Check the `README' file; it often has useful information that does not appear anywhere else in the directory.
Each linker target has an emulation. The emulation includes the default linker script, and certain emulations also modify certain types of linker behaviour.
Emulations are created during the build process by the shell script `genscripts.sh'.
The `genscripts.sh' script starts by reading a file in the `emulparams' directory. This is a shell script which sets various shell variables used by `genscripts.sh' and the other shell scripts it invokes.
The `genscripts.sh' script will invoke a shell script in the `scripttempl' directory in order to create default linker scripts written in the linker command language. The `scripttempl' script will be invoked 5 (or, in some cases, 6) times, with different assignments to shell variables, to create different default scripts. The choice of script is made based on the command line options.
After creating the scripts, `genscripts.sh' will invoke yet another shell script, this time in the `emultempl' directory. That shell script will create the emulation source file, which contains C code. This C code permits the linker emulation to override various linker behaviours. Most targets use the generic emulation code, which is in `emultempl/generic.em'.
To summarize, `genscripts.sh' reads three shell scripts: an emulation parameters script in the `emulparams' directory, a linker script generation script in the `scripttempl' directory, and an emulation source file generation script in the `emultempl' directory.
For example, the Sun 4 linker sets up variables in `emulparams/sun4.sh', creates linker scripts using `scripttempl/aout.sc', and creates the emulation code using `emultempl/sunos.em'.
Note that the linker can support several emulations simultaneously,
depending upon how it is configured. An emulation can be selected with
the -m option. The -V option will list all supported
emulations.
Each target selects a particular file in the `emulparams' directory
by setting the shell variable targ_emul in `configure.tgt'.
This shell variable is used by the `configure' script to control
building an emulation source file.
Certain conventions are enforced. Suppose the targ_emul variable
is set to emul in `configure.tgt'. The name of the emulation
shell script will be `emulparams/emul.sh'. The
`Makefile' must have a target named `eemul.c'; this
target must depend upon `emulparams/emul.sh', as well as the
appropriate scripts in the `scripttempl' and `emultempl'
directories. The `Makefile' target must invoke GENSCRIPTS
with two arguments: emul, and the value of the make variable
tdir_emul. The value of the latter variable will be set by
the `configure' script, and is used to set the default target
directory to search.
By convention, the `emulparams/emul.sh' shell script should only set shell variables. It may set shell variables which are to be interpreted by the `scripttempl' and the `emultempl' scripts. Certain shell variables are interpreted directly by the `genscripts.sh' script.
Here is a list of shell variables interpreted by `genscripts.sh', as well as some conventional shell variables interpreted by the `scripttempl' and `emultempl' scripts.
SCRIPT_NAME
SCRIPT_NAME is set to script, `genscripts.sh' will use
the script `scriptteml/script.sc'.
TEMPLATE_NAME
TEMPLATE_NAME is set to template, `genscripts.sh' will
use the script `emultempl/template.em'. If this variable is
not set, the default value is `generic'.
GENERATE_SHLIB_SCRIPT
OUTPUT_FORMAT
OUTPUT_FORMAT expression in the linker script.
ARCH
OUTPUT_ARCH expression in the linker script.
ENTRY
ENTRY expression in the linker script.
TEXT_START_ADDR
NONPAGED_TEXT_START_ADDR
TEXT_START_ADDR to its value before running the
`scripttempl' script for the -n and -N options
(see section 2.2 `scripttempl' scripts).
SEGMENT_SIZE
DATA_ALIGNMENT when running the `scripttempl' script.
TARGET_PAGE_SIZE
SEGMENT_SIZE is not defined, the `genscripts.sh' script
uses this to define it.
Each linker target uses a `scripttempl' script to generate the
default linker scripts. The name of the `scripttempl' script is
set by the SCRIPT_NAME variable in the `emulparams' script.
If SCRIPT_NAME is set to script, genscripts.sh will
invoke `scripttempl/script.sc'.
The `genscripts.sh' script will invoke the `scripttempl'
script 5 or 6 times. Each time it will set the shell variable
LD_FLAG to a different value. When the linker is run, the
options used will direct it to select a particular script. (Script
selection is controlled by the get_script emulation entry point;
this describes the conventional behaviour).
The `scripttempl' script should just write a linker script, written in the linker command language, to standard output. If the emulation name--the name of the `emulparams' file without the `.sc' extension--is emul, then the output will be directed to `ldscripts/emul.extension' in the build directory, where extension changes each time the `scripttempl' script is invoked.
Here is the list of values assigned to LD_FLAG.
(empty)
n
-n option. The output has an extension of `.xn'.
N
-N option. The output has an extension of `.xbn'.
r
-r option. The output has an extension of `.xr'.
u
-Ur option. The output has an extension of `.xu'.
shared
LD_FLAG set to
this value if GENERATE_SHLIB_SCRIPT is defined in the
`emulparams' file. The `emultempl' script must arrange to use
this script at the appropriate time, normally when the linker is invoked
with the -shared option. The output has an extension of
`.xs'.
Besides the shell variables set by the `emulparams' script, and the
LD_FLAG variable, the `genscripts.sh' script will set
certain variables for each run of the `scripttempl' script.
RELOCATING
-r and -Ur).
CONSTRUCTING
-r).
DATA_ALIGNMENT
ALIGN expression when the output should be
page aligned, or to `.' when generating the -N script.
CREATE_SHLIB
-shared
script.
The conventional way to write a `scripttempl' script is to first
set a few shell variables, and then write out a linker script using
cat with a here document. The linker script will use variable
substitutions, based on the above variables and those set in the
`emulparams' script, to control its behaviour.
When there are parts of the `scripttempl' script which should only
be run when doing a final relocation, they should be enclosed within a
variable substitution based on RELOCATING. For example, on many
targets special symbols such as _end should be defined when doing
a final link. Naturally, those symbols should not be defined when doing
a relocateable link using -r. The `scripttempl' script
could use a construct like this to define those symbols:
${RELOCATING+ _end = .;}
This will do the symbol assignment only if the RELOCATING
variable is defined.
The basic job of the linker script is to put the sections in the correct order, and at the correct memory addresses. For some targets, the linker script may have to do some other operations.
For example, on most MIPS platforms, the linker is responsible for
defining the special symbol _gp, used to initialize the
$gp register. It must be set to the start of the small data
section plus 0x8000. Naturally, it should only be defined when
doing a final relocation. This will typically be done like this:
${RELOCATING+ _gp = ALIGN(16) + 0x8000;}
This line would appear just before the sections which compose the small data section (`.sdata', `.sbss'). All those sections would be contiguous in memory.
Many COFF systems build constructor tables in the linker script. The
compiler will arrange to output the address of each global constructor
in a `.ctor' section, and the address of each global destructor in
a `.dtor' section (this is done by defining
ASM_OUTPUT_CONSTRUCTOR and ASM_OUTPUT_DESTRUCTOR in the
gcc configuration files). The gcc runtime support
routines expect the constructor table to be named __CTOR_LIST__.
They expect it to be a list of words, with the first word being the
count of the number of entries. There should be a trailing zero word.
(Actually, the count may be -1 if the trailing word is present, and the
trailing word may be omitted if the count is correct, but, as the
gcc behaviour has changed slightly over the years, it is safest
to provide both). Here is a typical way that might be handled in a
`scripttempl' file.
${CONSTRUCTING+ __CTOR_LIST__ = .;}
${CONSTRUCTING+ LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)}
${CONSTRUCTING+ *(.ctors)}
${CONSTRUCTING+ LONG(0)}
${CONSTRUCTING+ __CTOR_END__ = .;}
${CONSTRUCTING+ __DTOR_LIST__ = .;}
${CONSTRUCTING+ LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)}
${CONSTRUCTING+ *(.dtors)}
${CONSTRUCTING+ LONG(0)}
${CONSTRUCTING+ __DTOR_END__ = .;}
The use of CONSTRUCTING ensures that these linker script commands
will only appear when the linker is supposed to be building the
constructor and destructor tables. This example is written for a target
which uses 4 byte pointers.
Embedded systems often need to set a stack address. This is normally
best done by using the PROVIDE construct with a default stack
address. This permits the user to easily override the stack address
using the --defsym option. Here is an example:
${RELOCATING+ PROVIDE (__stack = 0x80000000);}
The value of the symbol __stack would then be used in the startup
code to initialize the stack pointer.
Each linker target uses an `emultempl' script to generate the
emulation code. The name of the `emultempl' script is set by the
TEMPLATE_NAME variable in the `emulparams' script. If the
TEMPLATE_NAME variable is not set, the default is
`generic'. If the value of TEMPLATE_NAME is template,
`genscripts.sh' will use `emultempl/template.em'.
Most targets use the generic `emultempl' script, `emultempl/generic.em'. A different `emultempl' script is only needed if the linker must support unusual actions, such as linking against shared libraries.
The `emultempl' script is normally written as a simple invocation
of cat with a here document. The document will use a few
variable substitutions. Typically each function names uses a
substitution involving EMULATION_NAME, for ease of debugging when
the linker supports multiple emulations.
Every function and variable in the emitted file should be static. The
only globally visible object must be named
ld_EMULATION_NAME_emulation, where EMULATION_NAME is
the name of the emulation set in `configure.tgt' (this is also the
name of the `emulparams' file without the `.sh' extension).
The `genscripts.sh' script will set the shell variable
EMULATION_NAME before invoking the `emultempl' script.
The ld_EMULATION_NAME_emulation variable must be a
struct ld_emulation_xfer_struct, as defined in `ldemul.h'.
It defines a set of function pointers which are invoked by the linker,
as well as strings for the emulation name (normally set from the shell
variable EMULATION_NAME and the default BFD target name (normally
set from the shell variable OUTPUT_FORMAT which is normally set
by the `emulparams' file).
The `genscripts.sh' script will set the shell variable
COMPILE_IN when it invokes the `emultempl' script for the
default emulation. In this case, the `emultempl' script should
include the linker scripts directly, and return them from the
get_scripts entry point. When the emulation is not the default,
the get_scripts entry point should just return a file name. See
`emultempl/generic.em' for an example of how this is done.
At some point, the linker emulation entry points should be documented.
This document was generated on 2 June 2000 using the texi2html translator version 1.51a.