 88open standards. By default GCC does not optimize the argument area.

-mshort-data-num

Generate smaller data references by making them relative to r0, which allows loading a value using a single instruction (rather than the usual two). You control which data references are affected by specifying num with this option. For example, if you specify `-mshort-data-512', then the data references affected are those involving displacements of less than 512 bytes. `-mshort-data-num' is not effective for num greater than 64k.

-mserialize-volatile

-mno-serialize-volatile

Do, or don't, generate code to guarantee sequential consistency of volatile memory references. By default, consistency is guaranteed. The order of memory references made by the MC88110 processor does not always match the order of the instructions requesting those references. In particular, a load instruction may execute before a preceding store instruction. Such reordering violates sequential consistency of volatile memory references, when there are multiple processors. When consistency must be guaranteed, GNU C generates special instructions, as needed, to force execution in the proper order. The MC88100 processor does not reorder memory references and so always provides sequential consistency. However, by default, GNU C generates the special instructions to guarantee consistency even when you use `-m88100', so that the code may be run on an MC88110 processor. If you intend to run your code only on the MC88100 processor, you may use `-mno-serialize-volatile'. The extra code generated to guarantee consistency may affect the performance of your application. If you know that you can safely forgo this guarantee, you may use `-mno-serialize-volatile'.

-msvr4

-msvr3

Turn on (`-msvr4') or off (`-msvr3') compiler extensions related to System V release 4 (SVr4). This controls the following:

1. Which variant of the assembler syntax to emit.
2. `-msvr4' makes the C preprocessor recognize `#pragma weak' that is used on System V release 4.
3. `-msvr4' makes GCC issue additional declaration directives used in SVr4.

`-msvr4' is the default for the m88k-motorola-sysv4 and m88k-dg-dgux m88k configurations. `-msvr3' is the default for all other m88k configurations.

-mversion-03.00

This option is obsolete, and is ignored.

-mno-check-zero-division

-mcheck-zero-division

Do, or don't, generate code to guarantee that integer division by zero will be detected. By default, detection is guaranteed. Some models of the MC88100 processor fail to trap upon integer division by zero under certain conditions. By default, when compiling code that might be run on such a processor, GNU C generates code that explicitly checks for zero-valued divisors and traps with exception number 503 when one is detected. Use of mno-check-zero-division suppresses such checking for code generated to run on an MC88100 processor. GNU C assumes that the MC88110 processor correctly detects all instances of integer division by zero. When `-m88110' is specified, both `-mcheck-zero-division' and `-mno-check-zero-division' are ignored, and no explicit checks for zero-valued divisors are generated.

-muse-div-instruction

Use the div instruction for signed integer division on the MC88100 processor. By default, the div instruction is not used. On the MC88100 processor the signed integer division instruction div) traps to the operating system on a negative operand. The operating system transparently completes the operation, but at a large cost in execution time. By default, when compiling code that might be run on an MC88100 processor, GNU C emulates signed integer division using the unsigned integer division instruction divu), thereby avoiding the large penalty of a trap to the operating system. Such emulation has its own, smaller, execution cost in both time and space. To the extent that your code's important signed integer division operations are performed on two nonnegative operands, it may be desirable to use the div instruction directly. On the MC88110 processor the div instruction (also known as the divs instruction) processes negative operands without trapping to the operating system. When `-m88110' is specified, `-muse-div-instruction' is ignored, and the div instruction is used for signed integer division. Note that the result of dividing INT_MIN by -1 is undefined. In particular, the behavior of such a division with and without `-muse-div-instruction' may differ.

-mtrap-large-shift

-mhandle-large-shift

Include code to detect bit-shifts of more than 31 bits; respectively, trap such shifts or emit code to handle them properly. By default GCC makes no special provision for large bit shifts.

-mwarn-passed-structs

Warn when a function passes a struct as an argument or result. Structure-passing conventions have changed during the evolution of the C language, and are often the source of portability problems. By default, GCC issues no such warning.

2.14.12 IBM RS/6000 and PowerPC Options

These `-m' options are defined for the IBM RS/6000 and PowerPC:

-mpower

-mno-power

-mpower2

-mno-power2

-mpowerpc

-mno-powerpc

-mpowerpc-gpopt

-mno-powerpc-gpopt

-mpowerpc-gfxopt

-mno-powerpc-gfxopt

-mpowerpc64

-mno-powerpc64

GCC supports two related instruction set architectures for the RS/6000 and PowerPC. The POWER instruction set are those instructions supported by the `rios' chip set used in the original RS/6000 systems and the PowerPC instruction set is the architecture of the Motorola MPC5xx, MPC6xx, MPC8xx microprocessors, and the IBM 4xx microprocessors. Neither architecture is a subset of the other. However there is a large common subset of instructions supported by both. An MQ register is included in processors supporting the POWER architecture. You use these options to specify which instructions are available on the processor you are using. The default value of these options is determined when configuring GCC. Specifying the `-mcpu=cpu_type' overrides the specification of these options. We recommend you use the `-mcpu=cpu_type' option rather than the options listed above. The `-mpower' option allows GCC to generate instructions that are found only in the POWER architecture and to use the MQ register. Specifying `-mpower2' implies `-power' and also allows GCC to generate instructions that are present in the POWER2 architecture but not the original POWER architecture. The `-mpowerpc' option allows GCC to generate instructions that are found only in the 32-bit subset of the PowerPC architecture. Specifying `-mpowerpc-gpopt' implies `-mpowerpc' and also allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying `-mpowerpc-gfxopt' implies `-mpowerpc' and also allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select. The `-mpowerpc64' option allows GCC to generate the additional 64-bit instructions that are found in the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to `-mno-powerpc64'. If you specify both `-mno-power' and `-mno-powerpc', GCC will use only the instructions in the common subset of both architectures plus some special AIX common-mode calls, and will not use the MQ register. Specifying both `-mpower' and `-mpowerpc' permits GCC to use any instruction from either architecture and to allow use of the MQ register; specify this for the Motorola MPC601.

-mnew-mnemonics

-mold-mnemonics

Select which mnemonics to use in the generated assembler code. `-mnew-mnemonics' requests output that uses the assembler mnemonics defined for the PowerPC architecture, while `-mold-mnemonics' requests the assembler mnemonics defined for the POWER architecture. Instructions defined in only one architecture have only one mnemonic; GCC uses that mnemonic irrespective of which of these options is specified. GCC defaults to the mnemonics appropriate for the architecture in use. Specifying `-mcpu=cpu_type' sometimes overrides the value of these option. Unless you are building a cross-compiler, you should normally not specify either `-mnew-mnemonics' or `-mold-mnemonics', but should instead accept the default.

-mcpu=cpu_type

Set architecture type, register usage, choice of mnemonics, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are `rs6000', `rios1', `rios2', `rsc', `601', `602', `603', `603e', `604', `604e', `620', `740', `750', `power', `power2', `powerpc', `403', `505', `801', `821', `823', and `860' and `common'. `-mcpu=power', `-mcpu=power2', and `-mcpu=powerpc' specify generic POWER, POWER2 and pure PowerPC (i.e., not MPC601) architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes. Specifying any of the following options: `-mcpu=rios1', `-mcpu=rios2', `-mcpu=rsc', `-mcpu=power', or `-mcpu=power2' enables the `-mpower' option and disables the `-mpowerpc' option; `-mcpu=601' enables both the `-mpower' and `-mpowerpc' options. All of `-mcpu=602', `-mcpu=603', `-mcpu=603e', `-mcpu=604', `-mcpu=620', enable the `-mpowerpc' option and disable the `-mpower' option. Exactly similarly, all of `-mcpu=403', `-mcpu=505', `-mcpu=821', `-mcpu=860' and `-mcpu=powerpc' enable the `-mpowerpc' option and disable the `-mpower' option. `-mcpu=common' disables both the `-mpower' and `-mpowerpc' options. AIX versions 4 or greater selects `-mcpu=common' by default, so that code will operate on all members of the RS/6000 and PowerPC families. In that case, GCC will use only the instructions in the common subset of both architectures plus some special AIX common-mode calls, and will not use the MQ register. GCC assumes a generic processor model for scheduling purposes. Specifying any of the options `-mcpu=rios1', `-mcpu=rios2', `-mcpu=rsc', `-mcpu=power', or `-mcpu=power2' also disables the `new-mnemonics' option. Specifying `-mcpu=601', `-mcpu=602', `-mcpu=603', `-mcpu=603e', `-mcpu=604', `620', `403', or `-mcpu=powerpc' also enables the `new-mnemonics' option. Specifying `-mcpu=403', `-mcpu=821', or `-mcpu=860' also enables the `-msoft-float' option.

-mtune=cpu_type

Set the instruction scheduling parameters for machine type cpu_type, but do not set the architecture type, register usage, choice of mnemonics like `-mcpu='cpu_type would. The same values for cpu_type are used for `-mtune='cpu_type as for `-mcpu='cpu_type. The `-mtune='cpu_type option overrides the `-mcpu='cpu_type option in terms of instruction scheduling parameters.

-mfull-toc

-mno-fp-in-toc

-mno-sum-in-toc

-mminimal-toc

Modify generation of the TOC (Table Of Contents), which is created for every executable file. The `-mfull-toc' option is selected by default. In that case, GCC will allocate at least one TOC entry for each unique non-automatic variable reference in your program. GCC will also place floating-point constants in the TOC. However, only 16,384 entries are available in the TOC. If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the `-mno-fp-in-toc' and `-mno-sum-in-toc' options. `-mno-fp-in-toc' prevents GCC from putting floating-point constants in the TOC and `-mno-sum-in-toc' forces GCC to generate code to calculate the sum of an address and a constant at run-time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space. If you still run out of space in the TOC even when you specify both of these options, specify `-mminimal-toc' instead. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC will produce code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently executed code.

-maix64

-maix32

Enable AIX 64-bit ABI and calling convention: 64-bit pointers, 64-bit long type, and the infrastructure needed to support them. Specifying `-maix64' implies `-mpowerpc64' and `-mpowerpc', while `-maix32' disables the 64-bit ABI and implies `-mno-powerpc64'. GCC defaults to `-maix32'.

-mxl-call

-mno-xl-call

On AIX, pass floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared. AIX XL compilers access floating point arguments which do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by AIX XL compilers without optimization.

-mthreads

Support AIX Threads. Link an application written to use pthreads with special libraries and startup code to enable the application to run.

-mpe

Support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message passing with special startup code to enable the application to run. The system must have PE installed in the standard location (`/usr/lpp/ppe.poe/'), or the `specs' file must be overridden with the `-specs=' option to specify the appropriate directory location. The Parallel Environment does not support threads, so the `-mpe' option and the `-mthreads' option are incompatible.

-msoft-float

-mhard-float

Generate code that does not use (uses) the floating-point register set. Software floating point emulation is provided if you use the `-msoft-float' option, and pass the option to GCC when linking.

-mmultiple

-mno-multiple

Generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use `-mmultiple' on little endian PowerPC systems, since those instructions do not work when the processor is in little endian mode. The exceptions are PPC740 and PPC750 which permit the instructions usage in little endian mode.

-mstring

-mno-string

Generate code that uses (does not use) the load string instructions and the store string word instructions to save multiple registers and do small block moves. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use `-mstring' on little endian PowerPC systems, since those instructions do not work when the processor is in little endian mode. The exceptions are PPC740 and PPC750 which permit the instructions usage in little endian mode.

-mupdate

-mno-update

Generate code that uses (does not use) the load or store instructions that update the base register to the address of the calculated memory location. These instructions are generated by default. If you use `-mno-update', there is a small window between the time that the stack pointer is updated and the address of the previous frame is stored, which means code that walks the stack frame across interrupts or signals may get corrupted data.

-mfused-madd

-mno-fused-madd

Generate code that uses (does not use) the floating point multiply and accumulate instructions. These instructions are generated by default if hardware floating is used.

-mno-bit-align

-mbit-align

On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit fields to be aligned to the base type of the bit field. For example, by default a structure containing nothing but 8 unsigned bitfields of length 1 would be aligned to a 4 byte boundary and have a size of 4 bytes. By using `-mno-bit-align', the structure would be aligned to a 1 byte boundary and be one byte in size.

-mno-strict-align

-mstrict-align

On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references will be handled by the system.

-mrelocatable

-mno-relocatable

On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a different address at runtime. If you use `-mrelocatable' on any module, all objects linked together must be compiled with `-mrelocatable' or `-mrelocatable-lib'.

-mrelocatable-lib

-mno-relocatable-lib

On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a different address at runtime. Modules compiled with `-mrelocatable-lib' can be linked with either modules compiled without `-mrelocatable' and `-mrelocatable-lib' or with modules compiled with the `-mrelocatable' options.

-mno-toc

-mtoc

On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program.

-mlittle

-mlittle-endian

On System V.4 and embedded PowerPC systems compile code for the processor in little endian mode. The `-mlittle-endian' option is the same as `-mlittle'.

-mbig

-mbig-endian

On System V.4 and embedded PowerPC systems compile code for the processor in big endian mode. The `-mbig-endian' option is the same as `-mbig'.

-mcall-sysv

On System V.4 and embedded PowerPC systems compile code using calling conventions that adheres to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC using `powerpc-*-eabiaix'.

-mcall-sysv-eabi

Specify both `-mcall-sysv' and `-meabi' options.

-mcall-sysv-noeabi

Specify both `-mcall-sysv' and `-mno-eabi' options.

-mcall-aix

On System V.4 and embedded PowerPC systems compile code using calling conventions that are similar to those used on AIX. This is the default if you configured GCC using `powerpc-*-eabiaix'.

-mcall-solaris

On System V.4 and embedded PowerPC systems compile code for the Solaris operating system.

-mcall-linux

On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.

-mprototype

-mno-prototype

On System V.4 and embedded PowerPC systems assume that all calls to variable argument functions are properly prototyped. Otherwise, the compiler must insert an instruction before every non prototyped call to set or clear bit 6 of the condition code register (CR) to indicate whether floating point values were passed in the floating point registers in case the function takes a variable arguments. With `-mprototype', only calls to prototyped variable argument functions will set or clear the bit.

-msim

On embedded PowerPC systems, assume that the startup module is called `sim-crt0.o' and that the standard C libraries are `libsim.a' and `libc.a'. This is the default for `powerpc-*-eabisim'. configurations.

-mmvme

On embedded PowerPC systems, assume that the startup module is called `crt0.o' and the standard C libraries are `libmvme.a' and `libc.a'.

-mads

On embedded PowerPC systems, assume that the startup module is called `crt0.o' and the standard C libraries are `libads.a' and `libc.a'.

-myellowknife

On embedded PowerPC systems, assume that the startup module is called `crt0.o' and the standard C libraries are `libyk.a' and `libc.a'.

-memb

On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags header to indicate that `eabi' extended relocations are used.

-meabi

-mno-eabi

On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded Applications Binary Interface (eabi) which is a set of modifications to the System V.4 specifications. Selecting -meabi means that the stack is aligned to an 8 byte boundary, a function __eabi is called to from main to set up the eabi environment, and the `-msdata' option can use both r2 and r13 to point to two separate small data areas. Selecting -mno-eabi means that the stack is aligned to a 16 byte boundary, do not call an initialization function from main, and the `-msdata' option will only use r13 to point to a single small data area. The `-meabi' option is on by default if you configured GCC using one of the `powerpc*-*-eabi*' options.

-msdata=eabi

On System V.4 and embedded PowerPC systems, put small initialized const global and static data in the `.sdata2' section, which is pointed to by register r2. Put small initialized non-const global and static data in the `.sdata' section, which is pointed to by register r13. Put small uninitialized global and static data in the `.sbss' section, which is adjacent to the `.sdata' section. The `-msdata=eabi' option is incompatible with the `-mrelocatable' option. The `-msdata=eabi' option also sets the `-memb' option.

-msdata=sysv

On System V.4 and embedded PowerPC systems, put small global and static data in the `.sdata' section, which is pointed to by register r13. Put small uninitialized global and static data in the `.sbss' section, which is adjacent to the `.sdata' section. The `-msdata=sysv' option is incompatible with the `-mrelocatable' option.

-msdata=default

-msdata

On System V.4 and embedded PowerPC systems, if `-meabi' is used, compile code the same as `-msdata=eabi', otherwise compile code the same as `-msdata=sysv'.

-msdata-data

On System V.4 and embedded PowerPC systems, put small global and static data in the `.sdata' section. Put small uninitialized global and static data in the `.sbss' section. Do not use register r13 to address small data however. This is the default behavior unless other `-msdata' options are used.

-msdata=none

-mno-sdata

On embedded PowerPC systems, put all initialized global and static data in the `.data' section, and all uninitialized data in the `.bss' section.

-G num

On embedded PowerPC systems, put global and static items less than or equal to num bytes into the small data or bss sections instead of the normal data or bss section. By default, num is 8. The `-G num' switch is also passed to the linker. All modules should be compiled with the same `-G num' value.

-mregnames

-mno-regnames

On System V.4 and embedded PowerPC systems do (do not) emit register names in the assembly language output using symbolic forms.

2.14.13 IBM RT Options

These `-m' options are defined for the IBM RT PC:

-min-line-mul

Use an in-line code sequence for integer multiplies. This is the default.

-mcall-lib-mul

Call lmul$$ for integer multiples.

-mfull-fp-blocks

Generate full-size floating point data blocks, including the minimum amount of scratch space recommended by IBM. This is the default.

-mminimum-fp-blocks

Do not include extra scratch space in floating point data blocks. This results in smaller code, but slower execution, since scratch space must be allocated dynamically.

-mfp-arg-in-fpregs

Use a calling sequence incompatible with the IBM calling convention in which floating point arguments are passed in floating point registers. Note that varargs.h and stdargs.h will not work with floating point operands if this option is specified.

-mfp-arg-in-gregs

Use the normal calling convention for floating point arguments. This is the default.

-mhc-struct-return

Return structures of more than one word in memory, rather than in a register. This provides compatibility with the MetaWare HighC (hc) compiler. Use the option `-fpcc-struct-return' for compatibility with the Portable C Compiler (pcc).

-mnohc-struct-return

Return some structures of more than one word in registers, when convenient. This is the default. For compatibility with the IBM-supplied compilers, use the option `-fpcc-struct-return' or the option `-mhc-struct-return'.

2.14.14 MIPS Options

These `-m' options are defined for the MIPS family of computers:

-mcpu=cpu type

Assume the defaults for the machine type cpu type when scheduling instructions. The choices for cpu type are `r2000', `r3000', `r3900', `r4000', `r4100', `r4300', `r4400', `r4600', `r4650', `r5000', `r6000', `r8000', and `orion'. Additionally, the `r2000', `r3000', `r4000', `r5000', and `r6000' can be abbreviated as `r2k' (or `r2K'), `r3k', etc. While picking a specific cpu type will schedule things appropriately for that particular chip, the compiler will not generate any code that does not meet level 1 of the MIPS ISA (instruction set architecture) without a `-mipsX' or `-mabi' switch being used.

-mips1

Issue instructions from level 1 of the MIPS ISA. This is the default. `r3000' is the default cpu type at this ISA level.

-mips2

Issue instructions from level 2 of the MIPS ISA (branch likely, square root instructions). `r6000' is the default cpu type at this ISA level.

-mips3

Issue instructions from level 3 of the MIPS ISA (64 bit instructions). `r4000' is the default cpu type at this ISA level.

-mips4

Issue instructions from level 4 of the MIPS ISA (conditional move, prefetch, enhanced FPU instructions). `r8000' is the default cpu type at this ISA level.

-mfp32

Assume that 32 32-bit floating point registers are available. This is the default.

-mfp64

Assume that 32 64-bit floating point registers are available. This is the default when the `-mips3' option is used.

-mgp32

Assume that 32 32-bit general purpose registers are available. This is the default.

-mgp64

Assume that 32 64-bit general purpose registers are available. This is the default when the `-mips3' option is used.

-mint64

Force int and long types to be 64 bits wide. See `-mlong32' for an explanation of the default, and the width of pointers.

-mlong64

Force long types to be 64 bits wide. See `-mlong32' for an explanation of the default, and the width of pointers.

-mlong32

Force long, int, and pointer types to be 32 bits wide. If none of `-mlong32', `-mlong64', or `-mint64' are set, the size of ints, longs, and pointers depends on the ABI and ISA choosen. For `-mabi=32', and `-mabi=n32', ints and longs are 32 bits wide. For `-mabi=64', ints are 32 bits, and longs are 64 bits wide. For `-mabi=eabi' and either `-mips1' or `-mips2', ints and longs are 32 bits wide. For `-mabi=eabi' and higher ISAs, ints are 32 bits, and longs are 64 bits wide. The width of pointer types is the smaller of the width of longs or the width of general purpose registers (which in turn depends on the ISA).

-mabi=32

-mabi=o64

-mabi=n32

-mabi=64

-mabi=eabi

Generate code for the indicated ABI. The default instruction level is `-mips1' for `32', `-mips3' for `n32', and `-mips4' otherwise. Conversely, with `-mips1' or `-mips2', the default ABI is `32'; otherwise, the default ABI is `64'.

-mmips-as

Generate code for the MIPS assembler, and invoke `mips-tfile' to add normal debug information. This is the default for all platforms except for the OSF/1 reference platform, using the OSF/rose object format. If the either of the `-gstabs' or `-gstabs+' switches are used, the `mips-tfile' program will encapsulate the stabs within MIPS ECOFF.

-mgas

Generate code for the GNU assembler. This is the default on the OSF/1 reference platform, using the OSF/rose object format. Also, this is the default if the configure option `--with-gnu-as' is used.

-msplit-addresses

-mno-split-addresses

Generate code to load the high and low parts of address constants separately. This allows gcc to optimize away redundant loads of the high order bits of addresses. This optimization requires GNU as and GNU ld. This optimization is enabled by default for some embedded targets where GNU as and GNU ld are standard.

-mrnames

-mno-rnames

The `-mrnames' switch says to output code using the MIPS software names for the registers, instead of the hardware names (ie, a0 instead of $4). The only known assembler that supports this option is the Algorithmics assembler.

-mgpopt

-mno-gpopt

The `-mgpopt' switch says to write all of the data declarations before the instructions in the text section, this allows the MIPS assembler to generate one word memory references instead of using two words for short global or static data items. This is on by default if optimization is selected.

-mstats

-mno-stats

For each non-inline function processed, the `-mstats' switch causes the compiler to emit one line to the standard error file to print statistics about the program (number of registers saved, stack size, etc.).

-mmemcpy

-mno-memcpy

The `-mmemcpy' switch makes all block moves call the appropriate string function (`memcpy' or `bcopy') instead of possibly generating inline code.

-mmips-tfile

-mno-mips-tfile

The `-mno-mips-tfile' switch causes the compiler not postprocess the object file with the `mips-tfile' program, after the MIPS assembler has generated it to add debug support. If `mips-tfile' is not run, then no local variables will be available to the debugger. In addition, `stage2' and `stage3' objects will have the temporary file names passed to the assembler embedded in the object file, which means the objects will not compare the same. The `-mno-mips-tfile' switch should only be used when there are bugs in the `mips-tfile' program that prevents compilation.

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

-mhard-float

Generate output containing floating point instructions. This is the default if you use the unmodified sources.

-mabicalls

-mno-abicalls

Emit (or do not emit) the pseudo operations `.abicalls', `.cpload', and `.cprestore' that some System V.4 ports use for position independent code.

-mlong-calls

-mno-long-calls

Do all calls with the `JALR' instruction, which requires loading up a function's address into a register before the call. You need to use this switch, if you call outside of the current 512 megabyte segment to functions that are not through pointers.

-mhalf-pic

-mno-half-pic

Put pointers to extern references into the data section and load them up, rather than put the references in the text section.

-membedded-pic

-mno-embedded-pic

Generate PIC code suitable for some embedded systems. All calls are made using PC relative address, and all data is addressed using the $gp register. No more than 65536 bytes of global data may be used. This requires GNU as and GNU ld which do most of the work. This currently only works on targets which use ECOFF; it does not work with ELF.

-membedded-data

-mno-embedded-data

Allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data. This gives slightly slower code than the default, but reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems.

-msingle-float

-mdouble-float

The `-msingle-float' switch tells gcc to assume that the floating point coprocessor only supports single precision operations, as on the `r4650' chip. The `-mdouble-float' switch permits gcc to use double precision operations. This is the default.

-mmad

-mno-mad

Permit use of the `mad', `madu' and `mul' instructions, as on the `r4650' chip.

-m4650

Turns on `-msingle-float', `-mmad', and, at least for now, `-mcpu=r4650'.

-mips16

-mno-mips16

Enable 16-bit instructions.

-mentry

Use the entry and exit pseudo ops. This option can only be used with `-mips16'.

-EL

Compile code for the processor in little endian mode. The requisite libraries are assumed to exist.

-EB

Compile code for the processor in big endian mode. The requisite libraries are assumed to exist.

-G num

Put global and static items less than or equal to num bytes into the small data or bss sections instead of the normal data or bss section. This allows the assembler to emit one word memory reference instructions based on the global pointer (gp or $28), instead of the normal two words used. By default, num is 8 when the MIPS assembler is used, and 0 when the GNU assembler is used. The `-G num' switch is also passed to the assembler and linker. All modules should be compiled with the same `-G num' value.

-nocpp

Tell the MIPS assembler to not run its preprocessor over user assembler files (with a `.s' suffix) when assembling them.

These options are defined by the macro TARGET_SWITCHES in the machine description. The default for the options is also defined by that macro, which enables you to change the defaults.

2.14.15 Intel 386 Options

These `-m' options are defined for the i386 family of computers:

-mcpu=cpu type

Assume the defaults for the machine type cpu type when scheduling instructions. The choices for cpu type are: `i386'`pentium'

`i486'	`i586'	`i686'
`pentiumpro'	`k6'

While picking a specific cpu type will schedule things appropriately for that particular chip, the compiler will not generate any code that does not run on the i386 without the `-march=cpu type' option being used. `i586' is equivalent to `pentium' and `i686' is equivalent to `pentiumpro'. `k6' is the AMD chip as opposed to the Intel ones. -march=cpu type Generate instructions for the machine type cpu type. The choices for cpu type are the same as for `-mcpu'. Moreover, specifying `-march=cpu type' implies `-mcpu=cpu type'.
-m386
-m486
-mpentium
-mpentiumpro Synonyms for -mcpu=i386, -mcpu=i486, -mcpu=pentium, and -mcpu=pentiumpro respectively. These synonyms are deprecated.
-mieee-fp
-mno-ieee-fp Control whether or not the compiler uses IEEE floating point comparisons. These handle correctly the case where the result of a comparison is unordered.
-msoft-float Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. On machines where a function returns floating point results in the 80387 register stack, some floating point opcodes may be emitted even if `-msoft-float' is used.
-mno-fp-ret-in-387 Do not use the FPU registers for return values of functions. The usual calling convention has functions return values of types float and double in an FPU register, even if there is no FPU. The idea is that the operating system should emulate an FPU. The option `-mno-fp-ret-in-387' causes such values to be returned in ordinary CPU registers instead.
-mno-fancy-math-387 Some 387 emulators do not support the sin, cos and sqrt instructions for the 387. Specify this option to avoid generating those instructions. This option is the default on FreeBSD. As of revision 2.6.1, these instructions are not generated unless you also use the `-ffast-math' switch.
-malign-double
-mno-align-double Control whether GCC aligns double, long double, and long long variables on a two word boundary or a one word boundary. Aligning double variables on a two word boundary will produce code that runs somewhat faster on a `Pentium' at the expense of more memory. Warning: if you use the `-malign-double' switch, structures containing the above types will be aligned differently than the published application binary interface specifications for the 386.
-msvr3-shlib
-mno-svr3-shlib Control whether GCC places uninitialized locals into bss or data. `-msvr3-shlib' places these locals into bss. These options are meaningful only on System V Release 3.
-mno-wide-multiply
-mwide-multiply Control whether GCC uses the mul and imul that produce 64 bit results in eax:edx from 32 bit operands to do long long multiplies and 32-bit division by constants.
-mrtd Use a different function-calling convention, in which functions that take a fixed number of arguments return with the ret num instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there. You can specify that an individual function is called with this calling sequence with the function attribute `stdcall'. You can also override the `-mrtd' option by using the function attribute `cdecl'. See section 4.23 Declaring Attributes of Functions. Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code will be generated for calls to those functions. In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)
-mreg-alloc=regs Control the default allocation order of integer registers. The string regs is a series of letters specifying a register. The supported letters are: a allocate EAX; b allocate EBX; c allocate ECX; d allocate EDX; S allocate ESI; D allocate EDI; B allocate EBP.
-mregparm=num Control how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for a specific function by using the function attribute `regparm'. See section 4.23 Declaring Attributes of Functions. Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules.
-malign-loops=num Align loops to a 2 raised to a num byte boundary. If `-malign-loops' is not specified, the default is 2 unless gas 2.8 (or later) is being used in which case the default is to align the loop on a 16 byte boundary if it is less than 8 bytes away.
-malign-jumps=num Align instructions that are only jumped to to a 2 raised to a num byte boundary. If `-malign-jumps' is not specified, the default is 2 if optimizing for a 386, and 4 if optimizing for a 486 unless gas 2.8 (or later) is being used in which case the default is to align the instruction on a 16 byte boundary if it is less than 8 bytes away.
-malign-functions=num Align the start of functions to a 2 raised to num byte boundary. If `-malign-functions' is not specified, the default is 2 if optimizing for a 386, and 4 if optimizing for a 486.
-mpreferred-stack-boundary=num Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If `-mpreferred-stack-boundary' is not specified, the default is 4 (16 bytes or 128 bits). The stack is required to be aligned on a 4 byte boundary. On Pentium and PentiumPro, double and long double values should be aligned to an 8 byte boundary (see `-malign-double') or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extention (SSE) data type __m128 suffers similar penalties if it is not 16 byte aligned. To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary will most likely misalign the stack. It is recommended that libraries that use callbacks always use the default setting. This extra alignment does consume extra stack space. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to `-mpreferred-stack-boundary=2'.

2.14.16 HPPA Options

These `-m' options are defined for the HPPA family of computers:

-march=architecture type

Generate code for the specified architecture. The choices for architecture type are `1.0' for PA 1.0, `1.1' for PA 1.1, and `2.0' for PA 2.0 processors. Refer to `/usr/lib/sched.models' on an HP-UX system to determine the proper architecture option for your machine. Code compiled for lower numbered architectures will run on higher numbered architectures, but not the other way around. PA 2.0 support currently requires gas snapshot 19990413 or later. The next release of binutils (current is 2.9.1) will probably contain PA 2.0 support.

-mpa-risc-1-0

-mpa-risc-1-1

-mpa-risc-2-0

Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.

-mbig-switch

Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table.

-mjump-in-delay

Fill delay slots of function calls with unconditional jump instructions by modifying the return pointer for the function call to be the target of the conditional jump.

-mdisable-fpregs

Prevent floating point registers from being used in any manner. This is necessary for compiling kernels which perform lazy context switching of floating point registers. If you use this option and attempt to perform floating point operations, the compiler will abort.

-mdisable-indexing

Prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH.

-mno-space-regs

Generate code that assumes the target has no space registers. This allows GCC to generate faster indirect calls and use unscaled index address modes. Such code is suitable for level 0 PA systems and kernels.

-mfast-indirect-calls

Generate code that assumes calls never cross space boundaries. This allows GCC to emit code which performs faster indirect calls. This option will not work in the presense of shared libraries or nested functions.

-mspace

Optimize for space rather than execution time. Currently this only enables out of line function prologues and epilogues. This option is incompatible with PIC code generation and profiling.

-mlong-load-store

Generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the `+k' option to the HP compilers.

-mportable-runtime

Use the portable calling conventions proposed by HP for ELF systems.

-mgas

Enable the use of assembler directives only GAS understands.

-mschedule=cpu type

Schedule code according to the constraints for the machine type cpu type. The choices for cpu type are `700' `7100', `7100LC', `7200', and `8000'. Refer to `/usr/lib/sched.models' on an HP-UX system to determine the proper scheduling option for your machine.

-mlinker-opt

Enable the optimization pass in the HPUX linker. Note this makes symbolic debugging impossible. It also triggers a bug in the HPUX 8 and HPUX 9 linkers in which they give bogus error messages when linking some programs.

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all HPPA targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded target `hppa1.1-*-pro' does provide software floating point support. `-msoft-float' changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile `libgcc.a', the library that comes with GCC, with `-msoft-float' in order for this to work.

2.14.17 Intel 960 Options

These `-m' options are defined for the Intel 960 implementations:

-mcpu type

Assume the defaults for the machine type cpu type for some of the other options, including instruction scheduling, floating point support, and addressing modes. The choices for cpu type are `ka', `kb', `mc', `ca', `cf', `sa', and `sb'. The default is `kb'.

-mnumerics

-msoft-float

The `-mnumerics' option indicates that the processor does support floating-point instructions. The `-msoft-float' option indicates that floating-point support should not be assumed.

-mleaf-procedures

-mno-leaf-procedures

Do (or do not) attempt to alter leaf procedures to be callable with the bal instruction as well as call. This will result in more efficient code for explicit calls when the bal instruction can be substituted by the assembler or linker, but less efficient code in other cases, such as calls via function pointers, or using a linker that doesn't support this optimization.

-mtail-call

-mno-tail-call

Do (or do not) make additional attempts (beyond those of the machine-independent portions of the compiler) to optimize tail-recursive calls into branches. You may not want to do this because the detection of cases where this is not valid is not totally complete. The default is `-mno-tail-call'.

-mcomplex-addr

-mno-complex-addr

Assume (or do not assume) that the use of a complex addressing mode is a win on this implementation of the i960. Complex addressing modes may not be worthwhile on the K-series, but they definitely are on the C-series. The default is currently `-mcomplex-addr' for all processors except the CB and CC.

-mcode-align

-mno-code-align

Align code to 8-byte boundaries for faster fetching (or don't bother). Currently turned on by default for C-series implementations only.

-mic-compat

-mic2.0-compat

-mic3.0-compat

Enable compatibility with iC960 v2.0 or v3.0.

-masm-compat

-mintel-asm

Enable compatibility with the iC960 assembler.

-mstrict-align

-mno-strict-align

Do not permit (do permit) unaligned accesses.

-mold-align

Enable structure-alignment compatibility with Intel's gcc release version 1.3 (based on gcc 1.37). This option implies `-mstrict-align'.

-mlong-double-64

Implement type `long double' as 64-bit floating point numbers. Without the option `long double' is implemented by 80-bit floating point numbers. The only reason we have it because there is no 128-bit `long double' support in `fp-bit.c' yet. So it is only useful for people using soft-float targets. Otherwise, we should recommend against use of it.

2.14.18 DEC Alpha Options

These `-m' options are defined for the DEC Alpha implementations:

-mno-soft-float

-msoft-float

Use (do not use) the hardware floating-point instructions for floating-point operations. When -msoft-float is specified, functions in `libgcc1.c' will be used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines will issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them. Note that Alpha implementations without floating-point operations are required to have floating-point registers.

-mfp-reg

-mno-fp-regs

Generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float. If the floating-point register set is not used, floating point operands are passed in integer registers as if they were integers and floating-point results are passed in $0 instead of $f0. This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with -mno-fp-regs must also be compiled with that option. A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers.

-mieee

The Alpha architecture implements floating-point hardware optimized for maximum performance. It is mostly compliant with the IEEE floating point standard. However, for full compliance, software assistance is required. This option generates code fully IEEE compliant code except that the inexact flag is not maintained (see below). If this option is turned on, the CPP macro _IEEE_FP is defined during compilation. The option is a shorthand for: `-D_IEEE_FP -mfp-trap-mode=su -mtrap-precision=i -mieee-conformant'. The resulting code is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this option -ieee_with_no_inexact.

-mieee-with-inexact

This is like `-mieee' except the generated code also maintains the IEEE inexact flag. Turning on this option causes the generated code to implement fully-compliant IEEE math. The option is a shorthand for `-D_IEEE_FP -D_IEEE_FP_INEXACT' plus the three following: `-mieee-conformant', `-mfp-trap-mode=sui', and `-mtrap-precision=i'. On some Alpha implementations the resulting code may execute significantly slower than the code generated by default. Since there is very little code that depends on the inexact flag, you should normally not specify this option. Other Alpha compilers call this option `-ieee_with_inexact'.

-mfp-trap-mode=trap mode

This option controls what floating-point related traps are enabled. Other Alpha compilers call this option `-fptm 'trap mode. The trap mode can be set to one of four values:

`n'

This is the default (normal) setting. The only traps that are enabled are the ones that cannot be disabled in software (e.g., division by zero trap).

`u'

In addition to the traps enabled by `n', underflow traps are enabled as well.

`su'

Like `su', but the instructions are marked to be safe for software completion (see Alpha architecture manual for details).

`sui'

Like `su', but inexact traps are enabled as well.

-mfp-rounding-mode=rounding mode

Selects the IEEE rounding mode. Other Alpha compilers call this option `-fprm 'rounding mode. The rounding mode can be one of:

`n'

Normal IEEE rounding mode. Floating point numbers are rounded towards the nearest machine number or towards the even machine number in case of a tie.

`m'

Round towards minus infinity.

`c'

Chopped rounding mode. Floating point numbers are rounded towards zero.

`d'

Dynamic rounding mode. A field in the floating point control register (fpcr, see Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. Thus, unless your program modifies the fpcr, `d' corresponds to round towards plus infinity.

-mtrap-precision=trap precision

In the Alpha architecture, floating point traps are imprecise. This means without software assistance it is impossible to recover from a floating trap and program execution normally needs to be terminated. GCC can generate code that can assist operating system trap handlers in determining the exact location that caused a floating point trap. Depending on the requirements of an application, different levels of precisions can be selected:

`p'

Program precision. This option is the default and means a trap handler can only identify which program caused a floating point exception.

`f'

Function precision. The trap handler can determine the function that caused a floating point exception.

`i'

Instruction precision. The trap handler can determine the exact instruction that caused a floating point exception.

Other Alpha compilers provide the equivalent options called `-scope_safe' and `-resumption_safe'.

-mieee-conformant

This option marks the generated code as IEEE conformant. You must not use this option unless you also specify `-mtrap-precision=i' and either `-mfp-trap-mode=su' or `-mfp-trap-mode=sui'. Its only effect is to emit the line `.eflag 48' in the function prologue of the generated assembly file. Under DEC Unix, this has the effect that IEEE-conformant math library routines will be linked in.

-mbuild-constants

Normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it cannot, it will output the constant as a literal and generate code to load it from the data segment at runtime. Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the maximum is six). You would typically use this option to build a shared library dynamic loader. Itself a shared library, it must relocate itself in memory before it can find the variables and constants in its own data segment.

-malpha-as

-mgas

Select whether to generate code to be assembled by the vendor-supplied assembler (`-malpha-as') or by the GNU assembler `-mgas'.

-mbwx

-mno-bwx

-mcix

-mno-cix

-mmax

-mno-max

Indicate whether GCC should generate code to use the optional BWX, CIX, and MAX instruction sets. The default is to use the instruction sets supported by the CPU type specified via `-mcpu=' option or that of the CPU on which GCC was built if none was specified.

-mcpu=cpu_type

Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. You can specify either the `EV' style name or the corresponding chip number. GCC supports scheduling parameters for the EV4 and EV5 family of processors and will choose the default values for the instruction set from the processor you specify. If you do not specify a processor type, GCC will default to the processor on which the compiler was built. Supported values for cpu_type are

`ev4'

`21064'

Schedules as an EV4 and has no instruction set extensions.

`ev5'

`21164'

Schedules as an EV5 and has no instruction set extensions.

`ev56'

`21164a'

Schedules as an EV5 and supports the BWX extension.

`pca56'

`21164pc'

`21164PC'

Schedules as an EV5 and supports the BWX and MAX extensions.

`ev6'

`21264'

Schedules as an EV5 (until Digital releases the scheduling parameters for the EV6) and supports the BWX, CIX, and MAX extensions.

-mmemory-latency=time

Sets the latency the scheduler should assume for typical memory references as seen by the application. This number is highly dependant on the memory access patterns used by the application and the size of the external cache on the machine. Valid options for time are

`number'

A decimal number representing clock cycles.

`L1'

`L2'

`L3'

`main'

The compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that L3 is only valid for EV5.

2.14.19 Clipper Options

These `-m' options are defined for the Clipper implementations:

-mc300

Produce code for a C300 Clipper processor. This is the default.

-mc400

Produce code for a C400 Clipper processor i.e. use floating point registers f8..f15.

2.14.20 H8/300 Options

These `-m' options are defined for the H8/300 implementations:

-mrelax

Shorten some address references at link time, when possible; uses the linker option `-relax'. See section `ld and the H8/300' in Using ld, for a fuller description.

-mh

Generate code for the H8/300H.

-ms

Generate code for the H8/S.

-mint32

Make int data 32 bits by default.

-malign-300

On the h8/300h, use the same alignment rules as for the h8/300. The default for the h8/300h is to align longs and floats on 4 byte boundaries. `-malign-300' causes them to be aligned on 2 byte boundaries. This option has no effect on the h8/300.

2.14.21 SH Options

These `-m' options are defined for the SH implementations:

-m1

Generate code for the SH1.

-m2

Generate code for the SH2.

-m3

Generate code for the SH3.

-m3e

Generate code for the SH3e.

-mb

Compile code for the processor in big endian mode.

-ml

Compile code for the processor in little endian mode.

-mdalign

Align doubles at 64 bit boundaries. Note that this changes the calling conventions, and thus some functions from the standard C library will not work unless you recompile it first with -mdalign.

-mrelax

Shorten some address references at link time, when possible; uses the linker option `-relax'.

2.14.22 Options for System V

These additional options are available on System V Release 4 for compatibility with other compilers on those systems:

-G

Create a shared object. It is recommended that `-symbolic' or `-shared' be used instead.

-Qy

Identify the versions of each tool used by the compiler, in a .ident assembler directive in the output.

-Qn

Refrain from adding .ident directives to the output file (this is the default).

-YP,dirs

Search the directories dirs, and no others, for libraries specified with `-l'.

-Ym,dir

Look in the directory dir to find the M4 preprocessor. The assembler uses this option.

2.14.23 TMS320C3x/C4x Options

These `-m' options are defined for TMS320C3x/C4x implementations:

-mcpu=cpu_type

Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are `c30', `c31', `c32', `c40', and `c44'. The default is `c40' to generate code for the TMS320C40.

-mbig-memory

-mbig

-msmall-memory

-msmall

Generates code for the big or small memory model. The small memory model assumed that all data fits into one 64K word page. At run-time the data page (DP) register must be set to point to the 64K page containing the .bss and .data program sections. The big memory model is the default and requires reloading of the DP register for every direct memory access.

-mbk

-mno-bk

Allow (disallow) allocation of general integer operands into the block count register BK.

-mdb

-mno-db

Enable (disable) generation of code using decrement and branch, DBcond(D), instructions. This is enabled by default for the C4x. To be on the safe side, this is disabled for the C3x, since the maximum iteration count on the C3x is 2^23 + 1 (but who iterates loops more than 2^23 times on the C3x?). Note that GCC will try to reverse a loop so that it can utilise the decrement and branch instruction, but will give up if there is more than one memory reference in the loop. Thus a loop where the loop counter is decremented can generate slightly more efficient code, in cases where the RPTB instruction cannot be utilised.

-mdp-isr-reload

-mparanoid

Force the DP register to be saved on entry to an interrupt service routine (ISR), reloaded to point to the data section, and restored on exit from the ISR. This should not be required unless someone has violated the small memory model by modifying the DP register, say within an object library.

-mmpyi

-mno-mpyi

For the C3x use the 24-bit MPYI instruction for integer multiplies instead of a library call to guarantee 32-bit results. Note that if one of the operands is a constant, then the multiplication will be performed using shifts and adds. If the -mmpyi option is not specified for the C3x, then squaring operations are performed inline instead of a library call.

-mfast-fix

-mno-fast-fix

The C3x/C4x FIX instruction to convert a floating point value to an integer value chooses the nearest integer less than or equal to the floating point value rather than to the nearest integer. Thus if the floating point number is negative, the result will be incorrectly truncated an additional code is necessary to detect and correct this case. This option can be used to disable generation of the additional code required to correct the result.

-mrptb

-mno-rptb

Enable (disable) generation of repeat block sequences using the RPTB instruction for zero overhead looping. The RPTB construct is only used for innermost loops that do not call functions or jump across the loop boundaries. There is no advantage having nested RPTB loops due to the overhead required to save and restore the RC, RS, and RE registers. This is enabled by default with -O2.

-mrpts=count

-mno-rpts

Enable (disable) the use of the single instruction repeat instruction RPTS. If a repeat block contains a single instruction, and the loop count can be guaranteed to be less than the value count, GCC will emit a RPTS instruction instead of a RPTB. If no value is specified, then a RPTS will be emitted even if the loop count cannot be determined at compile time. Note that the repeated instruction following RPTS does not have to be reloaded from memory each iteration, thus freeing up the CPU buses for oeprands. However, since interrupts are blocked by this instruction, it is disabled by default.

-mloop-unsigned

-mno-loop-unsigned

The maximum iteration count when using RPTS and RPTB (and DB on the C40) is 2^31 + 1 since these instructions test if the iteration count is negative to terminate the loop. If the iteration count is unsigned there is a possibility than the 2^31 + 1 maximum iteration count may be exceeded. This switch allows an unsigned iteration count.

-mti

Try to emit an assembler syntax that the TI assembler (asm30) is happy with. This also enforces compatibility with the API employed by the TI C3x C compiler. For example, long doubles are passed as structures rather than in floating point registers.

-mregparm

-mmemparm

Generate code that uses registers (stack) for passing arguments to functions. By default, arguments are passed in registers where possible rather than by pushing arguments on to the stack.

-mparallel-insns

-mno-parallel-insns

Allow the generation of parallel instructions. This is enabled by default with -O2.

-mparallel-mpy

-mno-parallel-mpy

Allow the generation of MPY||ADD and MPY||SUB parallel instructions, provided -mparallel-insns is also specified. These instructions have tight register constraints which can pessimize the code generation of large functions.

2.14.24 V850 Options

These `-m' options are defined for V850 implementations:

-mlong-calls

-mno-long-calls

Treat all calls as being far away (near). If calls are assumed to be far away, the compiler will always load the functions address up into a register, and call indirect through the pointer.

-mno-ep

-mep

Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into the ep register, and use the shorter sld and sst instructions. The `-mep' option is on by default if you optimize.

-mno-prolog-function

-mprolog-function

Do not use (do use) external functions to save and restore registers at the prolog and epilog of a function. The external functions are slower, but use less code space if more than one function saves the same number of registers. The `-mprolog-function' option is on by default if you optimize.

-mspace

Try to make the code as small as possible. At present, this just turns on the `-mep' and `-mprolog-function' options.

-mtda=n

Put static or global variables whose size is n bytes or less into the tiny data area that register ep points to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references).

-msda=n

Put static or global variables whose size is n bytes or less into the small data area that register gp points to. The small data area can hold up to 64 kilobytes.

-mzda=n

Put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.

-mv850

Specify that the target processor is the V850.

-mbig-switch

Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table.

2.14.25 ARC Options

These options are defined for ARC implementations:

-EL

Compile code for little endian mode. This is the default.

-EB

Compile code for big endian mode.

-mmangle-cpu

Prepend the name of the cpu to all public symbol names. In multiple-processor systems, there are many ARC variants with different instruction and register set characteristics. This flag prevents code compiled for one cpu to be linked with code compiled for another. No facility exists for handling variants that are "almost identical". This is an all or nothing option.

-mcpu=cpu

Compile code for ARC variant cpu. Which variants are supported depend on the configuration. All variants support `-mcpu=base', this is the default.

-mtext=text section

-mdata=data section

-mrodata=readonly data section

Put functions, data, and readonly data in text section, data section, and readonly data section respectively by default. This can be overridden with the section attribute. See section 4.29 Specifying Attributes of Variables.

2.14.26 NS32K Options

These are the `-m' options defined for the 32000 series. The default values for these options depends on which style of 32000 was selected when the compiler was configured; the defaults for the most common choices are given below.

-m32032

-m32032

Generate output for a 32032. This is the default when the compiler is configured for 32032 and 32016 based systems.

-m32332

-m32332

Generate output for a 32332. This is the default when the compiler is configured for 32332-based systems.

-m32532

-m32532

Generate output for a 32532. This is the default when the compiler is configured for 32532-based systems.

-m32081

Generate output containing 32081 instructions for floating point. This is the default for all systems.

-m32381

Generate output containing 32381 instructions for floating point. This also implies `-m32081'. The 32381 is only compatible with the 32332 and 32532 cpus. This is the default for the pc532-netbsd configuration.

-mmulti-add

Try and generate multiply-add floating point instructions polyF and dotF. This option is only available if the `-m32381' option is in effect. Using these instructions requires changes to to register allocation which generally has a negative impact on performance. This option should only be enabled when compiling code particularly likely to make heavy use of multiply-add instructions.

-mnomulti-add

Do not try and generate multiply-add floating point instructions polyF and dotF. This is the default on all platforms.

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries may not be available.

-mnobitfield

Do not use the bit-field instructions. On some machines it is faster to use shifting and masking operations. This is the default for the pc532.

-mbitfield

Do use the bit-field instructions. This is the default for all platforms except the pc532.

-mrtd

Use a different function-calling convention, in which functions that take a fixed number of arguments return pop their arguments on return with the ret instruction. This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code will be generated for calls to those functions. In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.) This option takes its name from the 680x0 rtd instruction.

-mregparam

Use a different function-calling convention where the first two arguments are passed in registers. This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

-mnoregparam

Do not pass any arguments in registers. This is the default for all targets.

-msb

It is OK to use the sb as an index register which is always loaded with zero. This is the default for the pc532-netbsd target.

-mnosb

The sb register is not available for use or has not been initialized to zero by the run time system. This is the default for all targets except the pc532-netbsd. It is also implied whenever `-mhimem' or `-fpic' is set.

-mhimem

Many ns32000 series addressing modes use displacements of up to 512MB. If an address is above 512MB then displacements from zero can not be used. This option causes code to be generated which can be loaded above 512MB. This may be useful for operating systems or ROM code.

-mnohimem

Assume code will be loaded in the first 512MB of virtual address space. This is the default for all platforms.

2.15 Options for Code Generation Conventions

These machine-independent options control the interface conventions used in code generation.

Most of them have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. In the table below, only one of the forms is listed--the one which is not the default. You can figure out the other form by either removing `no-' or adding it.

-fexceptions

Enable exception handling. Generates extra code needed to propagate exceptions. For some targets, this implies generation of frame unwind information for all functions. This can produce significant data size overhead, although it does not affect execution. If you do not specify this option, it is enabled by default for languages like C++ which normally require exception handling, and disabled for languages like C that do not normally require it. However, when compiling C code that needs to interoperate properly with exception handlers written in C++, you may need to enable this option. You may also wish to disable this option is you are compiling older C++ programs that don't use exception handling.

-fpcc-struct-return

Return "short" struct and union values in memory like longer ones, rather than in registers. This convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers. The precise convention for returning structures in memory depends on the target configuration macros. Short structures and unions are those whose size and alignment match that of some integer type.

-freg-struct-return

Use the convention that struct and union values are returned in registers when possible. This is more efficient for small structures than `-fpcc-struct-return'. If you specify neither `-fpcc-struct-return' nor its contrary `-freg-struct-return', GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to `-fpcc-struct-return', except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative.

-fshort-enums

Allocate to an enum type only as many bytes as it needs for the declared range of possible values. Specifically, the enum type will be equivalent to the smallest integer type which has enough room.

-fshort-double

Use the same size for double as for float.

-fshared-data

Requests that the data and non-const variables of this compilation be shared data rather than private data. The distinction makes sense only on certain operating systems, where shared data is shared between processes running the same program, while private data exists in one copy per process.

-fno-common

Allocate even uninitialized global variables in the bss section of the object file, rather than generating them as common blocks. This has the effect that if the same variable is declared (without extern) in two different compilations, you will get an error when you link them. The only reason this might be useful is if you wish to verify that the program will work on other systems which always work this way.

-fno-ident

Ignore the `#ident' directive.

-fno-gnu-linker

Do not output global initializations (such as C++ constructors and destructors) in the form used by the GNU linker (on systems where the GNU linker is the standard method of handling them). Use this option when you want to use a non-GNU linker, which also requires using the collect2 program to make sure the system linker includes constructors and destructors. (collect2 is included in the GCC distribution.) For systems which must use collect2, the compiler driver gcc is configured to do this automatically.

-finhibit-size-directive

Don't output a .size assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling `crtstuff.c'; you should not need to use it for anything else.

-fverbose-asm

Put extra commentary information in the generated assembly code to make it more readable. This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself). `-fno-verbose-asm', the default, causes the extra information to be omitted and is useful when comparing two assembler files.

-fvolatile

Consider all memory references through pointers to be volatile.

-fvolatile-global

Consider all memory references to extern and global data items to be volatile. GCC does not consider static data items to be volatile because of this switch.

-fvolatile-static

Consider all memory references to static data to be volatile.

-fpic

Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that `-fpic' does not work; in that case, recompile with `-fPIC' instead. (These maximums are 16k on the m88k, 8k on the Sparc, and 32k on the m68k and RS/6000. The 386 has no such limit.) Position-independent code requires special support, and therefore works only on certain machines. For the 386, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent.

-fPIC

If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on the m68k, m88k, and the Sparc. Position-independent code requires special support, and therefore works only on certain machines.

-ffixed-reg

Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role). reg must be the name of a register. The register names accepted are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file. This flag does not have a negative form, because it specifies a three-way choice.

-fcall-used-reg

Treat the register named reg as an allocable register that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way will not save and restore the register reg. It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results. This flag does not have a negative form, because it specifies a three-way choice.

-fcall-saved-reg

Treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way will save and restore the register reg if they use it. It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results. A different sort of disaster will result from the use of this flag for a register in which function values may be returned. This flag does not have a negative form, because it specifies a three-way choice.

-fpack-struct

Pack all structure members together without holes. Usually you would not want to use this option, since it makes the code suboptimal, and the offsets of structure members won't agree with system libraries.

-fcheck-memory-usage

Generate extra code to check each memory access. GCC will generate code that is suitable for a detector of bad memory accesses such as `Checker'. Normally, you should compile all, or none, of your code with this option. If you do mix code compiled with and without this option, you must ensure that all code that has side effects and that is called by code compiled with this option is, itself, compiled with this option. If you do not, you might get erroneous messages from the detector. If you use functions from a library that have side-effects (such as read), you might not be able to recompile the library and specify this option. In that case, you can enable the `-fprefix-function-name' option, which requests GCC to encapsulate your code and make other functions look as if they were compiled with `-fcheck-memory-usage'. This is done by calling "stubs", which are provided by the detector. If you cannot find or build stubs for every function you call, you might have to specify `-fcheck-memory-usage' without `-fprefix-function-name'. If you specify this option, you can not use the asm or __asm__ keywords in functions with memory checking enabled. The compiler cannot understand what the asm statement will do, and therefore cannot generate the appropriate code, so it is rejected. However, the function attribute no_check_memory_usage will disable memory checking within a function, and asm statements can be put inside such functions. Inline expansion of a non-checked function within a checked function is permitted; the inline function's memory accesses won't be checked, but the rest will. If you move your asm statements to non-checked inline functions, but they do access memory, you can add calls to the support code in your inline function, to indicate any reads, writes, or copies being done. These calls would be similar to those done in the stubs described above.

-fprefix-function-name

Request GCC to add a prefix to the symbols generated for function names. GCC adds a prefix to the names of functions defined as well as functions called. Code compiled with this option and code compiled without the option can't be linked together, unless stubs are used. If you compile the following code with `-fprefix-function-name'

extern void bar (int);
void
foo (int a)
{
  return bar (a + 5);
}

GCC will compile the code as if it was written:

extern void prefix_bar (int);
void
prefix_foo (int a)
{
  return prefix_bar (a + 5);
}

This option is designed to be used with `-fcheck-memory-usage'.

-finstrument-functions

Generate instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, the following profiling functions will be called with the address of the current function and its call site. (On some platforms, __builtin_return_address does not work beyond the current function, so the call site information may not be available to the profiling functions otherwise.)

void __cyg_profile_func_enter (void *this_fn, void *call_site);
void __cyg_profile_func_exit  (void *this_fn, void *call_site);

The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table. This instrumentation is also done for functions expanded inline in other functions. The profiling calls will indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size. If you use `extern inline' in your C code, an addressable version of such functions must be provided. (This is normally the case anyways, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.) A function may be given the attribute no_instrument_function, in which case this instrumentation will not be done. This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (perhaps signal handlers, if the profiling routines generate output or allocate memory).

-fstack-check

Generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag if you are running in an environment with multiple threads, but only rarely need to specify it in a single-threaded environment since stack overflow is automatically detected on nearly all systems if there is only one stack.

-fargument-alias

-fargument-noalias

-fargument-noalias-global

Specify the possible relationships among parameters and between parameters and global data. `-fargument-alias' specifies that arguments (parameters) may alias each other and may alias global storage. `-fargument-noalias' specifies that arguments do not alias each other, but may alias global storage. `-fargument-noalias-global' specifies that arguments do not alias each other and do not alias global storage. Each language will automatically use whatever option is required by the language standard. You should not need to use these options yourself.

-fleading-underscore

This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code. Be warned that you should know what you are doing when invoking this option, and that not all targets provide complete support for it.

2.16 Environment Variables Affecting GCC

This section describes several environment variables that affect how GCC operates. Some of them work by specifying directories or prefixes to use when searching for various kinds of files. Some are used to specify other aspects of the compilation environment.

Note that you can also specify places to search using options such as `-B', `-I' and `-L' (see section 2.12 Options for Directory Search). These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GCC. See section 17.1 Controlling the Compilation Driver, `gcc'.

LANG

LC_CTYPE

LC_MESSAGES

LC_ALL

These environment variables control the way that GCC uses localization information that allow GCC to work with different national conventions. GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has been configured to do so. These locale categories can be set to any value supported by your installation. A typical value is `en_UK' for English in the United Kingdom. The LC_CTYPE environment variable specifies character classification. GCC uses it to determine the character boundaries in a string; this is needed for some multibyte encodings that contain quote and escape characters that would otherwise be interpreted as a string end or escape. The LC_MESSAGES environment variable specifies the language to use in diagnostic messages. If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable. If none of these variables are set, GCC defaults to traditional C English behavior.

TMPDIR

If TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temporary files to hold the output of one stage of compilation which is to be used as input to the next stage: for example, the output of the preprocessor, which is the input to the compiler proper.

GCC_EXEC_PREFIX

If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish. If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram. The default value of GCC_EXEC_PREFIX is `prefix/lib/gcc-lib/' where prefix is the value of prefix when you ran the `configure' script. Other prefixes specified with `-B' take precedence over this prefix. This prefix is also used for finding files such as `crt0.o' that are used for linking. In addition, the prefix is used in an unusual way in finding the directories to search for header files. For each of the standard directories whose name normally begins with `/usr/local/lib/gcc-lib' (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. Thus, with `-Bfoo/', GCC will search `foo/bar' where it would normally search `/usr/local/lib/bar'. These alternate directories are searched first; the standard directories come next.

COMPILER_PATH

The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories thus specified when searching for subprograms, if it can't find the subprograms using GCC_EXEC_PREFIX.

LIBRARY_PATH

The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it can't find them using GCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching for ordinary libraries for the `-l' option (but directories specified with `-L' come first).

C_INCLUDE_PATH

CPLUS_INCLUDE_PATH

OBJC_INCLUDE_PATH

These environment variables pertain to particular languages. Each variable's value is a colon-separated list of directories, much like PATH. When GCC searches for header files, it tries the directories listed in the variable for the language you are using, after the directories specified with `-I' but before the standard header file directories.

DEPENDENCIES_OUTPUT

If this variable is set, its value specifies how to output dependencies for Make based on the header files processed by the compiler. This output looks much like the output from the `-M' option (see section 2.9 Options Controlling the Preprocessor), but it goes to a separate file, and is in addition to the usual results of compilation. The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name. Or the value can have the form `file target', in which case the rules are written to file file using target as the target name.

LANG

This variable is used to pass locale information to the compiler. One way in which this information is used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, the following values for LANG are recognized:

C-JIS

Recognize JIS characters.

C-SJIS

Recognize SJIS characters.

C-EUCJP

Recognize EUCJP characters.

If LANG is not defined, or if it has some other value, then the compiler will use mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters.

2.17 Running Protoize

The program protoize is an optional part of GNU C. You can use it to add prototypes to a program, thus converting the program to ANSI C in one respect. The companion program unprotoize does the reverse: it removes argument types from any prototypes that are found.

When you run these programs, you must specify a set of source files as command line arguments. The conversion programs start out by compiling these files to see what functions they define. The information gathered about a file foo is saved in a file named `foo.X'.

After scanning comes actual conversion. The specified files are all eligible to be converted; any files they include (whether sources or just headers) are eligible as well.

But not all the eligible files are converted. By default, protoize and unprotoize convert only source and header files in the current directory. You can specify additional directories whose files should be converted with the `-d directory' option. You can also specify particular files to exclude with the `-x file' option. A file is converted if it is eligible, its directory name matches one of the specified directory names, and its name within the directory has not been excluded.

Basic conversion with protoize consists of rewriting most function definitions and function declarations to specify the types of the arguments. The only ones not rewritten are those for varargs functions.

protoize optionally inserts prototype declarations at the beginning of the source file, to make them available for any calls that precede the function's definition. Or it can insert prototype declarations with block scope in the blocks where undeclared functions are called.

Basic conversion with unprotoize consists of rewriting most function declarations to remove any argument types, and rewriting function definitions to the old-style pre-ANSI form.

Both conversion programs print a warning for any function declaration or definition that they can't convert. You can suppress these warnings with `-q'.

The output from protoize or unprotoize replaces the original source file. The original file is renamed to a name ending with `.save'. If the `.save' file already exists, then the source file is simply discarded.

protoize and unprotoize both depend on GCC itself to scan the program and collect information about the functions it uses. So neither of these programs will work until GCC is installed.

Here is a table of the options you can use with protoize and unprotoize. Each option works with both programs unless otherwise stated.

-B directory

Look for the file `SYSCALLS.c.X' in directory, instead of the usual directory (normally `/usr/local/lib'). This file contains prototype information about standard system functions. This option applies only to protoize.

-c compilation-options

Use compilation-options as the options when running gcc to produce the `.X' files. The special option `-aux-info' is always passed in addition, to tell gcc to write a `.X' file. Note that the compilation options must be given as a single argument to protoize or unprotoize. If you want to specify several gcc options, you must quote the entire set of compilation options to make them a single word in the shell. There are certain gcc arguments that you cannot use, because they would produce the wrong kind of output. These include `-g', `-O', `-c', `-S', and `-o' If you include these in the compilation-options, they are ignored.

-C

Rename files to end in `.C' instead of `.c'. This is convenient if you are converting a C program to C++. This option applies only to protoize.

-g

Add explicit global declarations. This means inserting explicit declarations at the beginning of each source file for each function that is called in the file and was not declared. These declarations precede the first function definition that contains a call to an undeclared function. This option applies only to protoize.

-i string

Indent old-style parameter declarations with the string string. This option applies only to protoize. unprotoize converts prototyped function definitions to old-style function definitions, where the arguments are declared between the argument list and the initial `{'. By default, unprotoize uses five spaces as the indentation. If you want to indent with just one space instead, use `-i " "'.

-k

Keep the `.X' files. Normally, they are deleted after conversion is finished.

-l

Add explicit local declarations. protoize with `-l' inserts a prototype declaration for each function in each block which calls the function without any declaration. This option applies only to protoize.

-n

Make no real changes. This mode just prints information about the conversions that would have been done without `-n'.

-N

Make no `.save' files. The original files are simply deleted. Use this option with caution.

-p program

Use the program program as the compiler. Normally, the name `gcc' is used.

-q

Work quietly. Most warnings are suppressed.

-v

Print the version number, just like `-v' for gcc.

If you need special compiler options to compile one of your program's source files, then you should generate that file's `.X' file specially, by running gcc on that source file with the appropriate options and the option `-aux-info'. Then run protoize on the entire set of files. protoize will use the existing `.X' file because it is newer than the source file. For example:

gcc -Dfoo=bar file1.c -aux-info
protoize *.c

You need to include the special files along with the rest in the protoize command, even though their `.X' files already exist, because otherwise they won't get converted.

See section 7.11 Caveats of using protoize, for more information on how to use protoize successfully.

Note most of this information is out of date and superceded by the EGCS install procedures. It is provided for historical reference only.

3 Installing GNU CC

Here is the procedure for installing GNU CC on a GNU or Unix system. See section 3.6 Installing GNU CC on VMS, for VMS systems. In this section we assume you compile in the same directory that contains the source files; see section 3.3 Compilation in a Separate Directory, to find out how to compile in a separate directory on Unix systems.

You cannot install GNU C by itself on MSDOS; it will not compile under any MSDOS compiler except itself. You need to get the complete compilation package DJGPP, which includes binaries as well as sources, and includes all the necessary compilation tools and libraries.

1. If you have built GNU CC previously in the same directory for a different target machine, do `make distclean' to delete all files that might be invalid. One of the files this deletes is `Makefile'; if `make distclean' complains that `Makefile' does not exist, it probably means that the directory is already suitably clean.
2. On a System V release 4 system, make sure `/usr/bin' precedes `/usr/ucb' in PATH. The cc command in `/usr/ucb' uses libraries which have bugs.
3. Make sure the Bison parser generator is installed. (This is unnecessary if the Bison output files `c-parse.c' and `cexp.c' are more recent than `c-parse.y' and `cexp.y' and you do not plan to change the `.y' files.) Bison versions older than Sept 8, 1988 will produce incorrect output for `c-parse.c'.
4. If you have chosen a configuration for GNU CC which requires other GNU tools (such as GAS or the GNU linker) instead of the standard system tools, install the required tools in the build directory under the names `as', `ld' or whatever is appropriate. This will enable the compiler to find the proper tools for compilation of the program `enquire'. Alternatively, you can do subsequent compilation using a value of the PATH environment variable such that the necessary GNU tools come before the standard system tools.
5. Specify the host, build and target machine configurations. You do this when you run the `configure' script. The build machine is the system which you are using, the host machine is the system where you want to run the resulting compiler (normally the build machine), and the target machine is the system for which you want the compiler to generate code. If you are building a compiler to produce code for the machine it runs on (a native compiler), you normally do not need to specify any operands to `configure'; it will try to guess the type of machine you are on and use that as the build, host and target machines. So you don't need to specify a configuration when building a native compiler unless `configure' cannot figure out what your configuration is or guesses wrong. In those cases, specify the build machine's configuration name with the `--host' option; the host and target will default to be the same as the host machine. (If you are building a cross-compiler, see section 3.4 Building and Installing a Cross-Compiler.) Here is an example:

   ./configure --host=sparc-sun-sunos4.1
   

   A configuration name may be canonical or it may be more or less abbreviated. A canonical configuration name has three parts, separated by dashes. It looks like this: `cpu-company-system'. (The three parts may themselves contain dashes; `configure' can figure out which dashes serve which purpose.) For example, `m68k-sun-sunos4.1' specifies a Sun 3. You can also replace parts of the configuration by nicknames or aliases. For example, `sun3' stands for `m68k-sun', so `sun3-sunos4.1' is another way to specify a Sun 3. You can also use simply `sun3-sunos', since the version of SunOS is assumed by default to be version 4. You can specify a version number after any of the system types, and some of the CPU types. In most cases, the version is irrelevant, and will be ignored. So you might as well specify the version if you know it. See section 3.2 Configurations Supported by GNU CC, for a list of supported configuration names and notes on many of the configurations. You should check the notes in that section before proceeding any further with the installation of GNU CC.
6. When running configure, you may also need to specify certain additional options that describe variant hardware and software configurations. These are `--with-gnu-as', `--with-gnu-ld', `--with-stabs' and `--nfp'.

   `--with-gnu-as'

   If you will use GNU CC with the GNU assembler (GAS), you should declare this by using the `--with-gnu-as' option when you run `configure'. Using this option does not install GAS. It only modifies the output of GNU CC to work with GAS. Building and installing GAS is up to you. Conversely, if you do not wish to use GAS and do not specify `--with-gnu-as' when building GNU CC, it is up to you to make sure that GAS is not installed. GNU CC searches for a program named as in various directories; if the program it finds is GAS, then it runs GAS. If you are not sure where GNU CC finds the assembler it is using, try specifying `-v' when you run it. The systems where it makes a difference whether you use GAS are
   `hppa1.0-any-any', `hppa1.1-any-any', `i386-any-sysv', `i386-any-isc',
   `i860-any-bsd', `m68k-bull-sysv',
   `m68k-hp-hpux', `m68k-sony-bsd',
   `m68k-altos-sysv', `m68000-hp-hpux',
   `m68000-att-sysv', `any-lynx-lynxos', and `mips-any'). On any other system, `--with-gnu-as' has no effect. On the systems listed above (except for the HP-PA, for ISC on the 386, and for `mips-sgi-irix5.*'), if you use GAS, you should also use the GNU linker (and specify `--with-gnu-ld').

   `--with-gnu-ld'

   Specify the option `--with-gnu-ld' if you plan to use the GNU linker with GNU CC. This option does not cause the GNU linker to be installed; it just modifies the behavior of GNU CC to work with the GNU linker.

   `--with-stabs'

   On MIPS based systems and on Alphas, you must specify whether you want GNU CC to create the normal ECOFF debugging format, or to use BSD-style stabs passed through the ECOFF symbol table. The normal ECOFF debug format cannot fully handle languages other than C. BSD stabs format can handle other languages, but it only works with the GNU debugger GDB. Normally, GNU CC uses the ECOFF debugging format by default; if you prefer BSD stabs, specify `--with-stabs' when you configure GNU CC. No matter which default you choose when you configure GNU CC, the user can use the `-gcoff' and `-gstabs+' options to specify explicitly the debug format for a particular compilation. `--with-stabs' is meaningful on the ISC system on the 386, also, if `--with-gas' is used. It selects use of stabs debugging information embedded in COFF output. This kind of debugging information supports C++ well; ordinary COFF debugging information does not. `--with-stabs' is also meaningful on 386 systems running SVR4. It selects use of stabs debugging information embedded in ELF output. The C++ compiler currently (2.6.0) does not support the DWARF debugging information normally used on 386 SVR4 platforms; stabs provide a workable alternative. This requires gas and gdb, as the normal SVR4 tools can not generate or interpret stabs.

   `--nfp'

   On certain systems, you must specify whether the machine has a floating point unit. These systems include `m68k-sun-sunosn' and `m68k-isi-bsd'. On any other system, `--nfp' currently has no effect, though perhaps there are other systems where it could usefully make a difference.

   `--enable-haifa'

   `--disable-haifa'

   Use `--enable-haifa' to enable use of an experimental instruction scheduler (from IBM Haifa). This may or may not produce better code. Some targets on which it is known to be a win enable it by default; use `--disable-haifa' to disable it in these cases. configure will print out whether the Haifa scheduler is enabled when it is run.

   `--enable-threads=type'

   Certain systems, notably Linux-based GNU systems, can't be relied on to supply a threads facility for the Objective C runtime and so will default to single-threaded runtime. They may, however, have a library threads implementation available, in which case threads can be enabled with this option by supplying a suitable type, probably `posix'. The possibilities for type are `single', `posix', `win32', `solaris', `irix' and `mach'.

   `--enable-checking'

   When you specify this option, the compiler is built to perform checking of tree node types when referencing fields of that node. This does not change the generated code, but adds error checking within the compiler. This will slow down the compiler and may only work properly if you are building the compiler with GNU C. The `configure' script searches subdirectories of the source directory for other compilers that are to be integrated into GNU CC. The GNU compiler for C++, called G++ is in a subdirectory named `cp'. `configure' inserts rules into `Makefile' to build all of those compilers. Here we spell out what files will be set up by configure. Normally you need not be concerned with these files.

   • A file named `config.h' is created that contains a `#include' of the top-level config file for the machine you will run the compiler on (see section 18 The Configuration File). This file is responsible for defining information about the host machine. It includes `tm.h'. The top-level config file is located in the subdirectory `config'. Its name is always `xm-something.h'; usually `xm-machine.h', but there are some exceptions. If your system does not support symbolic links, you might want to set up `config.h' to contain a `#include' command which refers to the appropriate file.
   • A file named `tconfig.h' is created which includes the top-level config file for your target machine. This is used for compiling certain programs to run on that machine.
   • A file named `tm.h' is created which includes the machine-description macro file for your target machine. It should be in the subdirectory `config' and its name is often `machine.h'.

   `--enable-nls'

   `--disable-nls'

   The `--enable-nls' option enables Native Language Support (NLS), which lets GCC output diagnostics in languages other than American English. No translations are available yet, so the main users of this option now are those translating GCC's diagnostics who want to test their work. Once translations become available, Native Language Support will become enabled by default. The `--disable-nls' option disables NLS.

   `--with-included-gettext'

   If NLS is enabled, the GCC build procedure normally attempts to use the host's gettext libraries, and falls back on GCC's copy of the GNU gettext library only if the host libraries do not suffice. The `--with-included-gettext' option causes the build procedure to prefer its copy of GNU gettext.

   `--with-catgets'

   If NLS is enabled, and if the host lacks gettext but has the inferior catgets interface, the GCC build procedure normally ignores catgets and instead uses GCC's copy of the GNU gettext library. The `--with-catgets' option causes the build procedure to use the host's catgets in this situation.

7. In certain cases, you should specify certain other options when you run configure.
   • The standard directory for installing GNU CC is `/usr/local/lib'. If you want to install its files somewhere else, specify `--prefix=dir' when you run `configure'. Here dir is a directory name to use instead of `/usr/local' for all purposes with one exception: the directory `/usr/local/include' is searched for header files no matter where you install the compiler. To override this name, use the --with-local-prefix option below. The directory you specify need not exist, but its parent directory must exist.
   • Specify `--with-local-prefix=dir' if you want the compiler to search directory `dir/include' for locally installed header files instead of `/usr/local/include'. You should specify `--with-local-prefix' only if your site has a different convention (not `/usr/local') for where to put site-specific files. The default value for `--with-local-prefix' is `/usr/local' regardless of the value of `--prefix'. Specifying `--prefix' has no effect on which directory GNU CC searches for local header files. This may seem counterintuitive, but actually it is logical. The purpose of `--prefix' is to specify where to install GNU CC. The local header files in `/usr/local/include'---if you put any in that directory--are not part of GNU CC. They are part of other programs--perhaps many others. (GNU CC installs its own header files in another directory which is based on the `--prefix' value.) Do not specify `/usr' as the `--with-local-prefix'! The directory you use for `--with-local-prefix' must not contain any of the system's standard header files. If it did contain them, certain programs would be miscompiled (including GNU Emacs, on certain targets), because this would override and nullify the header file corrections made by the fixincludes script. Indications are that people who use this option use it based on mistaken ideas of what it is for. People use it as if it specified where to install part of GNU CC. Perhaps they make this assumption because installing GNU CC creates the directory.
8. Build the compiler. Just type `make LANGUAGES=c' in the compiler directory. `LANGUAGES=c' specifies that only the C compiler should be compiled. The makefile normally builds compilers for all the supported languages; currently, C, C++ and Objective C. However, C is the only language that is sure to work when you build with other non-GNU C compilers. In addition, building anything but C at this stage is a waste of time. In general, you can specify the languages to build by typing the argument `LANGUAGES="list"', where list is one or more words from the list `c', `c++', and `objective-c'. If you have any additional GNU compilers as subdirectories of the GNU CC source directory, you may also specify their names in this list. Ignore any warnings you may see about "statement not reached" in `insn-emit.c'; they are normal. Also, warnings about "unknown escape sequence" are normal in `genopinit.c' and perhaps some other files. Likewise, you should ignore warnings about "constant is so large that it is unsigned" in `insn-emit.c' and `insn-recog.c', a warning about a comparison always being zero in `enquire.o', and warnings about shift counts exceeding type widths in `cexp.y'. Any other compilation errors may represent bugs in the port to your machine or operating system, and should be investigated and reported (see section 8 Reporting Bugs). Some compilers fail to compile GNU CC because they have bugs or limitations. For example, the Microsoft compiler is said to run out of macro space. Some Ultrix compilers run out of expression space; then you need to break up the statement where the problem happens.
9. If you are building a cross-compiler, stop here. See section 3.4 Building and Installing a Cross-Compiler.
10. Move the first-stage object files and executables into a subdirectory with this command:

    make stage1
    

    The files are moved into a subdirectory named `stage1'. Once installation is complete, you may wish to delete these files with rm -r stage1.
11. If you have chosen a configuration for GNU CC which requires other GNU tools (such as GAS or the GNU linker) instead of the standard system tools, install the required tools in the `stage1' subdirectory under the names `as', `ld' or whatever is appropriate. This will enable the stage 1 compiler to find the proper tools in the following stage. Alternatively, you can do subsequent compilation using a value of the PATH environment variable such that the necessary GNU tools come before the standard system tools.
12. Recompile the compiler with itself, with this command:

    make CC="stage1/xgcc -Bstage1/" CFLAGS="-g -O2"
    

    This is called making the stage 2 compiler. The command shown above builds compilers for all the supported languages. If you don't want them all, you can specify the languages to build by typing the argument `LANGUAGES="list"'. list should contain one or more words from the list `c', `c++', `objective-c', and `proto'. Separate the words with spaces. `proto' stands for the programs protoize and unprotoize; they are not a separate language, but you use LANGUAGES to enable or disable their installation. If you are going to build the stage 3 compiler, then you might want to build only the C language in stage 2. Once you have built the stage 2 compiler, if you are short of disk space, you can delete the subdirectory `stage1'. On a 68000 or 68020 system lacking floating point hardware, unless you have selected a `tm.h' file that expects by default that there is no such hardware, do this instead:

    make CC="stage1/xgcc -Bstage1/" CFLAGS="-g -O2 -msoft-float"
    

13. If you wish to test the compiler by compiling it with itself one more time, install any other necessary GNU tools (such as GAS or the GNU linker) in the `stage2' subdirectory as you did in the `stage1' subdirectory, then do this:

    make stage2
    make CC="stage2/xgcc -Bstage2/" CFLAGS="-g -O2"
    

    This is called making the stage 3 compiler. Aside from the `-B' option, the compiler options should be the same as when you made the stage 2 compiler. But the LANGUAGES option need not be the same. The command shown above builds compilers for all the supported languages; if you don't want them all, you can specify the languages to build by typing the argument `LANGUAGES="list"', as described above. If you do not have to install any additional GNU tools, you may use the command

    make bootstrap LANGUAGES=language-list BOOT_CFLAGS=option-list
    

    instead of making `stage1', `stage2', and performing the two compiler builds.
14. Compare the latest object files with the stage 2 object files--they ought to be identical, aside from time stamps (if any). On some systems, meaningful comparison of object files is impossible; they always appear "different." This is currently true on Solaris and some systems that use ELF object file format. On some versions of Irix on SGI machines and DEC Unix (OSF/1) on Alpha systems, you will not be able to compare the files without specifying `-save-temps'; see the description of individual systems above to see if you get comparison failures. You may have similar problems on other systems. Use this command to compare the files:

    make compare
    

    This will mention any object files that differ between stage 2 and stage 3. Any difference, no matter how innocuous, indicates that the stage 2 compiler has compiled GNU CC incorrectly, and is therefore a potentially serious bug which you should investigate and report (see section 8 Reporting Bugs). If your system does not put time stamps in the object files, then this is a faster way to compare them (using the Bourne shell):

    for file in *.o; do
    cmp $file stage2/$file
    done
    

    If you have built the compiler with the `-mno-mips-tfile' option on MIPS machines, you will not be able to compare the files.
15. Install the compiler driver, the compiler's passes and run-time support with `make install'. Use the same value for CC, CFLAGS and LANGUAGES that you used when compiling the files that are being installed. One reason this is necessary is that some versions of Make have bugs and recompile files gratuitously when you do this step. If you use the same variable values, those files will be recompiled properly. For example, if you have built the stage 2 compiler, you can use the following command:

    make install CC="stage2/xgcc -Bstage2/" CFLAGS="-g -O" LANGUAGES="list"
    

    This copies the files `cc1', `cpp' and `libgcc.a' to files `cc1', `cpp' and `libgcc.a' in the directory `/usr/local/lib/gcc-lib/target/version', which is where the compiler driver program looks for them. Here target is the canonicalized form of target machine type specified when you ran `configure', and version is the version number of GNU CC. This naming scheme permits various versions and/or cross-compilers to coexist. It also copies the executables for compilers for other languages (e.g., `cc1plus' for C++) to the same directory. This also copies the driver program `xgcc' into `/usr/local/bin/gcc', so that it appears in typical execution search paths. It also copies `gcc.1' into `/usr/local/man/man1' and info pages into `/usr/local/info'. On some systems, this command causes recompilation of some files. This is usually due to bugs in make. You should either ignore this problem, or use GNU Make. Warning: there is a bug in alloca in the Sun library. To avoid this bug, be sure to install the executables of GNU CC that were compiled by GNU CC. (That is, the executables from stage 2 or 3, not stage 1.) They use alloca as a built-in function and never the one in the library. (It is usually better to install GNU CC executables from stage 2 or 3, since they usually run faster than the ones compiled with some other compiler.)
16. If you're going to use C++, you need to install the C++ runtime library. This includes all I/O functionality, special class libraries, etc. The standard C++ runtime library for GNU CC is called `libstdc++'. An obsolescent library `libg++' may also be available, but it's necessary only for older software that hasn't been converted yet; if you don't know whether you need `libg++' then you probably don't need it. Here's one way to build and install `libstdc++' for GNU CC:
    • Build and install GNU CC, so that invoking `gcc' obtains the GNU CC that was just built.
    • Obtain a copy of a compatible `libstdc++' distribution. For example, the `libstdc++-2.8.0.tar.gz' distribution should be compatible with GCC 2.8.0. GCC distributors normally distribute `libstdc++' as well.
    • Set the `CXX' environment variable to `gcc' while running the `libstdc++' distribution's `configure' command. Use the same `configure' options that you used when you invoked GCC's `configure' command.
    • Invoke `make' to build the C++ runtime.
    • Invoke `make install' to install the C++ runtime.
    To summarize, after building and installing GNU CC, invoke the following shell commands in the topmost directory of the C++ library distribution. For configure-options, use the same options that you used to configure GNU CC.

    $ CXX=gcc ./configure configure-options
    $ make
    $ make install
    

17. GNU CC includes a runtime library for Objective-C because it is an integral part of the language. You can find the files associated with the library in the subdirectory `objc'. The GNU Objective-C Runtime Library requires header files for the target's C library in order to be compiled,and also requires the header files for the target's thread library if you want thread support. See section 3.4.5 Cross-Compilers and Header Files, for discussion about header files issues for cross-compilation. When you run `configure', it picks the appropriate Objective-C thread implementation file for the target platform. In some situations, you may wish to choose a different back-end as some platforms support multiple thread implementations or you may wish to disable thread support completely. You do this by specifying a value for the OBJC_THREAD_FILE makefile variable on the command line when you run make, for example:

    make CC="stage2/xgcc -Bstage2/" CFLAGS="-g -O2" OBJC_THREAD_FILE=thr-single
    

    Below is a list of the currently available back-ends.
    • thr-single Disable thread support, should work for all platforms.
    • thr-decosf1 DEC OSF/1 thread support.
    • thr-irix SGI IRIX thread support.
    • thr-mach Generic MACH thread support, known to work on NEXTSTEP.
    • thr-os2 IBM OS/2 thread support.
    • thr-posix Generix POSIX thread support.
    • thr-pthreads PCThreads on Linux-based GNU systems.
    • thr-solaris SUN Solaris thread support.
    • thr-win32 Microsoft Win32 API thread support.

3.1 Files Created by configure

Here we spell out what files will be set up by configure. Normally you need not be concerned with these files.

• A file named `config.h' is created that contains a `#include' of the top-level config file for the machine you will run the compiler on (see section 18 The Configuration File). This file is responsible for defining information about the host machine. It includes `tm.h'. The top-level config file is located in the subdirectory `config'. Its name is always `xm-something.h'; usually `xm-machine.h', but there are some exceptions. If your system does not support symbolic links, you might want to set up `config.h' to contain a `#include' command which refers to the appropriate file.
• A file named `tconfig.h' is created which includes the top-level config file for your target machine. This is used for compiling certain programs to run on that machine.
• A file named `tm.h' is created which includes the machine-description macro file for your target machine. It should be in the subdirectory `config' and its name is often `machine.h'.
• The command file `configure' also constructs the file `Makefile' by adding some text to the template file `Makefile.in'. The additional text comes from files in the `config' directory, named `t-target' and `x-host'. If these files do not exist, it means nothing needs to be added for a given target or host.

3.2 Configurations Supported by GNU CC

Here are the possible CPU types:

1750a, a29k, alpha, arm, cn, clipper, dsp16xx, elxsi, h8300, hppa1.0, hppa1.1, i370, i386, i486, i586, i860, i960, m32r, m68000, m68k, m88k, mips, mipsel, mips64, mips64el, ns32k, powerpc, powerpcle, pyramid, romp, rs6000, sh, sparc, sparclite, sparc64, vax, we32k.

Here are the recognized company names. As you can see, customary abbreviations are used rather than the longer official names.

acorn, alliant, altos, apollo, apple, att, bull, cbm, convergent, convex, crds, dec, dg, dolphin, elxsi, encore, harris, hitachi, hp, ibm, intergraph, isi, mips, motorola, ncr, next, ns, omron, plexus, sequent, sgi, sony, sun, tti, unicom, wrs.

The company name is meaningful only to disambiguate when the rest of the information supplied is insufficient. You can omit it, writing just `cpu-system', if it is not needed. For example, `vax-ultrix4.2' is equivalent to `vax-dec-ultrix4.2'.

Here is a list of system types:

386bsd, aix, acis, amigaos, aos, aout, aux, bosx, bsd, clix, coff, ctix, cxux, dgux, dynix, ebmon, ecoff, elf, esix, freebsd, hms, genix, gnu, linux-gnu, hiux, hpux, iris, irix, isc, luna, lynxos, mach, minix, msdos, mvs, netbsd, newsos, nindy, ns, osf, osfrose, ptx, riscix, riscos, rtu, sco, sim, solaris, sunos, sym, sysv, udi, ultrix, unicos, uniplus, unos, vms, vsta, vxworks, winnt, xenix.

You can omit the system type; then `configure' guesses the operating system from the CPU and company.

You can add a version number to the system type; this may or may not make a difference. For example, you can write `bsd4.3' or `bsd4.4' to distinguish versions of BSD. In practice, the version number is most needed for `sysv3' and `sysv4', which are often treated differently.

If you specify an impossible combination such as `i860-dg-vms', then you may get an error message from `configure', or it may ignore part of the information and do the best it can with the rest. `configure' always prints the canonical name for the alternative that it used. GNU CC does not support all possible alternatives.

Often a particular model of machine has a name. Many machine names are recognized as aliases for CPU/company combinations. Thus, the machine name `sun3', mentioned above, is an alias for `m68k-sun'. Sometimes we accept a company name as a machine name, when the name is popularly used for a particular machine. Here is a table of the known machine names:

3300, 3b1, 3bn, 7300, altos3068, altos, apollo68, att-7300, balance, convex-cn, crds, decstation-3100, decstation, delta, encore, fx2800, gmicro, hp7nn, hp8nn, hp9k2nn, hp9k3nn, hp9k7nn, hp9k8nn, iris4d, iris, isi68, m3230, magnum, merlin, miniframe, mmax, news-3600, news800, news, next, pbd, pc532, pmax, powerpc, powerpcle, ps2, risc-news, rtpc, sun2, sun386i, sun386, sun3, sun4, symmetry, tower-32, tower.

Remember that a machine name specifies both the cpu type and the company name. If you want to install your own homemade configuration files, you can use `local' as the company name to access them. If you use configuration `cpu-local', the configuration name without the cpu prefix is used to form the configuration file names.

Thus, if you specify `m68k-local', configuration uses files `m68k.md', `local.h', `m68k.c', `xm-local.h', `t-local', and `x-local', all in the directory `config/m68k'.

Here is a list of configurations that have special treatment or special things you must know:

`1750a-*-*'

MIL-STD-1750A processors. The MIL-STD-1750A cross configuration produces output for as1750, an assembler/linker available under the GNU Public License for the 1750A. as1750 can be obtained at ftp://ftp.fta-berlin.de/pub/crossgcc/1750gals/. A similarly licensed simulator for the 1750A is available from same address. You should ignore a fatal error during the building of libgcc (libgcc is not yet implemented for the 1750A.) The as1750 assembler requires the file `ms1750.inc', which is found in the directory `config/1750a'. GNU CC produced the same sections as the Fairchild F9450 C Compiler, namely:

Normal

The program code section.

Static

The read/write (RAM) data section.

Konst

The read-only (ROM) constants section.

Init

Initialization section (code to copy KREL to SREL).

The smallest addressable unit is 16 bits (BITS_PER_UNIT is 16). This means that type `char' is represented with a 16-bit word per character. The 1750A's "Load/Store Upper/Lower Byte" instructions are not used by GNU CC.

`alpha-*-osf1'

Systems using processors that implement the DEC Alpha architecture and are running the DEC Unix (OSF/1) operating system, for example the DEC Alpha AXP systems.CC.) GNU CC writes a `.verstamp' directive to the assembler output file unless it is built as a cross-compiler. It gets the version to use from the system header file `/usr/include/stamp.h'. If you install a new version of DEC Unix, you should rebuild GCC to pick up the new version stamp. Note that since the Alpha is a 64-bit architecture, cross-compilers from 32-bit machines will not generate code as efficient as that generated when the compiler is running on a 64-bit machine because many optimizations that depend on being able to represent a word on the target in an integral value on the host cannot be performed. Building cross-compilers on the Alpha for 32-bit machines has only been tested in a few cases and may not work properly. make compare may fail on old versions of DEC Unix unless you add `-save-temps' to CFLAGS. On these systems, the name of the assembler input file is stored in the object file, and that makes comparison fail if it differs between the stage1 and stage2 compilations. The option `-save-temps' forces a fixed name to be used for the assembler input file, instead of a randomly chosen name in `/tmp'. Do not add `-save-temps' unless the comparisons fail without that option. If you add `-save-temps', you will have to manually delete the `.i' and `.s' files after each series of compilations. GNU CC now supports both the native (ECOFF) debugging format used by DBX and GDB and an encapsulated STABS format for use only with GDB. See the discussion of the `--with-stabs' option of `configure' above for more information on these formats and how to select them. There is a bug in DEC's assembler that produces incorrect line numbers for ECOFF format when the `.align' directive is used. To work around this problem, GNU CC will not emit such alignment directives while writing ECOFF format debugging information even if optimization is being performed. Unfortunately, this has the very undesirable side-effect that code addresses when `-O' is specified are different depending on whether or not `-g' is also specified. To avoid this behavior, specify `-gstabs+' and use GDB instead of DBX. DEC is now aware of this problem with the assembler and hopes to provide a fix shortly.

`arc-*-elf'

Argonaut ARC processor. This configuration is intended for embedded systems.

`arm-*-aout'

Advanced RISC Machines ARM-family processors. These are often used in embedded applications. There are no standard Unix configurations. This configuration corresponds to the basic instruction sequences and will produce `a.out' format object modules. You may need to make a variant of the file `arm.h' for your particular configuration.

`arm-*-linuxaout'

Any of the ARM family processors running the Linux-based GNU system with the `a.out' binary format (ELF is not yet supported). You must use version 2.8.1.0.7 or later of the GNU/Linux binutils, which you can download from `sunsite.unc.edu:/pub/Linux/GCC' and other mirror sites for Linux-based GNU systems.

`arm-*-riscix'

The ARM2 or ARM3 processor running RISC iX, Acorn's port of BSD Unix. If you are running a version of RISC iX prior to 1.2 then you must specify the version number during configuration. Note that the assembler shipped with RISC iX does not support stabs debugging information; a new version of the assembler, with stabs support included, is now available from Acorn and via ftp `ftp.acorn.com:/pub/riscix/as+xterm.tar.Z'. To enable stabs debugging, pass `--with-gnu-as' to configure. You will need to install GNU `sed' before you can run configure.

`a29k'

AMD Am29k-family processors. These are normally used in embedded applications. There are no standard Unix configurations. This configuration corresponds to AMD's standard calling sequence and binary interface and is compatible with other 29k tools. You may need to make a variant of the file `a29k.h' for your particular configuration.

`a29k-*-bsd'

AMD Am29050 used in a system running a variant of BSD Unix.

`decstation-*'

MIPS-based DECstations can support three different personalities: Ultrix, DEC OSF/1, and OSF/rose. (Alpha-based DECstation products have a configuration name beginning with `alpha-dec'.) To configure GCC for these platforms use the following configurations:

`decstation-ultrix'

Ultrix configuration.

`decstation-osf1'

Dec's version of OSF/1.

`decstation-osfrose'

Open Software Foundation reference port of OSF/1 which uses the OSF/rose object file format instead of ECOFF. Normally, you would not select this configuration.

The MIPS C compiler needs to be told to increase its table size for switch statements with the `-Wf,-XNg1500' option in order to compile `cp/parse.c'. If you use the `-O2' optimization option, you also need to use `-Olimit 3000'. Both of these options are automatically generated in the `Makefile' that the shell script `configure' builds. If you override the CC make variable and use the MIPS compilers, you may need to add `-Wf,-XNg1500 -Olimit 3000'.

`elxsi-elxsi-bsd'

The Elxsi's C compiler has known limitations that prevent it from compiling GNU C. Please contact mrs@cygnus.com for more details.

`dsp16xx'

A port to the AT&T DSP1610 family of processors.

`h8300-*-*'

Hitachi H8/300 series of processors. The calling convention and structure layout has changed in release 2.6. All code must be recompiled. The calling convention now passes the first three arguments in function calls in registers. Structures are no longer a multiple of 2 bytes.

`hppa*-*-*'

There are several variants of the HP-PA processor which run a variety of operating systems. GNU CC must be configured to use the correct processor type and operating system, or GNU CC will not function correctly. The easiest way to handle this problem is to not specify a target when configuring GNU CC, the `configure' script will try to automatically determine the right processor type and operating system. `-g' does not work on HP-UX, since that system uses a peculiar debugging format which GNU CC does not know about. However, `-g' will work if you also use GAS and GDB in conjunction with GCC. We highly recommend using GAS for all HP-PA configurations. You should be using GAS-2.6 (or later) along with GDB-4.16 (or later). These can be retrieved from all the traditional GNU ftp archive sites. On some versions of HP-UX, you will need to install GNU `sed'. You will need to be install GAS into a directory before /bin, /usr/bin, and /usr/ccs/bin in your search path. You should install GAS before you build GNU CC. To enable debugging, you must configure GNU CC with the `--with-gnu-as' option before building.

`i370-*-*'

This port is very preliminary and has many known bugs. We hope to have a higher-quality port for this machine soon.

`i386-*-linux-gnuoldld'

Use this configuration to generate `a.out' binaries on Linux-based GNU systems if you do not have gas/binutils version 2.5.2 or later installed. This is an obsolete configuration.

`i386-*-linux-gnuaout'

Use this configuration to generate `a.out' binaries on Linux-based GNU systems. This configuration is being superseded. You must use gas/binutils version 2.5.2 or later.

`i386-*-linux-gnu'

Use this configuration to generate ELF binaries on Linux-based GNU systems. You must use gas/binutils version 2.5.2 or later.

`i386-*-sco'

Compilation with RCC is recommended. Also, it may be a good idea to link with GNU malloc instead of the malloc that comes with the system.

`i386-*-sco3.2v4'

Use this configuration for SCO release 3.2 version 4.

`i386-*-sco3.2v5*'

Use this for the SCO OpenServer Release family including 5.0.0, 5.0.2, 5.0.4, 5.0.5, Internet FastStart 1.0, and Internet FastStart 1.1. GNU CC can generate COFF binaries if you specify `-mcoff' or ELF binaries, the default. A full `make bootstrap' is recommended so that an ELF compiler that builds ELF is generated. You must have TLS597 from ftp://ftp.sco.com/TLS installed for ELF C++ binaries to work correctly on releases before 5.0.4. The native SCO assembler that is provided with the OS at no charge is normally required. If, however, you must be able to use the GNU assembler (perhaps you have complex asms) you must configure this package `--with-gnu-as'. To do this, install (cp or symlink) gcc/as to your copy of the GNU assembler. You must use a recent version of GNU binutils; version 2.9.1 seems to work well. If you select this option, you will be unable to build COFF images. Trying to do so will result in non-obvious failures. In general, the "--with-gnu-as" option isn't as well tested as the native assembler. NOTE: If you are building C++, you must follow the instructions about invoking `make bootstrap' because the native OpenServer compiler may build a `cc1plus' that will not correctly parse many valid C++ programs. You must do a `make bootstrap' if you are building with the native compiler.

`i386-*-isc'

It may be a good idea to link with GNU malloc instead of the malloc that comes with the system. In ISC version 4.1, `sed' core dumps when building `deduced.h'. Use the version of `sed' from version 4.0.

`i386-*-esix'

It may be good idea to link with GNU malloc instead of the malloc that comes with the system.

`i386-ibm-aix'

You need to use GAS version 2.1 or later, and LD from GNU binutils version 2.2 or later.

`i386-sequent-bsd'

Go to the Berkeley universe before compiling.

`i386-sequent-ptx1*'

`i386-sequent-ptx2*'

You must install GNU `sed' before running `configure'.

`i386-sun-sunos4'

You may find that you need another version of GNU CC to begin bootstrapping with, since the current version when built with the system's own compiler seems to get an infinite loop compiling part of `libgcc2.c'. GNU CC version 2 compiled with GNU CC (any version) seems not to have this problem. See section 3.5 Installing GNU CC on the Sun, for information on installing GNU CC on Sun systems.

`i[345]86-*-winnt3.5'

This version requires a GAS that has not yet been released. Until it is, you can get a prebuilt binary version via anonymous ftp from `cs.washington.edu:pub/gnat' or `cs.nyu.edu:pub/gnat'. You must also use the Microsoft header files from the Windows NT 3.5 SDK. Find these on the CDROM in the `/mstools/h' directory dated 9/4/94. You must use a fixed version of Microsoft linker made especially for NT 3.5, which is also is available on the NT 3.5 SDK CDROM. If you do not have this linker, can you also use the linker from Visual C/C++ 1.0 or 2.0. Installing GNU CC for NT builds a wrapper linker, called `ld.exe', which mimics the behaviour of Unix `ld' in the specification of libraries (`-L' and `-l'). `ld.exe' looks for both Unix and Microsoft named libraries. For example, if you specify `-lfoo', `ld.exe' will look first for `libfoo.a' and then for `foo.lib'. You may install GNU CC for Windows NT in one of two ways, depending on whether or not you have a Unix-like shell and various Unix-like utilities.

1. If you do not have a Unix-like shell and few Unix-like utilities, you will use a DOS style batch script called `configure.bat'. Invoke it as configure winnt from an MSDOS console window or from the program manager dialog box. `configure.bat' assumes you have already installed and have in your path a Unix-like `sed' program which is used to create a working `Makefile' from `Makefile.in'. `Makefile' uses the Microsoft Nmake program maintenance utility and the Visual C/C++ V8.00 compiler to build GNU CC. You need only have the utilities `sed' and `touch' to use this installation method, which only automatically builds the compiler itself. You must then examine what `fixinc.winnt' does, edit the header files by hand and build `libgcc.a' manually.
2. The second type of installation assumes you are running a Unix-like shell, have a complete suite of Unix-like utilities in your path, and have a previous version of GNU CC already installed, either through building it via the above installation method or acquiring a pre-built binary. In this case, use the `configure' script in the normal fashion.

`i860-intel-osf1'

This is the Paragon. If you have version 1.0 of the operating system, see section 7.2 Installation Problems, for special things you need to do to compensate for peculiarities in the system.

`*-lynx-lynxos'

LynxOS 2.2 and earlier comes with GNU CC 1.x already installed as `/bin/gcc'. You should compile with this instead of `/bin/cc'. You can tell GNU CC to use the GNU assembler and linker, by specifying `--with-gnu-as --with-gnu-ld' when configuring. These will produce COFF format object files and executables; otherwise GNU CC will use the installed tools, which produce `a.out' format executables.

`m32r-*-elf'

Mitsubishi M32R processor. This configuration is intended for embedded systems.

`m68000-hp-bsd'

HP 9000 series 200 running BSD. Note that the C compiler that comes with this system cannot compile GNU CC; contact law@cygnus.com to get binaries of GNU CC for bootstrapping.

`m68k-altos'

Altos 3068. You must use the GNU assembler, linker and debugger. Also, you must fix a kernel bug. Details in the file `README.ALTOS'.

`m68k-apple-aux'

Apple Macintosh running A/UX. You may configure GCC to use either the system assembler and linker or the GNU assembler and linker. You should use the GNU configuration if you can, especially if you also want to use GNU C++. You enabled that configuration with + the `--with-gnu-as' and `--with-gnu-ld' options to configure. Note the C compiler that comes with this system cannot compile GNU CC. You can find binaries of GNU CC for bootstrapping on jagubox.gsfc.nasa.gov. You will also a patched version of `/bin/ld' there that raises some of the arbitrary limits found in the original.

`m68k-att-sysv'

AT&T 3b1, a.k.a. 7300 PC. Special procedures are needed to compile GNU CC with this machine's standard C compiler, due to bugs in that compiler. You can bootstrap it more easily with previous versions of GNU CC if you have them. Installing GNU CC on the 3b1 is difficult if you do not already have GNU CC running, due to bugs in the installed C compiler. However, the following procedure might work. We are unable to test it.

1. Comment out the `#include "config.h"' line near the start of `cccp.c' and do `make cpp'. This makes a preliminary version of GNU cpp.
2. Save the old `/lib/cpp' and copy the preliminary GNU cpp to that file name.
3. Undo your change in `cccp.c', or reinstall the original version, and do `make cpp' again.
4. Copy this final version of GNU cpp into `/lib/cpp'.
5. Replace every occurrence of obstack_free in the file `tree.c' with _obstack_free.
6. Run make to get the first-stage GNU CC.
7. Reinstall the original version of `/lib/cpp'.
8. Now you can compile GNU CC with itself and install it in the normal fashion.

`m68k-bull-sysv'

Bull DPX/2 series 200 and 300 with BOS-2.00.45 up to BOS-2.01. GNU CC works either with native assembler or GNU assembler. You can use GNU assembler with native coff generation by providing `--with-gnu-as' to the configure script or use GNU assembler with dbx-in-coff encapsulation by providing `--with-gnu-as --stabs'. For any problem with native assembler or for availability of the DPX/2 port of GAS, contact F.Pierresteguy@frcl.bull.fr.

`m68k-crds-unox'

Use `configure unos' for building on Unos. The Unos assembler is named casm instead of as. For some strange reason linking `/bin/as' to `/bin/casm' changes the behavior, and does not work. So, when installing GNU CC, you should install the following script as `as' in the subdirectory where the passes of GCC are installed:

#!/bin/sh
casm $*

The default Unos library is named `libunos.a' instead of `libc.a'. To allow GNU CC to function, either change all references to `-lc' in `gcc.c' to `-lunos' or link `/lib/libc.a' to `/lib/libunos.a'. When compiling GNU CC with the standard compiler, to overcome bugs in the support of alloca, do not use `-O' when making stage 2. Then use the stage 2 compiler with `-O' to make the stage 3 compiler. This compiler will have the same characteristics as the usual stage 2 compiler on other systems. Use it to make a stage 4 compiler and compare that with stage 3 to verify proper compilation. (Perhaps simply defining ALLOCA in `x-crds' as described in the comments there will make the above paragraph superfluous. Please inform us of whether this works.) Unos uses memory segmentation instead of demand paging, so you will need a lot of memory. 5 Mb is barely enough if no other tasks are running. If linking `cc1' fails, try putting the object files into a library and linking from that library.

`m68k-hp-hpux'

HP 9000 series 300 or 400 running HP-UX. HP-UX version 8.0 has a bug in the assembler that prevents compilation of GNU CC. To fix it, get patch PHCO_4484 from HP. In addition, if you wish to use gas `--with-gnu-as' you must use gas version 2.1 or later, and you must use the GNU linker version 2.1 or later. Earlier versions of gas relied upon a program which converted the gas output into the native HP-UX format, but that program has not been kept up to date. gdb does not understand that native HP-UX format, so you must use gas if you wish to use gdb.

`m68k-sun'

Sun 3. We do not provide a configuration file to use the Sun FPA by default, because programs that establish signal handlers for floating point traps inherently cannot work with the FPA. See section 3.5 Installing GNU CC on the Sun, for information on installing GNU CC on Sun systems.

`m88k-*-svr3'

Motorola m88k running the AT&T/Unisoft/Motorola V.3 reference port. These systems tend to use the Green Hills C, revision 1.8.5, as the standard C compiler. There are apparently bugs in this compiler that result in object files differences between stage 2 and stage 3. If this happens, make the stage 4 compiler and compare it to the stage 3 compiler. If the stage 3 and stage 4 object files are identical, this suggests you encountered a problem with the standard C compiler; the stage 3 and 4 compilers may be usable. It is best, however, to use an older version of GNU CC for bootstrapping if you have one.

`m88k-*-dgux'

Motorola m88k running DG/UX. To build 88open BCS native or cross compilers on DG/UX, specify the configuration name as `m88k-*-dguxbcs' and build in the 88open BCS software development environment. To build ELF native or cross compilers on DG/UX, specify `m88k-*-dgux' and build in the DG/UX ELF development environment. You set the software development environment by issuing `sde-target' command and specifying either `m88kbcs' or `m88kdguxelf' as the operand. If you do not specify a configuration name, `configure' guesses the configuration based on the current software development environment.

`m88k-tektronix-sysv3'

Tektronix XD88 running UTekV 3.2e. Do not turn on optimization while building stage1 if you bootstrap with the buggy Green Hills compiler. Also, The bundled LAI System V NFS is buggy so if you build in an NFS mounted directory, start from a fresh reboot, or avoid NFS all together. Otherwise you may have trouble getting clean comparisons between stages.

`mips-mips-bsd'

MIPS machines running the MIPS operating system in BSD mode. It's possible that some old versions of the system lack the functions memcpy, memcmp, and memset. If your system lacks these, you must remove or undo the definition of TARGET_MEM_FUNCTIONS in `mips-bsd.h'. The MIPS C compiler needs to be told to increase its table size for switch statements with the `-Wf,-XNg1500' option in order to compile `cp/parse.c'. If you use the `-O2' optimization option, you also need to use `-Olimit 3000'. Both of these options are automatically generated in the `Makefile' that the shell script `configure' builds. If you override the CC make variable and use the MIPS compilers, you may need to add `-Wf,-XNg1500 -Olimit 3000'.

`mips-mips-riscos*'

The MIPS C compiler needs to be told to increase its table size for switch statements with the `-Wf,-XNg1500' option in order to compile `cp/parse.c'. If you use the `-O2' optimization option, you also need to use `-Olimit 3000'. Both of these options are automatically generated in the `Makefile' that the shell script `configure' builds. If you override the CC make variable and use the MIPS compilers, you may need to add `-Wf,-XNg1500 -Olimit 3000'. MIPS computers running RISC-OS can support four different personalities: default, BSD 4.3, System V.3, and System V.4 (older versions of RISC-OS don't support V.4). To configure GCC for these platforms use the following configurations:

`mips-mips-riscosrev'

Default configuration for RISC-OS, revision rev.

`mips-mips-riscosrevbsd'

BSD 4.3 configuration for RISC-OS, revision rev.

`mips-mips-riscosrevsysv4'

System V.4 configuration for RISC-OS, revision rev.

`mips-mips-riscosrevsysv'

System V.3 configuration for RISC-OS, revision rev.

The revision rev mentioned above is the revision of RISC-OS to use. You must reconfigure GCC when going from a RISC-OS revision 4 to RISC-OS revision 5. This has the effect of avoiding a linker bug (see section 7.2 Installation Problems, for more details).

`mips-sgi-*'

In order to compile GCC on an SGI running IRIX 4, the "c.hdr.lib" option must be installed from the CD-ROM supplied from Silicon Graphics. This is found on the 2nd CD in release 4.0.1. In order to compile GCC on an SGI running IRIX 5, the "compiler_dev.hdr" subsystem must be installed from the IDO CD-ROM supplied by Silicon Graphics. make compare may fail on version 5 of IRIX unless you add `-save-temps' to CFLAGS. On these systems, the name of the assembler input file is stored in the object file, and that makes comparison fail if it differs between the stage1 and stage2 compilations. The option `-save-temps' forces a fixed name to be used for the assembler input file, instead of a randomly chosen name in `/tmp'. Do not add `-save-temps' unless the comparisons fail without that option. If you do you `-save-temps', you will have to manually delete the `.i' and `.s' files after each series of compilations. The MIPS C compiler needs to be told to increase its table size for switch statements with the `-Wf,-XNg1500' option in order to compile `cp/parse.c'. If you use the `-O2' optimization option, you also need to use `-Olimit 3000'. Both of these options are automatically generated in the `Makefile' that the shell script `configure' builds. If you override the CC make variable and use the MIPS compilers, you may need to add `-Wf,-XNg1500 -Olimit 3000'. On Irix version 4.0.5F, and perhaps on some other versions as well, there is an assembler bug that reorders instructions incorrectly. To work around it, specify the target configuration `mips-sgi-irix4loser'. This configuration inhibits assembler optimization. In a compiler configured with target `mips-sgi-irix4', you can turn off assembler optimization by using the `-noasmopt' option. This compiler option passes the option `-O0' to the assembler, to inhibit reordering. The `-noasmopt' option can be useful for testing whether a problem is due to erroneous assembler reordering. Even if a problem does not go away with `-noasmopt', it may still be due to assembler reordering--perhaps GNU CC itself was miscompiled as a result. To enable debugging under Irix 5, you must use GNU as 2.5 or later, and use the `--with-gnu-as' configure option when configuring gcc. GNU as is distributed as part of the binutils package.

`mips-sony-sysv'

Sony MIPS NEWS. This works in NEWSOS 5.0.1, but not in 5.0.2 (which uses ELF instead of COFF). Support for 5.0.2 will probably be provided soon by volunteers. In particular, the linker does not like the code generated by GCC when shared libraries are linked in.

`ns32k-encore'

Encore ns32000 system. Encore systems are supported only under BSD.

`ns32k-*-genix'

National Semiconductor ns32000 system. Genix has bugs in alloca and malloc; you must get the compiled versions of these from GNU Emacs.

`ns32k-sequent'

Go to the Berkeley universe before compiling.

`ns32k-utek'

UTEK ns32000 system ("merlin"). The C compiler that comes with this system cannot compile GNU CC; contact `tektronix!reed!mason' to get binaries of GNU CC for bootstrapping.

`romp-*-aos'

`romp-*-mach'

The only operating systems supported for the IBM RT PC are AOS and MACH. GNU CC does not support AIX running on the RT. We recommend you compile GNU CC with an earlier version of itself; if you compile GNU CC with hc, the Metaware compiler, it will work, but you will get mismatches between the stage 2 and stage 3 compilers in various files. These errors are minor differences in some floating-point constants and can be safely ignored; the stage 3 compiler is correct.

`rs6000-*-aix'

`powerpc-*-aix'

Various early versions of each release of the IBM XLC compiler will not bootstrap GNU CC. Symptoms include differences between the stage2 and stage3 object files, and errors when compiling `libgcc.a' or `enquire'. Known problematic releases include: xlc-1.2.1.8, xlc-1.3.0.0 (distributed with AIX 3.2.5), and xlc-1.3.0.19. Both xlc-1.2.1.28 and xlc-1.3.0.24 (PTF 432238) are known to produce working versions of GNU CC, but most other recent releases correctly bootstrap GNU CC. Release 4.3.0 of AIX and ones prior to AIX 3.2.4 include a version of the IBM assembler which does not accept debugging directives: assembler updates are available as PTFs. Also, if you are using AIX 3.2.5 or greater and the GNU assembler, you must have a version modified after October 16th, 1995 in order for the GNU C compiler to build. See the file `README.RS6000' for more details on any of these problems. GNU CC does not yet support the 64-bit PowerPC instructions. Objective C does not work on this architecture because it makes assumptions that are incompatible with the calling conventions. AIX on the RS/6000 provides support (NLS) for environments outside of the United States. Compilers and assemblers use NLS to support locale-specific representations of various objects including floating-point numbers ("." vs "," for separating decimal fractions). There have been problems reported where the library linked with GNU CC does not produce the same floating-point formats that the assembler accepts. If you have this problem, set the LANG environment variable to "C" or "En_US". Due to changes in the way that GNU CC invokes the binder (linker) for AIX 4.1, you may now receive warnings of duplicate symbols from the link step that were not reported before. The assembly files generated by GNU CC for AIX have always included multiple symbol definitions for certain global variable and function declarations in the original program. The warnings should not prevent the linker from producing a correct library or runnable executable. By default, AIX 4.1 produces code that can be used on either Power or PowerPC processors. You can specify a default version for the `-mcpu='cpu_type switch by using the configure option `--with-cpu-'cpu_type.

`powerpc-*-elf'

`powerpc-*-sysv4'

PowerPC system in big endian mode, running System V.4. You can specify a default version for the `-mcpu='cpu_type switch by using the configure option `--with-cpu-'cpu_type.

`powerpc-*-linux-gnu'

PowerPC system in big endian mode, running the Linux-based GNU system. You can specify a default version for the `-mcpu='cpu_type switch by using the configure option `--with-cpu-'cpu_type.

`powerpc-*-eabiaix'

Embedded PowerPC system in big endian mode with -mcall-aix selected as the default. You can specify a default version for the `-mcpu='cpu_type switch by using the configure option `--with-cpu-'cpu_type.

`powerpc-*-eabisim'

Embedded PowerPC system in big endian mode for use in running under the PSIM simulator. You can specify a default version for the `-mcpu='cpu_type switch by using the configure option `--with-cpu-'cpu_type.

`powerpc-*-eabi'

Embedded PowerPC system in big endian mode. You can specify a default version for the `-mcpu='cpu_type switch by using the configure option `--with-cpu-'cpu_type.

`powerpcle-*-elf'

`powerpcle-*-sysv4'

PowerPC system in little endian mode, running System V.4. You can specify a default version for the `-mcpu='cpu_type switch by using the configure option `--with-cpu-'cpu_type.

`powerpcle-*-solaris2*'

PowerPC system in little endian mode, running Solaris 2.5.1 or higher. You can specify a default version for the `-mcpu='cpu_type switch by using the configure option `--with-cpu-'cpu_type. Beta versions of the Sun 4.0 compiler do not seem to be able to build GNU CC correctly. There are also problems with the host assembler and linker that are fixed by using the GNU versions of these tools.

`powerpcle-*-eabisim'

Embedded PowerPC system in little endian mode for use in running under the PSIM simulator.

`powerpcle-*-eabi'

Embedded PowerPC system in little endian mode. You can specify a default version for the `-mcpu='cpu_type switch by using the configure option `--with-cpu-'cpu_type.

`powerpcle-*-winnt'

`powerpcle-*-pe'

PowerPC system in little endian mode running Windows NT. You can specify a default version for the `-mcpu='cpu_type switch by using the configure option `--with-cpu-'cpu_type.

`vax-dec-ultrix'

Don't try compiling with Vax C (vcc). It produces incorrect code in some cases (for example, when alloca is used). Meanwhile, compiling `cp/parse.c' with pcc does not work because of an internal table size limitation in that compiler. To avoid this problem, compile just the GNU C compiler first, and use it to recompile building all the languages that you want to run.

`sparc-sun-*'

See section 3.5 Installing GNU CC on the Sun, for information on installing GNU CC on Sun systems.

`vax-dec-vms'

See section 3.6 Installing GNU CC on VMS, for details on how to install GNU CC on VMS.

`we32k-*-*'

These computers are also known as the 3b2, 3b5, 3b20 and other similar names. (However, the 3b1 is actually a 68000; see section 3.2 Configurations Supported by GNU CC.) Don't use `-g' when compiling with the system's compiler. The system's linker seems to be unable to handle such a large program with debugging information. The system's compiler runs out of capacity when compiling `stmt.c' in GNU CC. You can work around this by building `cpp' in GNU CC first, then use that instead of the system's preprocessor with the system's C compiler to compile `stmt.c'. Here is how:

mv /lib/cpp /lib/cpp.att
cp cpp /lib/cpp.gnu
echo '/lib/cpp.gnu -traditional ${1+"$@"}' > /lib/cpp
chmod +x /lib/cpp

The system's compiler produces bad code for some of the GNU CC optimization files. So you must build the stage 2 compiler without optimization. Then build a stage 3 compiler with optimization. That executable should work. Here are the necessary commands:

make LANGUAGES=c CC=stage1/xgcc CFLAGS="-Bstage1/ -g"
make stage2
make CC=stage2/xgcc CFLAGS="-Bstage2/ -g -O"

You may need to raise the ULIMIT setting to build a C++ compiler, as the file `cc1plus' is larger than one megabyte.

3.3 Compilation in a Separate Directory

If you wish to build the object files and executables in a directory other than the one containing the source files, here is what you must do differently:

1. Make sure you have a version of Make that supports the VPATH feature. (GNU Make supports it, as do Make versions on most BSD systems.)
2. If you have ever run `configure' in the source directory, you must undo the configuration. Do this by running:

   make distclean
   

3. Go to the directory in which you want to build the compiler before running `configure':

   mkdir gcc-sun3
   cd gcc-sun3
   

   On systems that do not support symbolic links, this directory must be on the same file system as the source code directory.
4. Specify where to find `configure' when you run it:

   ../gcc/configure ...
   

   This also tells configure where to find the compiler sources; configure takes the directory from the file name that was used to invoke it. But if you want to be sure, you can specify the source directory with the `--srcdir' option, like this:

   ../gcc/configure --srcdir=../gcc other options
   

   The directory you specify with `--srcdir' need not be the same as the one that configure is found in.

Now, you can run make in that directory. You need not repeat the configuration steps shown above, when ordinary source files change. You must, however, run configure again when the configuration files change, if your system does not support symbolic links.

3.4 Building and Installing a Cross-Compiler

GNU CC can function as a cross-compiler for many machines, but not all.

• Cross-compilers for the Mips as target using the Mips assembler currently do not work, because the auxiliary programs `mips-tdump.c' and `mips-tfile.c' can't be compiled on anything but a Mips. It does work to cross compile for a Mips if you use the GNU assembler and linker.
• Cross-compilers between machines with different floating point formats have not all been made to work. GNU CC now has a floating point emulator with which these can work, but each target machine description needs to be updated to take advantage of it.
• Cross-compilation between machines of different word sizes is somewhat problematic and sometimes does not work.

Since GNU CC generates assembler code, you probably need a cross-assembler that GNU CC can run, in order to produce object files. If you want to link on other than the target machine, you need a cross-linker as well. You also need header files and libraries suitable for the target machine that you can install on the host machine.

3.4.1 Steps of Cross-Compilation

To compile and run a program using a cross-compiler involves several steps:

• Run the cross-compiler on the host machine to produce assembler files for the target machine. This requires header files for the target machine.
• Assemble the files produced by the cross-compiler. You can do this either with an assembler on the target machine, or with a cross-assembler on the host machine.
• Link those files to make an executable. You can do this either with a linker on the target machine, or with a cross-linker on the host machine. Whichever machine you use, you need libraries and certain startup files (typically `crt....o') for the target machine.

It is most convenient to do all of these steps on the same host machine, since then you can do it all with a single invocation of GNU CC. This requires a suitable cross-assembler and cross-linker. For some targets, the GNU assembler and linker are available.

3.4.2 Configuring a Cross-Compiler

To build GNU CC as a cross-compiler, you start out by running `configure'. Use the `--target=target' to specify the target type. If `configure' was unable to correctly identify the system you are running on, also specify the `--build=build' option. For example, here is how to configure for a cross-compiler that produces code for an HP 68030 system running BSD on a system that `configure' can correctly identify:

./configure --target=m68k-hp-bsd4.3

3.4.3 Tools and Libraries for a Cross-Compiler

If you have a cross-assembler and cross-linker available, you should install them now. Put them in the directory `/usr/local/target/bin'. Here is a table of the tools you should put in this directory:

`as'

This should be the cross-assembler.

`ld'

This should be the cross-linker.

`ar'

This should be the cross-archiver: a program which can manipulate archive files (linker libraries) in the target machine's format.

`ranlib'

This should be a program to construct a symbol table in an archive file.

The installation of GNU CC will find these programs in that directory, and copy or link them to the proper place to for the cross-compiler to find them when run later.

The easiest way to provide these files is to build the Binutils package and GAS. Configure them with the same `--host' and `--target' options that you use for configuring GNU CC, then build and install them. They install their executables automatically into the proper directory. Alas, they do not support all the targets that GNU CC supports.

If you want to install libraries to use with the cross-compiler, such as a standard C library, put them in the directory `/usr/local/target/lib'; installation of GNU CC copies all the files in that subdirectory into the proper place for GNU CC to find them and link with them. Here's an example of copying some libraries from a target machine:

ftp target-machine
lcd /usr/local/target/lib
cd /lib
get libc.a
cd /usr/lib
get libg.a
get libm.a
quit

The precise set of libraries you'll need, and their locations on the target machine, vary depending on its operating system.

Many targets require "start files" such as `crt0.o' and `crtn.o' which are linked into each executable; these too should be placed in `/usr/local/target/lib'. There may be several alternatives for `crt0.o', for use with profiling or other compilation options. Check your target's definition of STARTFILE_SPEC to find out what start files it uses. Here's an example of copying these files from a target machine:

ftp target-machine
lcd /usr/local/target/lib
prompt
cd /lib
mget *crt*.o
cd /usr/lib
mget *crt*.o
quit

3.4.4 `libgcc.a' and Cross-Compilers

Code compiled by GNU CC uses certain runtime support functions implicitly. Some of these functions can be compiled successfully with GNU CC itself, but a few cannot be. These problem functions are in the source file `libgcc1.c'; the library made from them is called `libgcc1.a'.

When you build a native compiler, these functions are compiled with some other compiler--the one that you use for bootstrapping GNU CC. Presumably it knows how to open code these operations, or else knows how to call the run-time emulation facilities that the machine comes with. But this approach doesn't work for building a cross-compiler. The compiler that you use for building knows about the host system, not the target system.

So, when you build a cross-compiler you have to supply a suitable library `libgcc1.a' that does the job it is expected to do.

To compile `libgcc1.c' with the cross-compiler itself does not work. The functions in this file are supposed to implement arithmetic operations that GNU CC does not know how to open code for your target machine. If these functions are compiled with GNU CC itself, they will compile into infinite recursion.

On any given target, most of these functions are not needed. If GNU CC can open code an arithmetic operation, it will not call these functions to perform the operation. It is possible that on your target machine, none of these functions is needed. If so, you can supply an empty library as `libgcc1.a'.

Many targets need library support only for multiplication and division. If you are linking with a library that contains functions for multiplication and division, you can tell GNU CC to call them directly by defining the macros MULSI3_LIBCALL, and the like. These macros need to be defined in the target description macro file. For some targets, they are defined already. This may be sufficient to avoid the need for libgcc1.a; if so, you can supply an empty library.

Some targets do not have floating point instructions; they need other functions in `libgcc1.a', which do floating arithmetic. Recent versions of GNU CC have a file which emulates floating point. With a certain amount of work, you should be able to construct a floating point emulator that can be used as `libgcc1.a'. Perhaps future versions will contain code to do this automatically and conveniently. That depends on whether someone wants to implement it.

Some embedded targets come with all the necessary `libgcc1.a' routines written in C or assembler. These targets build `libgcc1.a' automatically and you do not need to do anything special for them. Other embedded targets do not need any `libgcc1.a' routines since all the necessary operations are supported by the hardware.

If your target system has another C compiler, you can configure GNU CC as a native compiler on that machine, build just `libgcc1.a' with `make libgcc1.a' on that machine, and use the resulting file with the cross-compiler. To do this, execute the following on the target machine:

cd target-build-dir
./configure --host=sparc --target=sun3
make libgcc1.a

And then this on the host machine:

ftp target-machine
binary
cd target-build-dir
get libgcc1.a
quit

Another way to provide the functions you need in `libgcc1.a' is to define the appropriate perform_... macros for those functions. If these definitions do not use the C arithmetic operators that they are meant to implement, you should be able to compile them with the cross-compiler you are building. (If these definitions already exist for your target file, then you are all set.)

To build `libgcc1.a' using the perform macros, use `LIBGCC1=libgcc1.a OLDCC=./xgcc' when building the compiler. Otherwise, you should place your replacement library under the name `libgcc1.a' in the directory in which you will build the cross-compiler, before you run make.

3.4.5 Cross-Compilers and Header Files

If you are cross-compiling a standalone program or a program for an embedded system, then you may not need any header files except the few that are part of GNU CC (and those of your program). However, if you intend to link your program with a standard C library such as `libc.a', then you probably need to compile with the header files that go with the library you use.

The GNU C compiler does not come with these files, because (1) they are system-specific, and (2) they belong in a C library, not in a compiler.

If the GNU C library supports your target machine, then you can get the header files from there (assuming you actually use the GNU library when you link your program).

If your target machine comes with a C compiler, it probably comes with suitable header files also. If you make these files accessible from the host machine, the cross-compiler can use them also.

Otherwise, you're on your own in finding header files to use when cross-compiling.

When you have found suitable header files, put them in the directory `/usr/local/target/include', before building the cross compiler. Then installation will run fixincludes properly and install the corrected versions of the header files where the compiler will use them.

Provide the header files before you build the cross-compiler, because the build stage actually runs the cross-compiler to produce parts of `libgcc.a'. (These are the parts that can be compiled with GNU CC.) Some of them need suitable header files.

Here's an example showing how to copy the header files from a target machine. On the target machine, do this:

(cd /usr/include; tar cf - .) > tarfile

Then, on the host machine, do this:

ftp target-machine
lcd /usr/local/target/include
get tarfile
quit
tar xf tarfile

3.4.6 Actually Building the Cross-Compiler

Now you can proceed just as for compiling a single-machine compiler through the step of building stage 1. If you have not provided some sort of `libgcc1.a', then compilation will give up at the point where it needs that file, printing a suitable error message. If you do provide `libgcc1.a', then building the compiler will automatically compile and link a test program called `libgcc1-test'; if you get errors in the linking, it means that not all of the necessary routines in `libgcc1.a' are available.

You must provide the header file `float.h'. One way to do this is to compile `enquire' and run it on your target machine. The job of `enquire' is to run on the target machine and figure out by experiment the nature of its floating point representation. `enquire' records its findings in the header file `float.h'. If you can't produce this file by running `enquire' on the target machine, then you will need to come up with a suitable `float.h' in some other way (or else, avoid using it in your programs).

Do not try to build stage 2 for a cross-compiler. It doesn't work to rebuild GNU CC as a cross-compiler using the cross-compiler, because that would produce a program that runs on the target machine, not on the host. For example, if you compile a 386-to-68030 cross-compiler with itself, the result will not be right either for the 386 (because it was compiled into 68030 code) or for the 68030 (because it was configured for a 386 as the host). If you want to compile GNU CC into 68030 code, whether you compile it on a 68030 or with a cross-compiler on a 386, you must specify a 68030 as the host when you configure it.

To install the cross-compiler, use `make install', as usual.

3.5 Installing GNU CC on the Sun

On Solaris, do not use the linker or other tools in `/usr/ucb' to build GNU CC. Use /usr/ccs/bin.

If the assembler reports `Error: misaligned data' when bootstrapping, you are probably using an obsolete version of the GNU assembler. Upgrade to the latest version of GNU binutils, or use the Solaris assembler.

Make sure the environment variable FLOAT_OPTION is not set when you compile `libgcc.a'. If this option were set to f68881 when `libgcc.a' is compiled, the resulting code would demand to be linked with a special startup file and would not link properly without special pains.

There is a bug in alloca in certain versions of the Sun library. To avoid this bug, install the binaries of GNU CC that were compiled by GNU CC. They use alloca as a built-in function and never the one in the library.

Some versions of the Sun compiler crash when compiling GNU CC. The problem is a segmentation fault in cpp. This problem seems to be due to the bulk of data in the environment variables. You may be able to avoid it by using the following command to compile GNU CC with Sun CC:

make CC="TERMCAP=x OBJS=x LIBFUNCS=x STAGESTUFF=x cc"

SunOS 4.1.3 and 4.1.3_U1 have bugs that can cause intermittent core dumps when compiling GNU CC. A common symptom is an internal compiler error which does not recur if you run it again. To fix the problem, install Sun recommended patch 100726 (for SunOS 4.1.3) or 101508 (for SunOS 4.1.3_U1), or upgrade to a later SunOS release.

3.6 Installing GNU CC on VMS

The VMS version of GNU CC is distributed in a backup saveset containing both source code and precompiled binaries.

To install the `gcc' command so you can use the compiler easily, in the same manner as you use the VMS C compiler, you must install the VMS CLD file for GNU CC as follows:

1. Define the VMS logical names `GNU_CC' and `GNU_CC_INCLUDE' to point to the directories where the GNU CC executables (`gcc-cpp.exe', `gcc-cc1.exe', etc.) and the C include files are kept respectively. This should be done with the commands:

   $ assign /system /translation=concealed -
     disk:[gcc.] gnu_cc
   $ assign /system /translation=concealed -
     disk:[gcc.include.] gnu_cc_include
   

   with the appropriate disk and directory names. These commands can be placed in your system startup file so they will be executed whenever the machine is rebooted. You may, if you choose, do this via the `GCC_INSTALL.COM' script in the `[GCC]' directory.
2. Install the `GCC' command with the command line:

   $ set command /table=sys$common:[syslib]dcltables -
     /output=sys$common:[syslib]dcltables gnu_cc:[000000]gcc
   $ install replace sys$common:[syslib]dcltables
   

3. To install the help file, do the following:

   $ library/help sys$library:helplib.hlb gcc.hlp
   

   Now you can invoke the compiler with a command like `gcc /verbose file.c', which is equivalent to the command `gcc -v -c file.c' in Unix.

If you wish to use GNU C++ you must first install GNU CC, and then perform the following steps:

1. Define the VMS logical name `GNU_GXX_INCLUDE' to point to the directory where the preprocessor will search for the C++ header files. This can be done with the command:

   $ assign /system /translation=concealed -
     disk:[gcc.gxx_include.] gnu_gxx_include
   

   with the appropriate disk and directory name. If you are going to be using a C++ runtime library, this is where its install procedure will install its header files.
2. Obtain the file `gcc-cc1plus.exe', and place this in the same directory that `gcc-cc1.exe' is kept. The GNU C++ compiler can be invoked with a command like `gcc /plus /verbose file.cc', which is equivalent to the command `g++ -v -c file.cc' in Unix.

We try to put corresponding binaries and sources on the VMS distribution tape. But sometimes the binaries will be from an older version than the sources, because we don't always have time to update them. (Use the `/version' option to determine the version number of the binaries and compare it with the source file `version.c' to tell whether this is so.) In this case, you should use the binaries you get to recompile the sources. If you must recompile, here is how:

1. Execute the command procedure `vmsconfig.com' to set up the files `tm.h', `config.h', `aux-output.c', and `md.', and to create files `tconfig.h' and `hconfig.h'. This procedure also creates several linker option files used by `make-cc1.com' and a data file used by `make-l2.com'.

   $ @vmsconfig.com
   

2. Setup the logical names and command tables as defined above. In addition, define the VMS logical name `GNU_BISON' to point at the to the directories where the Bison executable is kept. This should be done with the command:

   $ assign /system /translation=concealed -
     disk:[bison.] gnu_bison
   

   You may, if you choose, use the `INSTALL_BISON.COM' script in the `[BISON]' directory.
3. Install the `BISON' command with the command line:

   $ set command /table=sys$common:[syslib]dcltables -
     /output=sys$common:[syslib]dcltables -
     gnu_bison:[000000]bison
   $ install replace sys$common:[syslib]dcltables
   

4. Type `@make-gcc' to recompile everything (alternatively, submit the file `make-gcc.com' to a batch queue). If you wish to build the GNU C++ compiler as well as the GNU CC compiler, you must first edit `make-gcc.com' and follow the instructions that appear in the comments.
5. In order to use GCC, you need a library of functions which GCC compiled code will call to perform certain tasks, and these functions are defined in the file `libgcc2.c'. To compile this you should use the command procedure `make-l2.com', which will generate the library `libgcc2.olb'. `libgcc2.olb' should be built using the compiler built from the same distribution that `libgcc2.c' came from, and `make-gcc.com' will automatically do all of this for you. To install the library, use the following commands:

   $ library gnu_cc:[000000]gcclib/delete=(new,eprintf)
   $ library gnu_cc:[000000]gcclib/delete=L_*
   $ library libgcc2/extract=*/output=libgcc2.obj
   $ library gnu_cc:[000000]gcclib libgcc2.obj
   

   The first command simply removes old modules that will be replaced with modules from `libgcc2' under different module names. The modules new and eprintf may not actually be present in your `gcclib.olb'---if the VMS librarian complains about those modules not being present, simply ignore the message and continue on with the next command. The second command removes the modules that came from the previous version of the library `libgcc2.c'. Whenever you update the compiler on your system, you should also update the library with the above procedure.
6. You may wish to build GCC in such a way that no files are written to the directory where the source files reside. An example would be the when the source files are on a read-only disk. In these cases, execute the following DCL commands (substituting your actual path names):

   $ assign dua0:[gcc.build_dir.]/translation=concealed, -
            dua1:[gcc.source_dir.]/translation=concealed  gcc_build
   $ set default gcc_build:[000000]
   

   where the directory `dua1:[gcc.source_dir]' contains the source code, and the directory `dua0:[gcc.build_dir]' is meant to contain all of the generated object files and executables. Once you have done this, you can proceed building GCC as described above. (Keep in mind that `gcc_build' is a rooted logical name, and thus the device names in each element of the search list must be an actual physical device name rather than another rooted logical name).
7. If you are building GNU CC with a previous version of GNU CC, you also should check to see that you have the newest version of the assembler. In particular, GNU CC version 2 treats global constant variables slightly differently from GNU CC version 1, and GAS version 1.38.1 does not have the patches required to work with GCC version 2. If you use GAS 1.38.1, then extern const variables will not have the read-only bit set, and the linker will generate warning messages about mismatched psect attributes for these variables. These warning messages are merely a nuisance, and can safely be ignored. If you are compiling with a version of GNU CC older than 1.33, specify `/DEFINE=("inline=")' as an option in all the compilations. This requires editing all the gcc commands in `make-cc1.com'. (The older versions had problems supporting inline.) Once you have a working 1.33 or newer GNU CC, you can change this file back.
8. If you want to build GNU CC with the VAX C compiler, you will need to make minor changes in `make-cccp.com' and `make-cc1.com' to choose alternate definitions of CC, CFLAGS, and LIBS. See comments in those files. However, you must also have a working version of the GNU assembler (GNU as, aka GAS) as it is used as the back-end for GNU CC to produce binary object modules and is not included in the GNU CC sources. GAS is also needed to compile `libgcc2' in order to build `gcclib' (see above); `make-l2.com' expects to be able to find it operational in `gnu_cc:[000000]gnu-as.exe'. To use GNU CC on VMS, you need the VMS driver programs `gcc.exe', `gcc.com', and `gcc.cld'. They are distributed with the VMS binaries (`gcc-vms') rather than the GNU CC sources. GAS is also included in `gcc-vms', as is Bison. Once you have successfully built GNU CC with VAX C, you should use the resulting compiler to rebuild itself. Before doing this, be sure to restore the CC, CFLAGS, and LIBS definitions in `make-cccp.com' and `make-cc1.com'. The second generation compiler will be able to take advantage of many optimizations that must be suppressed when building with other compilers.

Under previous versions of GNU CC, the generated code would occasionally give strange results when linked with the sharable `VAXCRTL' library. Now this should work.

Even with this version, however, GNU CC itself should not be linked with the sharable `VAXCRTL'. The version of qsort in `VAXCRTL' has a bug (known to be present in VMS versions V4.6 through V5.5) which causes the compiler to fail.

The executables are generated by `make-cc1.com' and `make-cccp.com' use the object library version of `VAXCRTL' in order to make use of the qsort routine in `gcclib.olb'. If you wish to link the compiler executables with the shareable image version of `VAXCRTL', you should edit the file `tm.h' (created by `vmsconfig.com') to define the macro QSORT_WORKAROUND.

QSORT_WORKAROUND is always defined when GNU CC is compiled with VAX C, to avoid a problem in case `gcclib.olb' is not yet available.

3.7 collect2

GNU CC uses a utility called collect2 on nearly all systems to arrange to call various initialization functions at start time.

The program collect2 works by linking the program once and looking through the linker output file for symbols with particular names indicating they are constructor functions. If it finds any, it creates a new temporary `.c' file containing a table of them, compiles it, and links the program a second time including that file.

The actual calls to the constructors are carried out by a subroutine called __main, which is called (automatically) at the beginning of the body of main (provided main was compiled with GNU CC). Calling __main is necessary, even when compiling C code, to allow linking C and C++ object code together. (If you use `-nostdlib', you get an unresolved reference to __main, since it's defined in the standard GCC library. Include `-lgcc' at the end of your compiler command line to resolve this reference.)

The program collect2 is installed as ld in the directory where the passes of the compiler are installed. When collect2 needs to find the real ld, it tries the following file names:

• `real-ld' in the directories listed in the compiler's search directories.
• `real-ld' in the directories listed in the environment variable PATH.
• The file specified in the REAL_LD_FILE_NAME configuration macro, if specified.
• `ld' in the compiler's search directories, except that collect2 will not execute itself recursively.
• `ld' in PATH.

"The compiler's search directories" means all the directories where gcc searches for passes of the compiler. This includes directories that you specify with `-B'.

Cross-compilers search a little differently:

• `real-ld' in the compiler's search directories.
• `target-real-ld' in PATH.
• The file specified in the REAL_LD_FILE_NAME configuration macro, if specified.
• `ld' in the compiler's search directories.
• `target-ld' in PATH.

collect2 explicitly avoids running ld using the file name under which collect2 itself was invoked. In fact, it remembers up a list of such names--in case one copy of collect2 finds another copy (or version) of collect2 installed as ld in a second place in the search path.

collect2 searches for the utilities nm and strip using the same algorithm as above for ld.

3.8 Standard Header File Directories

GCC_INCLUDE_DIR means the same thing for native and cross. It is where GNU CC stores its private include files, and also where GNU CC stores the fixed include files. A cross compiled GNU CC runs fixincludes on the header files in `$(tooldir)/include'. (If the cross compilation header files need to be fixed, they must be installed before GNU CC is built. If the cross compilation header files are already suitable for ANSI C and GNU CC, nothing special need be done).

GPLUSPLUS_INCLUDE_DIR means the same thing for native and cross. It is where g++ looks first for header files. The C++ library installs only target independent header files in that directory.

LOCAL_INCLUDE_DIR is used only for a native compiler. It is normally `/usr/local/include'. GNU CC searches this directory so that users can install header files in `/usr/local/include'.

CROSS_INCLUDE_DIR is used only for a cross compiler. GNU CC doesn't install anything there.

TOOL_INCLUDE_DIR is used for both native and cross compilers. It is the place for other packages to install header files that GNU CC will use. For a cross-compiler, this is the equivalent of `/usr/include'. When you build a cross-compiler, fixincludes processes any header files in this directory.

4 Extensions to the C Language Family

GNU C provides several language features not found in ANSI standard C. (The `-pedantic' option directs GNU CC to print a warning message if any of these features is used.) To test for the availability of these features in conditional compilation, check for a predefined macro __GNUC__, which is always defined under GNU CC.

These extensions are available in C and Objective C. Most of them are also available in C++. See section 5 Extensions to the C++ Language, for extensions that apply only to C++.

4.1 Statements and Declarations in Expressions

A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression.

Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces. For example:

({ int y = foo (); int z;
   if (y > 0) z = y;
   else z = - y;
   z; })

is a valid (though slightly more complex than necessary) expression for the absolute value of foo ().

The last thing in the compound statement should be an expression followed by a semicolon; the value of this subexpression serves as the value of the entire construct. (If you use some other kind of statement last within the braces, the construct has type void, and thus effectively no value.)

This feature is especially useful in making macro definitions "safe" (so that they evaluate each operand exactly once). For example, the "maximum" function is commonly defined as a macro in standard C as follows:

#define max(a,b) ((a) > (b) ? (a) : (b))

But this definition computes either a or b twice, with bad results if the operand has side effects. In GNU C, if you know the type of the operands (here let's assume int), you can define the macro safely as follows:

#define maxint(a,b) \
  ({int _a = (a), _b = (b); _a > _b ? _a : _b; })

Embedded statements are not allowed in constant expressions, such as the value of an enumeration constant, the width of a bit field, or the initial value of a static variable.

If you don't know the type of the operand, you can still do this, but you must use typeof (see section 4.7 Referring to a Type with typeof) or type naming (see section 4.6 Naming an Expression's Type).

4.2 Locally Declared Labels

Each statement expression is a scope in which local labels can be declared. A local label is simply an identifier; you can jump to it with an ordinary goto statement, but only from within the statement expression it belongs to.

A local label declaration looks like this:

__label__ label;

or

__label__ label1, label2, ...;

Local label declarations must come at the beginning of the statement expression, right after the `({', before any ordinary declarations.

The label declaration defines the label name, but does not define the label itself. You must do this in the usual way, with label:, within the statements of the statement expression.

The local label feature is useful because statement expressions are often used in macros. If the macro contains nested loops, a goto can be useful for breaking out of them. However, an ordinary label whose scope is the whole function cannot be used: if the macro can be expanded several times in one function, the label will be multiply defined in that function. A local label avoids this problem. For example:

#define SEARCH(array, target)                     \
({                                               \
  __label__ found;                                \
  typeof (target) _SEARCH_target = (target);      \
  typeof (*(array)) *_SEARCH_array = (array);     \
  int i, j;                                       \
  int value;                                      \
  for (i = 0; i < max; i++)                       \
    for (j = 0; j < max; j++)                     \
      if (_SEARCH_array[i][j] == _SEARCH_target)  \
        { value = i; goto found; }              \
  value = -1;                                     \
 found:                                           \
  value;                                          \
})

4.3 Labels as Values

You can get the address of a label defined in the current function (or a containing function) with the unary operator `&&'. The value has type void *. This value is a constant and can be used wherever a constant of that type is valid. For example:

void *ptr;
...
ptr = &&foo;

To use these values, you need to be able to jump to one. This is done with the computed goto statement(1), goto *exp;. For example,

goto *ptr;

Any expression of type void * is allowed.

One way of using these constants is in initializing a static array that will serve as a jump table:

static void *array[] = { &&foo, &&bar, &&hack };

Then you can select a label with indexing, like this:

goto *array[i];

Note that this does not check whether the subscript is in bounds--array indexing in C never does that.

Such an array of label values serves a purpose much like that of the switch statement. The switch statement is cleaner, so use that rather than an array unless the problem does not fit a switch statement very well.

Another use of label values is in an interpreter for threaded code. The labels within the interpreter function can be stored in the threaded code for super-fast dispatching.

You can use this mechanism to jump to code in a different function. If you do that, totally unpredictable things will happen. The best way to avoid this is to store the label address only in automatic variables and never pass it as an argument.

4.4 Nested Functions

A nested function is a function defined inside another function. (Nested functions are not supported for GNU C++.) The nested function's name is local to the block where it is defined. For example, here we define a nested function named square, and call it twice:

foo (double a, double b)
{
  double square (double z) { return z * z; }

  return square (a) + square (b);
}

The nested function can access all the variables of the containing function that are visible at the point of its definition. This is called lexical scoping. For example, here we show a nested function which uses an inherited variable named offset:

bar (int *array, int offset, int size)
{
  int access (int *array, int index)
    { return array[index + offset]; }
  int i;
  ...
  for (i = 0; i < size; i++)
    ... access (array, i) ...
}

Nested function definitions are permitted within functions in the places where variable definitions are allowed; that is, in any block, before the first statement in the block.

It is possible to call the nested function from outside the scope of its name by storing its address or passing the address to another function:

hack (int *array, int size)
{
  void store (int index, int value)
    { array[index] = value; }

  intermediate (store, size);
}

Here, the function intermediate receives the address of store as an argument. If intermediate calls store, the arguments given to store are used to store into array. But this technique works only so long as the containing function (hack, in this example) does not exit.

If you try to call the nested function through its address after the containing function has exited, all hell will break loose. If you try to call it after a containing scope level has exited, and if it refers to some of the variables that are no longer in scope, you may be lucky, but it's not wise to take the risk. If, however, the nested function does not refer to anything that has gone out of scope, you should be safe.

GNU CC implements taking the address of a nested function using a technique called trampolines. A paper describing them is available as `http://master.debian.org/~karlheg/Usenix88-lexic.pdf'.

A nested function can jump to a label inherited from a containing function, provided the label was explicitly declared in the containing function (see section 4.2 Locally Declared Labels). Such a jump returns instantly to the containing function, exiting the nested function which did the goto and any intermediate functions as well. Here is an example:

bar (int *array, int offset, int size)
{
  __label__ failure;
  int access (int *array, int index)
    {
      if (index > size)
        goto failure;
      return array[index + offset];
    }
  int i;
  ...
  for (i = 0; i < size; i++)
    ... access (array, i) ...
  ...
  return 0;

 /* Control comes here from access
    if it detects an error.  */
 failure:
  return -1;
}

A nested function always has internal linkage. Declaring one with extern is erroneous. If you need to declare the nested function before its definition, use auto (which is otherwise meaningless for function declarations).

bar (int *array, int offset, int size)
{
  __label__ failure;
  auto int access (int *, int);
  ...
  int access (int *array, int index)
    {
      if (index > size)
        goto failure;
      return array[index + offset];
    }
  ...
}

4.5 Constructing Function Calls

Using the built-in functions described below, you can record the arguments a function received, and call another function with the same arguments, without knowing the number or types of the arguments.

You can also record the return value of that function call, and later return that value, without knowing what data type the function tried to return (as long as your caller expects that data type).

__builtin_apply_args ()

This built-in function returns a pointer of type void * to data describing how to perform a call with the same arguments as were passed to the current function. The function saves the arg pointer register, structure value address, and all registers that might be used to pass arguments to a function into a block of memory allocated on the stack. Then it returns the address of that block.

__builtin_apply (function, arguments, size)

This built-in function invokes function (type void (*)()) with a copy of the parameters described by arguments (type void *) and size (type int). The value of arguments should be the value returned by __builtin_apply_args. The argument size specifies the size of the stack argument data, in bytes. This function returns a pointer of type void * to data describing how to return whatever value was returned by function. The data is saved in a block of memory allocated on the stack. It is not always simple to compute the proper value for size. The value is used by __builtin_apply to compute the amount of data that should be pushed on the stack and copied from the incoming argument area.

__builtin_return (result)

This built-in function returns the value described by result from the containing function. You should specify, for result, a value returned by __builtin_apply.

4.6 Naming an Expression's Type

You can give a name to the type of an expression using a typedef declaration with an initializer. Here is how to define name as a type name for the type of exp:

typedef name = exp;

This is useful in conjunction with the statements-within-expressions feature. Here is how the two together can be used to define a safe "maximum" macro that operates on any arithmetic type:

#define max(a,b) \
  ({typedef _ta = (a), _tb = (b);  \
    _ta _a = (a); _tb _b = (b);     \
    _a > _b ? _a : _b; })

The reason for using names that start with underscores for the local variables is to avoid conflicts with variable names that occur within the expressions that are substituted for a and b. Eventually we hope to design a new form of declaration syntax that allows you to declare variables whose scopes start only after their initializers; this will be a more reliable way to prevent such conflicts.

4.7 Referring to a Type with typeof

Another way to refer to the type of an expression is with typeof. The syntax of using of this keyword looks like sizeof, but the construct acts semantically like a type name defined with typedef.

There are two ways of writing the argument to typeof: with an expression or with a type. Here is an example with an expression:

typeof (x[0](1))

This assumes that x is an array of functions; the type described is that of the values of the functions.

Here is an example with a typename as the argument:

typeof (int *)

Here the type described is that of pointers to int.

If you are writing a header file that must work when included in ANSI C programs, write __typeof__ instead of typeof. See section 4.35 Alternate Keywords.

A typeof-construct can be used anywhere a typedef name could be used. For example, you can use it in a declaration, in a cast, or inside of sizeof or typeof.

• This declares y with the type of what x points to.

  typeof (*x) y;
  

• This declares y as an array of such values.

  typeof (*x) y[4];
  

• This declares y as an array of pointers to characters:

  typeof (typeof (char *)[4]) y;
  

  It is equivalent to the following traditional C declaration:

  char *y[4];
  

  To see the meaning of the declaration using typeof, and why it might be a useful way to write, let's rewrite it with these macros:

  #define pointer(T)  typeof(T *)
  #define array(T, N) typeof(T [N])
  

  Now the declaration can be rewritten this way:

  array (pointer (char), 4) y;
  

  Thus, array (pointer (char), 4) is the type of arrays of 4 pointers to char.

4.8 Generalized Lvalues

Compound expressions, conditional expressions and casts are allowed as lvalues provided their operands are lvalues. This means that you can take their addresses or store values into them.

Standard C++ allows compound expressions and conditional expressions as lvalues, and permits casts to reference type, so use of this extension is deprecated for C++ code.

For example, a compound expression can be assigned, provided the last expression in the sequence is an lvalue. These two expressions are equivalent:

(a, b) += 5
a, (b += 5)

Similarly, the address of the compound expression can be taken. These two expressions are equivalent:

&(a, b)
a, &b

A conditional expression is a valid lvalue if its type is not void and the true and false branches are both valid lvalues. For example, these two expressions are equivalent:

(a ? b : c) = 5
(a ? b = 5 : (c = 5))

A cast is a valid lvalue if its operand is an lvalue. A simple assignment whose left-hand side is a cast works by converting the right-hand side first to the specified type, then to the type of the inner left-hand side expression. After this is stored, the value is converted back to the specified type to become the value of the assignment. Thus, if a has type char *, the following two expressions are equivalent:

(int)a = 5
(int)(a = (char *)(int)5)

An assignment-with-arithmetic operation such as `+=' applied to a cast performs the arithmetic using the type resulting from the cast, and then continues as in the previous case. Therefore, these two expressions are equivalent:

(int)a += 5
(int)(a = (char *)(int) ((int)a + 5))

You cannot take the address of an lvalue cast, because the use of its address would not work out coherently. Suppose that &(int)f were permitted, where f has type float. Then the following statement would try to store an integer bit-pattern where a floating point number belongs:

*&(int)f = 1;

This is quite different from what (int)f = 1 would do--that would convert 1 to floating point and store it. Rather than cause this inconsistency, we think it is better to prohibit use of `&' on a cast.

If you really do want an int * pointer with the address of f, you can simply write (int *)&f.

4.9 Conditionals with Omitted Operands

The middle operand in a conditional expression may be omitted. Then if the first operand is nonzero, its value is the value of the conditional expression.

Therefore, the expression

x ? : y

has the value of x if that is nonzero; otherwise, the value of y.

This example is perfectly equivalent to

x ? x : y

In this simple case, the ability to omit the middle operand is not especially useful. When it becomes useful is when the first operand does, or may (if it is a macro argument), contain a side effect. Then repeating the operand in the middle would perform the side effect twice. Omitting the middle operand uses the value already computed without the undesirable effects of recomputing it.

4.10 Double-Word Integers

GNU C supports data types for integers that are twice as long as int. Simply write long long int for a signed integer, or unsigned long long int for an unsigned integer. To make an integer constant of type long long int, add the suffix LL to the integer. To make an integer constant of type unsigned long long int, add the suffix ULL to the integer.

You can use these types in arithmetic like any other integer types. Addition, subtraction, and bitwise boolean operations on these types are open-coded on all types of machines. Multiplication is open-coded if the machine supports fullword-to-doubleword a widening multiply instruction. Division and shifts are open-coded only on machines that provide special support. The operations that are not open-coded use special library routines that come with GNU CC.

There may be pitfalls when you use long long types for function arguments, unless you declare function prototypes. If a function expects type int for its argument, and you pass a value of type long long int, confusion will result because the caller and the subroutine will disagree about the number of bytes for the argument. Likewise, if the function expects long long int and you pass int. The best way to avoid such problems is to use prototypes.

4.11 Complex Numbers

GNU C supports complex data types. You can declare both complex integer types and complex floating types, using the keyword __complex__.

For example, `__complex__ double x;' declares x as a variable whose real part and imaginary part are both of type double. `__complex__ short int y;' declares y to have real and imaginary parts of type short int; this is not likely to be useful, but it shows that the set of complex types is complete.

To write a constant with a complex data type, use the suffix `i' or `j' (either one; they are equivalent). For example, 2.5fi has type __complex__ float and 3i has type __complex__ int. Such a constant always has a pure imaginary value, but you can form any complex value you like by adding one to a real constant.

To extract the real part of a complex-valued expression exp, write __real__ exp. Likewise, use __imag__ to extract the imaginary part.

The operator `~' performs complex conjugation when used on a value with a complex type.

GNU CC can allocate complex automatic variables in a noncontiguous fashion; it's even possible for the real part to be in a register while the imaginary part is on the stack (or vice-versa). None of the supported debugging info formats has a way to represent noncontiguous allocation like this, so GNU CC describes a noncontiguous complex variable as if it were two separate variables of noncomplex type. If the variable's actual name is foo, the two fictitious variables are named foo$real and foo$imag. You can examine and set these two fictitious variables with your debugger.

A future version of GDB will know how to recognize such pairs and treat them as a single variable with a complex type.

4.12 Hex Floats

GNU CC recognizes floating-point numbers written not only in the usual decimal notation, such as 1.55e1, but also numbers such as 0x1.fp3 written in hexadecimal format. In that format the 0x hex introducer and the p or P exponent field are mandatory. The exponent is a decimal number that indicates the power of 2 by which the significand part will be multiplied. Thus 0x1.f is 1 15/16, p3 multiplies it by 8, and the value of 0x1.fp3 is the same as 1.55e1.

Unlike for floating-point numbers in the decimal notation the exponent is always required in the hexadecimal notation. Otherwise the compiler would not be able to resolve the ambiguity of, e.g., 0x1.f. This could mean 1.0f or 1.9375 since f is also the extension for floating-point constants of type float.

4.13 Arrays of Length Zero

Zero-length arrays are allowed in GNU C. They are very useful as the last element of a structure which is really a header for a variable-length object:

struct line {
  int length;
  char contents[0];
};

{
  struct line *thisline = (struct line *)
    malloc (sizeof (struct line) + this_length);
  thisline->length = this_length;
}

In standard C, you would have to give contents a length of 1, which means either you waste space or complicate the argument to malloc.

4.14 Arrays of Variable Length

Variable-length automatic arrays are allowed in GNU C. These arrays are declared like any other automatic arrays, but with a length that is not a constant expression. The storage is allocated at the point of declaration and deallocated when the brace-level is exited. For example:

FILE *
concat_fopen (char *s1, char *s2, char *mode)
{
  char str[strlen (s1) + strlen (s2) + 1];
  strcpy (str, s1);
  strcat (str, s2);
  return fopen (str, mode);
}

Jumping or breaking out of the scope of the array name deallocates the storage. Jumping into the scope is not allowed; you get an error message for it.

You can use the function alloca to get an effect much like variable-length arrays. The function alloca is available in many other C implementations (but not in all). On the other hand, variable-length arrays are more elegant.

There are other differences between these two methods. Space allocated with alloca exists until the containing function returns. The space for a variable-length array is deallocated as soon as the array name's scope ends. (If you use both variable-length arrays and alloca in the same function, deallocation of a variable-length array will also deallocate anything more recently allocated with alloca.)

You can also use variable-length arrays as arguments to functions:

struct entry
tester (int len, char data[len][len])
{
  ...
}

The length of an array is computed once when the storage is allocated and is remembered for the scope of the array in case you access it with sizeof.

If you want to pass the array first and the length afterward, you can use a forward declaration in the parameter list--another GNU extension.

struct entry
tester (int len; char data[len][len], int len)
{
  ...
}

The `int len' before the semicolon is a parameter forward declaration, and it serves the purpose of making the name len known when the declaration of data is parsed.

You can write any number of such parameter forward declarations in the parameter list. They can be separated by commas or semicolons, but the last one must end with a semicolon, which is followed by the "real" parameter declarations. Each forward declaration must match a "real" declaration in parameter name and data type.

4.15 Macros with Variable Numbers of Arguments

In GNU C, a macro can accept a variable number of arguments, much as a function can. The syntax for defining the macro looks much like that used for a function. Here is an example:

#define eprintf(format, args...)  \
 fprintf (stderr, format , ## args)

Here args is a rest argument: it takes in zero or more arguments, as many as the call contains. All of them plus the commas between them form the value of args, which is substituted into the macro body where args is used. Thus, we have this expansion:

eprintf ("%s:%d: ", input_file_name, line_number)
==>
fprintf (stderr, "%s:%d: " , input_file_name, line_number)

Note that the comma after the string constant comes from the definition of eprintf, whereas the last comma comes from the value of args.

The reason for using `##' is to handle the case when args matches no arguments at all. In this case, args has an empty value. In this case, the second comma in the definition becomes an embarrassment: if it got through to the expansion of the macro, we would get something like this:

fprintf (stderr, "success!\n" , )

which is invalid C syntax. `##' gets rid of the comma, so we get the following instead:

fprintf (stderr, "success!\n")

This is a special feature of the GNU C preprocessor: `##' before a rest argument that is empty discards the preceding sequence of non-whitespace characters from the macro definition. (If another macro argument precedes, none of it is discarded.)

It might be better to discard the last preprocessor token instead of the last preceding sequence of non-whitespace characters; in fact, we may someday change this feature to do so. We advise you to write the macro definition so that the preceding sequence of non-whitespace characters is just a single token, so that the meaning will not change if we change the definition of this feature.

4.16 Non-Lvalue Arrays May Have Subscripts

Subscripting is allowed on arrays that are not lvalues, even though the unary `&' operator is not. For example, this is valid in GNU C though not valid in other C dialects:

struct foo {int a[4];};

struct foo f();

bar (int index)
{
  return f().a[index];
}

4.17 Arithmetic on void- and Function-Pointers

In GNU C, addition and subtraction operations are supported on pointers to void and on pointers to functions. This is done by treating the size of a void or of a function as 1.

A consequence of this is that sizeof is also allowed on void and on function types, and returns 1.

The option `-Wpointer-arith' requests a warning if these extensions are used.

4.18 Non-Constant Initializers

As in standard C++, the elements of an aggregate initializer for an automatic variable are not required to be constant expressions in GNU C. Here is an example of an initializer with run-time varying elements:

foo (float f, float g)
{
  float beat_freqs[2] = { f-g, f+g };
  ...
}

4.19 Constructor Expressions

GNU C supports constructor expressions. A constructor looks like a cast containing an initializer. Its value is an object of the type specified in the cast, containing the elements specified in the initializer.

Usually, the specified type is a structure. Assume that struct foo and structure are declared as shown:

struct foo {int a; char b[2];} structure;

Here is an example of constructing a struct foo with a constructor:

structure = ((struct foo) {x + y, 'a', 0});

This is equivalent to writing the following:

{
  struct foo temp = {x + y, 'a', 0};
  structure = temp;
}

You can also construct an array. If all the elements of the constructor are (made up of) simple constant expressions, suitable for use in initializers, then the constructor is an lvalue and can be coerced to a pointer to its first element, as shown here:

char **foo = (char *[]) { "x", "y", "z" };

Array constructors whose elements are not simple constants are not very useful, because the constructor is not an lvalue. There are only two valid ways to use it: to subscript it, or initialize an array variable with it. The former is probably slower than a switch statement, while the latter does the same thing an ordinary C initializer would do. Here is an example of subscripting an array constructor:

output = ((int[]) { 2, x, 28 }) [input];

Constructor expressions for scalar types and union types are is also allowed, but then the constructor expression is equivalent to a cast.

4.20 Labeled Elements in Initializers

Standard C requires the elements of an initializer to appear in a fixed order, the same as the order of the elements in the array or structure being initialized.

In GNU C you can give the elements in any order, specifying the array indices or structure field names they apply to. This extension is not implemented in GNU C++.

To specify an array index, write `[index]' or `[index] =' before the element value. For example,

int a[6] = { [4] 29, [2] = 15 };

is equivalent to

int a[6] = { 0, 0, 15, 0, 29, 0 };

The index values must be constant expressions, even if the array being initialized is automatic.

To initialize a range of elements to the same value, write `[first ... last] = value'. For example,

int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };

Note that the length of the array is the highest value specified plus one.

In a structure initializer, specify the name of a field to initialize with `fieldname:' before the element value. For example, given the following structure,

struct point { int x, y; };

the following initialization

struct point p = { y: yvalue, x: xvalue };

is equivalent to

struct point p = { xvalue, yvalue };

Another syntax which has the same meaning is `.fieldname ='., as shown here:

struct point p = { .y = yvalue, .x = xvalue };

You can also use an element label (with either the colon syntax or the period-equal syntax) when initializing a union, to specify which element of the union should be used. For example,

union foo { int i; double d; };

union foo f = { d: 4 };

will convert 4 to a double to store it in the union using the second element. By contrast, casting 4 to type union foo would store it into the union as the integer i, since it is an integer. (See section 4.22 Cast to a Union Type.)

You can combine this technique of naming elements with ordinary C initialization of successive elements. Each initializer element that does not have a label applies to the next consecutive element of the array or structure. For example,

int a[6] = { [1] = v1, v2, [4] = v4 };

is equivalent to

int a[6] = { 0, v1, v2, 0, v4, 0 };

Labeling the elements of an array initializer is especially useful when the indices are characters or belong to an enum type. For example:

int whitespace[256]
  = { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
      ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };

4.21 Case Ranges

You can specify a range of consecutive values in a single case label, like this:

case low ... high:

This has the same effect as the proper number of individual case labels, one for each integer value from low to high, inclusive.

This feature is especially useful for ranges of ASCII character codes:

case 'A' ... 'Z':

Be careful: Write spaces around the ..., for otherwise it may be parsed wrong when you use it with integer values. For example, write this:

case 1 ... 5:

rather than this:

case 1...5:

4.22 Cast to a Union Type

A cast to union type is similar to other casts, except that the type specified is a union type. You can specify the type either with union tag or with a typedef name. A cast to union is actually a constructor though, not a cast, and hence does not yield an lvalue like normal casts. (See section 4.19 Constructor Expressions.)

The types that may be cast to the union type are those of the members of the union. Thus, given the following union and variables:

union foo { int i; double d; };
int x;
double y;

both x and y can be cast to type union foo.

Using the cast as the right-hand side of an assignment to a variable of union type is equivalent to storing in a member of the union:

union foo u;
...
u = (union foo) x  ==  u.i = x
u = (union foo) y  ==  u.d = y

You can also use the union cast as a function argument:

void hack (union foo);
...
hack ((union foo) x);

4.23 Declaring Attributes of Functions

In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls and check your code more carefully.

The keyword __attribute__ allows you to specify special attributes when making a declaration. This keyword is followed by an attribute specification inside double parentheses. Nine attributes, noreturn, const, format, no_instrument_function, section, constructor, destructor, unused and weak are currently defined for functions. Other attributes, including section are supported for variables declarations (see section 4.29 Specifying Attributes of Variables) and for types (see section 4.30 Specifying Attributes of Types).

You may also specify attributes with `__' preceding and following each keyword. This allows you to use them in header files without being concerned about a possible macro of the same name. For example, you may use __noreturn__ instead of noreturn.

noreturn

A few standard library functions, such as abort and exit, cannot return. GNU CC knows this automatically. Some programs define their own functions that never return. You can declare them noreturn to tell the compiler this fact. For example,

void fatal () __attribute__ ((noreturn));

void
fatal (...)
{
  ... /* Print error message. */ ...
  exit (1);
}

The noreturn keyword tells the compiler to assume that fatal cannot return. It can then optimize without regard to what would happen if fatal ever did return. This makes slightly better code. More importantly, it helps avoid spurious warnings of uninitialized variables. Do not assume that registers saved by the calling function are restored before calling the noreturn function. It does not make sense for a noreturn function to have a return type other than void. The attribute noreturn is not implemented in GNU C versions earlier than 2.5. An alternative way to declare that a function does not return, which works in the current version and in some older versions, is as follows:

typedef void voidfn ();

volatile voidfn fatal;

const

Many functions do not examine any values except their arguments, and have no effects except the return value. Such a function can be subject to common subexpression elimination and loop optimization just as an arithmetic operator would be. These functions should be declared with the attribute const. For example,

int square (int) __attribute__ ((const));

says that the hypothetical function square is safe to call fewer times than the program says. The attribute const is not implemented in GNU C versions earlier than 2.5. An alternative way to declare that a function has no side effects, which works in the current version and in some older versions, is as follows:

typedef int intfn ();

extern const intfn square;

This approach does not work in GNU C++ from 2.6.0 on, since the language specifies that the `const' must be attached to the return value. Note that a function that has pointer arguments and examines the data pointed to must not be declared const. Likewise, a function that calls a non-const function usually must not be const. It does not make sense for a const function to return void.

format (archetype, string-index, first-to-check)

The format attribute specifies that a function takes printf, scanf, or strftime style arguments which should be type-checked against a format string. For example, the declaration:

extern int
my_printf (void *my_object, const char *my_format, ...)
      __attribute__ ((format (printf, 2, 3)));

causes the compiler to check the arguments in calls to my_printf for consistency with the printf style format string argument my_format. The parameter archetype determines how the format string is interpreted, and should be either printf, scanf, or strftime. The parameter string-index specifies which argument is the format string argument (starting from 1), while first-to-check is the number of the first argument to check against the format string. For functions where the arguments are not available to be checked (such as vprintf), specify the third parameter as zero. In this case the compiler only checks the format string for consistency. In the example above, the format string (my_format) is the second argument of the function my_print, and the arguments to check start with the third argument, so the correct parameters for the format attribute are 2 and 3. The format attribute allows you to identify your own functions which take format strings as arguments, so that GNU CC can check the calls to these functions for errors. The compiler always checks formats for the ANSI library functions printf, fprintf, sprintf, scanf, fscanf, sscanf, strftime, vprintf, vfprintf and vsprintf whenever such warnings are requested (using `-Wformat'), so there is no need to modify the header file `stdio.h'.

format_arg (string-index)

The format_arg attribute specifies that a function takes printf or scanf style arguments, modifies it (for example, to translate it into another language), and passes it to a printf or scanf style function. For example, the declaration:

extern char *
my_dgettext (char *my_domain, const char *my_format)
      __attribute__ ((format_arg (2)));

causes the compiler to check the arguments in calls to my_dgettext whose result is passed to a printf, scanf, or strftime type function for consistency with the printf style format string argument my_format. The parameter string-index specifies which argument is the format string argument (starting from 1). The format-arg attribute allows you to identify your own functions which modify format strings, so that GNU CC can check the calls to printf, scanf, or strftime function whose operands are a call to one of your own function. The compiler always treats gettext, dgettext, and dcgettext in this manner.

no_instrument_function

If `-finstrument-functions' is given, profiling function calls will be generated at entry and exit of most user-compiled functions. Functions with this attribute will not be so instrumented.

section ("section-name")

Normally, the compiler places the code it generates in the text section. Sometimes, however, you need additional sections, or you need certain particular functions to appear in special sections. The section attribute specifies that a function lives in a particular section. For example, the declaration:

extern void foobar (void) __attribute__ ((section ("bar")));

puts the function foobar in the bar section. Some file formats do not support arbitrary sections so the section attribute is not available on all platforms. If you need to map the entire contents of a module to a particular section, consider using the facilities of the linker instead.

constructor

destructor

The constructor attribute causes the function to be called automatically before execution enters main (). Similarly, the destructor attribute causes the function to be called automatically after main () has completed or exit () has been called. Functions with these attributes are useful for initializing data that will be used implicitly during the execution of the program. These attributes are not currently implemented for Objective C.

unused

This attribute, attached to a function, means that the function is meant to be possibly unused. GNU CC will not produce a warning for this function. GNU C++ does not currently support this attribute as definitions without parameters are valid in C++.

weak

The weak attribute causes the declaration to be emitted as a weak symbol rather than a global. This is primarily useful in defining library functions which can be overridden in user code, though it can also be used with non-function declarations. Weak symbols are supported for ELF targets, and also for a.out targets when using the GNU assembler and linker.

alias ("target")

The alias attribute causes the declaration to be emitted as an alias for another symbol, which must be specified. For instance,

void __f () { /* do something */; }
void f () __attribute__ ((weak, alias ("__f")));

declares `f' to be a weak alias for `__f'. In C++, the mangled name for the target must be used. Not all target machines support this attribute.

no_check_memory_usage

If `-fcheck-memory-usage' is given, calls to support routines will be generated before most memory accesses, to permit support code to record usage and detect uses of uninitialized or unallocated storage. Since the compiler cannot handle them properly, asm statements are not allowed. Declaring a function with this attribute disables the memory checking code for that function, permitting the use of asm statements without requiring separate compilation with different options, and allowing you to write support routines of your own if you wish, without getting infinite recursion if they get compiled with this option.

regparm (number)

On the Intel 386, the regparm attribute causes the compiler to pass up to number integer arguments in registers EAX, EDX, and ECX instead of on the stack. Functions that take a variable number of arguments will continue to be passed all of their arguments on the stack.

stdcall

On the Intel 386, the stdcall attribute causes the compiler to assume that the called function will pop off the stack space used to pass arguments, unless it takes a variable number of arguments. The PowerPC compiler for Windows NT currently ignores the stdcall attribute.

cdecl

On the Intel 386, the cdecl attribute causes the compiler to assume that the calling function will pop off the stack space used to pass arguments. This is useful to override the effects of the `-mrtd' switch. The PowerPC compiler for Windows NT currently ignores the cdecl attribute.

longcall

On the RS/6000 and PowerPC, the longcall attribute causes the compiler to always call the function via a pointer, so that functions which reside further than 64 megabytes (67,108,864 bytes) from the current location can be called.

dllimport

On the PowerPC running Windows NT, the dllimport attribute causes the compiler to call the function via a global pointer to the function pointer that is set up by the Windows NT dll library. The pointer name is formed by combining __imp_ and the function name.

dllexport

On the PowerPC running Windows NT, the dllexport attribute causes the compiler to provide a global pointer to the function pointer, so that it can be called with the dllimport attribute. The pointer name is formed by combining __imp_ and the function name.

exception (except-func [, except-arg])

On the PowerPC running Windows NT, the exception attribute causes the compiler to modify the structured exception table entry it emits for the declared function. The string or identifier except-func is placed in the third entry of the structured exception table. It represents a function, which is called by the exception handling mechanism if an exception occurs. If it was specified, the string or identifier except-arg is placed in the fourth entry of the structured exception table.

function_vector

Use this option on the H8/300 and H8/300H to indicate that the specified function should be called through the function vector. Calling a function through the function vector will reduce code size, however; the function vector has a limited size (maximum 128 entries on the H8/300 and 64 entries on the H8/300H) and shares space with the interrupt vector. You must use GAS and GLD from GNU binutils version 2.7 or later for this option to work correctly.

interrupt_handler

Use this option on the H8/300 and H8/300H to indicate that the specified function is an interrupt handler. The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present.

eightbit_data

Use this option on the H8/300 and H8/300H to indicate that the specified variable should be placed into the eight bit data section. The compiler will generate more efficient code for certain operations on data in the eight bit data area. Note the eight bit data area is limited to 256 bytes of data. You must use GAS and GLD from GNU binutils version 2.7 or later for this option to work correctly.

tiny_data

Use this option on the H8/300H to indicate that the specified variable should be placed into the tiny data section. The compiler will generate more efficient code for loads and stores on data in the tiny data section. Note the tiny data area is limited to slightly under 32kbytes of data.

interrupt

Use this option on the M32R/D to indicate that the specified function is an interrupt handler. The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present.

model (model-name)

Use this attribute on the M32R/D to set the addressability of an object, and the code generated for a function. The identifier model-name is one of small, medium, or large, representing each of the code models. Small model objects live in the lower 16MB of memory (so that their addresses can be loaded with the ld24 instruction), and are callable with the bl instruction. Medium model objects may live anywhere in the 32 bit address space (the compiler will generate seth/add3 instructions to load their addresses), and are callable with the bl instruction. Large model objects may live anywhere in the 32 bit address space (the compiler will generate seth/add3 instructions to load their addresses), and may not be reachable with the bl instruction (the compiler will generate the much slower seth/add3/jl instruction sequence).