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     * Ld-Internals: (ldint).	The GNU linker internals.
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This file documents the internals of the GNU linker ld.

Copyright (C) 1992, 93, 94, 95, 1996 Free Software Foundation, Inc. Contributed by Cygnus Support.

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy or distribute modified versions of this manual under the terms of the GPL (for which purpose this text may be regarded as a program in the language TeX).

This file documents the internals of the GNU linker ld.  It is a collection of miscellaneous information with little form at this point. Mostly, it is a repository into which you can put information about GNU ld as you discover it (or as you design changes to ld).

The README File
***************

Check the README file; it often has useful information that does not appear anywhere else in the directory.

How linker emulations are generated
***********************************

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.

emulparams scripts
==================

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
     This is the name of the scripttempl script to use.  If SCRIPT_NAME is set to SCRIPT, genscripts.sh will use the script scriptteml/SCRIPT.sc.

TEMPLATE_NAME
     This is the name of the emultemlp script to use.  If 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
     If this is set to a nonempty string, genscripts.sh will invoke the scripttempl script an extra time to create a shared library script.  linker scripts.

OUTPUT_FORMAT
     This is normally set to indicate the BFD output format use (e.g., "a.out-sunos-big".  The scripttempl script will normally use it in an OUTPUT_FORMAT expression in the linker script.

ARCH
     This is normally set to indicate the architecture to use (e.g., sparc).  The scripttempl script will normally use it in an OUTPUT_ARCH expression in the linker script.

ENTRY
     Some scripttempl scripts use this to set the entry address, in an ENTRY expression in the linker script.

TEXT_START_ADDR
     Some scripttempl scripts use this to set the start address of the .text section.

NONPAGED_TEXT_START_ADDR
     If this is defined, the genscripts.sh script sets TEXT_START_ADDR to its value before running the scripttempl script for the -n and -N options (see linker scripts).

SEGMENT_SIZE
     The genscripts.sh script uses this to set the default value of DATA_ALIGNMENT when running the scripttempl script.

TARGET_PAGE_SIZE
     If SEGMENT_SIZE is not defined, the genscripts.sh script uses this to define it.

scripttempl scripts
===================

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)
     The script generated is used by default (when none of the following cases apply).  The output has an extension of .x.

n
     The script generated is used when the linker is invoked with the -n option.  The output has an extension of .xn.

N
     The script generated is used when the linker is invoked with the -N option.  The output has an extension of .xbn.

r
     The script generated is used when the linker is invoked with the -r option.  The output has an extension of .xr.

u
     The script generated is used when the linker is invoked with the -Ur option.  The output has an extension of .xu.

shared
     The scripttempl script is only invoked with 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
     This will be set to a non-empty string when the linker is doing a final relocation (e.g., all scripts other than -r and -Ur).

CONSTRUCTING
     This will be set to a non-empty string when the linker is building global constructor and destructor tables (e.g., all scripts other than -r).

DATA_ALIGNMENT
     This will be set to an ALIGN expression when the output should be page aligned, or to . when generating the -N script.

CREATE_SHLIB
     This will be set to a non-empty string when generating a -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.

emultempl scripts
=================

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.