This is ldint.info, produced by makeinfo version 7.1 from ldint.texi.

This file documents the internals of the GNU linker ld.

   Copyright © 1992-2024 Free Software Foundation, Inc.  Contributed by
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INFO-DIR-SECTION Software development
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* Ld-Internals: (ldint).	The GNU linker internals.
END-INFO-DIR-ENTRY


File: ldint.info,  Node: Top,  Next: README,  Up: (dir)

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’).

   This document is distributed under the terms of the GNU Free
Documentation License.  A copy of the license is included in the section
entitled "GNU Free Documentation License".

* Menu:

* README::			The README File
* Emulations::			How linker emulations are generated
* Emulation Walkthrough::	A Walkthrough of a Typical Emulation
* Architecture Specific::	Some Architecture Specific Notes
* GNU Free Documentation License::  GNU Free Documentation License


File: ldint.info,  Node: README,  Next: Emulations,  Prev: Top,  Up: Top

1 The ‘README’ File
*******************

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


File: ldint.info,  Node: Emulations,  Next: Emulation Walkthrough,  Prev: README,  Up: Top

2 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.

* Menu:

* emulation parameters::        ‘emulparams’ scripts
* linker scripts::              ‘scripttempl’ scripts
* linker emulations::           ‘emultempl’ scripts


File: ldint.info,  Node: emulation parameters,  Next: linker scripts,  Up: Emulations

2.1 ‘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
     ‘scripttempl/SCRIPT.sc’.

‘TEMPLATE_NAME’
     This is the name of the ‘emultempl’ 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.  *note 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.

‘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.

‘ALIGNMENT’
     Some ‘scripttempl’ scripts set this to a number to pass to ‘ALIGN’
     to set the required alignment for the ‘end’ symbol.


File: ldint.info,  Node: linker scripts,  Next: linker emulations,  Prev: emulation parameters,  Up: Emulations

2.2 ‘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 to
9 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’.

‘c’
     The ‘scripttempl’ script is only invoked with ‘LD_FLAG’ set to this
     value if ‘GENERATE_COMBRELOC_SCRIPT’ is defined in the ‘emulparams’
     file or if ‘SCRIPT_NAME’ is ‘elf’.  The ‘emultempl’ script must
     arrange to use this script at the appropriate time, normally when
     the linker is invoked with the ‘-z combreloc’ option.  The output
     has an extension of ‘.xc’.

‘cshared’
     The ‘scripttempl’ script is only invoked with ‘LD_FLAG’ set to this
     value if ‘GENERATE_COMBRELOC_SCRIPT’ is defined in the ‘emulparams’
     file or if ‘SCRIPT_NAME’ is ‘elf’ and ‘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 -z combreloc’ option.  The
     output has an extension of ‘.xsc’.

‘wshared’
     Like _cshared_ but in addition to needing the ‘-shared’ and ‘-z
     combreloc’ options, it also needs the ‘-z relro’ option.  If the
     ‘-z now’ option is also used then the output has an extension of
     ‘.xsw’, otherwise it is ‘.xsco’.

‘pie’
     The ‘scripttempl’ script is only invoked with ‘LD_FLAG’ set to this
     value if ‘GENERATE_PIE_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
     ‘-pie’ option.  The output has an extension of ‘.xd’.

     In addition if the ‘scripttempl’ script is also invoked with
     ‘GENERATE_RELRO_SCRIPT’ defined then a second script is created
     with an extension of ‘.xdo’ to match the ‘-pie’ and ‘-z relro’
     options.

‘cpie’
     Like ‘pie’ but this version is used when the ‘scripttempl’ script
     is invoked with both ‘GENERATE_PIE_SCRIPT’ and
     ‘GENERATE_COMBRELOC_SCRIPT’ defined.  The ‘emultempl’ script must
     arrange to use this script at the appropriate time, normally when
     the linker is invoked with the ‘-pie’ and ‘-z combreloc’ options.
     The output has an extension of ‘.xdc’.

‘wpie’
     Like ‘cpie’, but in addition the ‘-z relro’ and ‘-z now’ options
     need to be defined.  The output has an extension of ‘.xdw’.

‘textonly’
‘pietextonly’
‘cpietextonly’
‘wpietextonly’

‘ctextonly’
‘wtextonly’
‘sharedtextonly’
‘csharedtextonly’
‘wsharedtextonly’

‘rotextonly’
‘roctextonly’
‘rowtextonly’
‘rosharedtextonly’
‘rocsharedtextonly’
‘rowsharedtextonly’
‘ropietextonly’
‘rocpietextonly’
‘rowpietextonly’

‘auto_import’
     The ‘scripttempl’ script is only invoked with ‘LD_FLAG’ set to this
     value if ‘GENERATE_AUTO_IMPORT_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 ‘--enable-auto-import’ option.  The output has an
     extension of ‘.xa’.

   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.

‘COMBRELOC’
     This will be set to a non-empty string when generating ‘-z
     combreloc’ scripts to a temporary file name which can be used
     during script generation.

   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
relocatable 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.


File: ldint.info,  Node: linker emulations,  Prev: linker scripts,  Up: Emulations

2.3 ‘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.


File: ldint.info,  Node: Emulation Walkthrough,  Next: Architecture Specific,  Prev: Emulations,  Up: Top

3 A Walkthrough of a Typical Emulation
**************************************

This chapter is to help people who are new to the way emulations
interact with the linker, or who are suddenly thrust into the position
of having to work with existing emulations.  It will discuss the files
you need to be aware of.  It will tell you when the given "hooks" in the
emulation will be called.  It will, hopefully, give you enough
information about when and how things happen that you'll be able to get
by.  As always, the source is the definitive reference to this.

   The starting point for the linker is in ‘ldmain.c’ where ‘main’ is
defined.  The bulk of the code that's emulation specific will initially
be in ‘emultempl/EMULATION.em’ but will end up in ‘eEMULATION.c’ when
the build is done.  Most of the work to select and interface with
emulations is in ‘ldemul.h’ and ‘ldemul.c’.  Specifically, ‘ldemul.h’
defines the ‘ld_emulation_xfer_struct’ structure your emulation exports.

   Your emulation file exports a symbol ‘ld_EMULATION_NAME_emulation’.
If your emulation is selected (it usually is, since usually there's only
one), ‘ldemul.c’ sets the variable LD_EMULATION to point to it.
‘ldemul.c’ also defines a number of API functions that interface to your
emulation, like ‘ldemul_after_parse’ which simply calls your
‘ld_EMULATION_emulation.after_parse’ function.  For the rest of this
section, the functions will be mentioned, but you should assume the
indirect reference to your emulation also.

   We will also skip or gloss over parts of the link process that don't
relate to emulations, like setting up internationalization.

   After initialization, ‘main’ selects an emulation by pre-scanning the
command-line arguments.  It calls ‘ldemul_choose_target’ to choose a
target.  If you set ‘choose_target’ to ‘ldemul_default_target’, it picks
your ‘target_name’ by default.

   ‘main’ calls ‘ldemul_before_parse’, then ‘parse_args’.  ‘parse_args’
calls ‘ldemul_parse_args’ for each arg, which must update the ‘getopt’
globals if it recognizes the argument.  If the emulation doesn't
recognize it, then parse_args checks to see if it recognizes it.

   Now that the emulation has had access to all its command-line
options, ‘main’ calls ‘ldemul_set_symbols’.  This can be used for any
initialization that may be affected by options.  It is also supposed to
set up any variables needed by the emulation script.

   ‘main’ now calls ‘ldemul_get_script’ to get the emulation script to
use (based on arguments, no doubt, *note Emulations::) and runs it.
While parsing, ‘ldgram.y’ may call ‘ldemul_hll’ or ‘ldemul_syslib’ to
handle the ‘HLL’ or ‘SYSLIB’ commands.  It may call
‘ldemul_unrecognized_file’ if you asked the linker to link a file it
doesn't recognize.  It will call ‘ldemul_recognized_file’ for each file
it does recognize, in case the emulation wants to handle some files
specially.  All the while, it's loading the files (possibly calling
‘ldemul_open_dynamic_archive’) and symbols and stuff.  After it's done
reading the script, ‘main’ calls ‘ldemul_after_parse’.  Use the
after-parse hook to set up anything that depends on stuff the script
might have set up, like the entry point.

   ‘main’ next calls ‘lang_process’ in ‘ldlang.c’.  This appears to be
the main core of the linking itself, as far as emulation hooks are
concerned(*).  It first opens the output file's BFD, calling
‘ldemul_set_output_arch’, and calls
‘ldemul_create_output_section_statements’ in case you need to use other
means to find or create object files (i.e.  shared libraries found on a
path, or fake stub objects).  Despite the name, nobody creates output
sections here.

   (*) In most cases, the BFD library does the bulk of the actual
linking, handling symbol tables, symbol resolution, relocations, and
building the final output file.  See the BFD reference for all the
details.  Your emulation is usually concerned more with managing things
at the file and section level, like "put this here, add this section",
etc.

   Next, the objects to be linked are opened and BFDs created for them,
and ‘ldemul_after_open’ is called.  At this point, you have all the
objects and symbols loaded, but none of the data has been placed yet.

   Next comes the Big Linking Thingy (except for the parts BFD does).
All input sections are mapped to output sections according to the
script.  If a section doesn't get mapped by default,
‘ldemul_place_orphan’ will get called to figure out where it goes.  Next
it figures out the offsets for each section, calling
‘ldemul_before_allocation’ before and ‘ldemul_after_allocation’ after
deciding where each input section ends up in the output sections.

   The last part of ‘lang_process’ is to figure out all the symbols'
values.  After assigning final values to the symbols, ‘ldemul_finish’ is
called, and after that, any undefined symbols are turned into fatal
errors.

   OK, back to ‘main’, which calls ‘ldwrite’ in ‘ldwrite.c’.  ‘ldwrite’
calls BFD's final_link, which does all the relocation fixups and writes
the output bfd to disk, and we're done.

   In summary,

   • ‘main()’ in ‘ldmain.c’
   • ‘emultempl/EMULATION.em’ has your code
   • ‘ldemul_choose_target’ (defaults to your ‘target_name’)
   • ‘ldemul_before_parse’
   • Parse argv, calls ‘ldemul_parse_args’ for each
   • ‘ldemul_set_symbols’
   • ‘ldemul_get_script’
   • parse script

        • may call ‘ldemul_hll’ or ‘ldemul_syslib’
        • may call ‘ldemul_open_dynamic_archive’

   • ‘ldemul_after_parse’
   • ‘lang_process()’ in ‘ldlang.c’

        • create ‘output_bfd’
        • ‘ldemul_set_output_arch’
        • ‘ldemul_create_output_section_statements’
        • read objects, create input bfds - all symbols exist, but have
          no values
        • may call ‘ldemul_unrecognized_file’
        • will call ‘ldemul_recognized_file’
        • ‘ldemul_after_open’
        • map input sections to output sections
        • may call ‘ldemul_place_orphan’ for remaining sections
        • ‘ldemul_before_allocation’
        • gives input sections offsets into output sections, places
          output sections
        • ‘ldemul_after_allocation’ - section addresses valid
        • assigns values to symbols
        • ‘ldemul_finish’ - symbol values valid

   • output bfd is written to disk


File: ldint.info,  Node: Architecture Specific,  Next: GNU Free Documentation License,  Prev: Emulation Walkthrough,  Up: Top

4 Some Architecture Specific Notes
**********************************

This is the place for notes on the behavior of ‘ld’ on specific
platforms.  Currently, only Intel x86 is documented (and of that, only
the auto-import behavior for DLLs).

* Menu:

* ix86::                        Intel x86


File: ldint.info,  Node: ix86,  Up: Architecture Specific

4.1 Intel x86
=============

     ‘ld’ can create DLLs that operate with various runtimes available
     on a common x86 operating system.  These runtimes include native
     (using the mingw "platform"), cygwin, and pw.

_auto-import from DLLs_
       1. With this feature on, DLL clients can import variables from
          DLL without any concern from their side (for example, without
          any source code modifications).  Auto-import can be enabled
          using the ‘--enable-auto-import’ flag, or disabled via the
          ‘--disable-auto-import’ flag.  Auto-import is disabled by
          default.

       2. This is done completely in bounds of the PE specification (to
          be fair, there's a minor violation of the spec at one point,
          but in practice auto-import works on all known variants of
          that common x86 operating system) So, the resulting DLL can be
          used with any other PE compiler/linker.

       3. Auto-import is fully compatible with standard import method,
          in which variables are decorated using attribute modifiers.
          Libraries of either type may be mixed together.

       4. Overhead (space): 8 bytes per imported symbol, plus 20 for
          each reference to it; Overhead (load time): negligible;
          Overhead (virtual/physical memory): should be less than effect
          of DLL relocation.

     Motivation

     The obvious and only way to get rid of dllimport insanity is to
     make client access variable directly in the DLL, bypassing the
     extra dereference imposed by ordinary DLL runtime linking.  I.e.,
     whenever client contains something like

     ‘mov dll_var,%eax,’

     address of dll_var in the command should be relocated to point into
     loaded DLL. The aim is to make OS loader do so, and than make ld
     help with that.  Import section of PE made following way: there's a
     vector of structures each describing imports from particular DLL.
     Each such structure points to two other parallel vectors: one
     holding imported names, and one which will hold address of
     corresponding imported name.  So, the solution is de-vectorize
     these structures, making import locations be sparse and pointing
     directly into code.

     Implementation

     For each reference of data symbol to be imported from DLL (to set
     of which belong symbols with name <sym>, if __imp_<sym> is found in
     implib), the import fixup entry is generated.  That entry is of
     type IMAGE_IMPORT_DESCRIPTOR and stored in .idata$3 subsection.
     Each fixup entry contains pointer to symbol's address within .text
     section (marked with __fuN_<sym> symbol, where N is integer),
     pointer to DLL name (so, DLL name is referenced by multiple
     entries), and pointer to symbol name thunk.  Symbol name thunk is
     singleton vector (__nm_th_<symbol>) pointing to
     IMAGE_IMPORT_BY_NAME structure (__nm_<symbol>) directly containing
     imported name.  Here comes that "om the edge" problem mentioned
     above: PE specification rambles that name vector
     (OriginalFirstThunk) should run in parallel with addresses vector
     (FirstThunk), i.e.  that they should have same number of elements
     and terminated with zero.  We violate this, since FirstThunk points
     directly into machine code.  But in practice, OS loader implemented
     the sane way: it goes thru OriginalFirstThunk and puts addresses to
     FirstThunk, not something else.  It once again should be noted that
     dll and symbol name structures are reused across fixup entries and
     should be there anyway to support standard import stuff, so
     sustained overhead is 20 bytes per reference.  Other question is
     whether having several IMAGE_IMPORT_DESCRIPTORS for the same DLL is
     possible.  Answer is yes, it is done even by native compiler/linker
     (libth32's functions are in fact resident in windows9x
     kernel32.dll, so if you use it, you have two
     IMAGE_IMPORT_DESCRIPTORS for kernel32.dll).  Yet other question is
     whether referencing the same PE structures several times is valid.
     The answer is why not, prohibiting that (detecting violation) would
     require more work on behalf of loader than not doing it.


File: ldint.info,  Node: GNU Free Documentation License,  Prev: Architecture Specific,  Up: Top

5 GNU Free Documentation License
********************************

                     Version 1.3, 3 November 2008

     Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
     <http://fsf.org/>

     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.

  0. PREAMBLE

     The purpose of this License is to make a manual, textbook, or other
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     This License is a kind of "copyleft", which means that derivative
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     It complements the GNU General Public License, which is a copyleft
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     We have designed this License in order to use it for manuals for
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  4. MODIFICATIONS

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       B. List on the Title Page, as authors, one or more persons or
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       C. State on the Title page the name of the publisher of the
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       H. Include an unaltered copy of this License.

       I. Preserve the section Entitled "History", Preserve its Title,
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       K. For any section Entitled "Acknowledgements" or "Dedications",
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       O. Preserve any Warranty Disclaimers.

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  5. COMBINING DOCUMENTS

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  6. COLLECTIONS OF DOCUMENTS

     You may make a collection consisting of the Document and other
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  7. AGGREGATION WITH INDEPENDENT WORKS

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  9. TERMINATION

     You may not copy, modify, sublicense, or distribute the Document
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  10. FUTURE REVISIONS OF THIS LICENSE

     The Free Software Foundation may publish new, revised versions of
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     versions will be similar in spirit to the present version, but may
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  11. RELICENSING

     "Massive Multiauthor Collaboration Site" (or "MMC Site") means any
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     The operator of an MMC Site may republish an MMC contained in the
     site under CC-BY-SA on the same site at any time before August 1,
     2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents
====================================================

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

       Copyright (C)  YEAR  YOUR NAME.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.3
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

   If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts."  line with this:

         with the Invariant Sections being LIST THEIR TITLES, with
         the Front-Cover Texts being LIST, and with the Back-Cover Texts
         being LIST.

   If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

   If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
software license, such as the GNU General Public License, to permit
their use in free software.



Tag Table:
Node: Top1088
Node: README1893
Node: Emulations2129
Node: emulation parameters4452
Node: linker scripts7844
Node: linker emulations18228
Node: Emulation Walkthrough20714
Node: Architecture Specific27493
Node: ix8627921
Node: GNU Free Documentation License32269

End Tag Table


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