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+\input texinfo
+@setfilename ldint.info
+
+@ifinfo
+@format
+START-INFO-DIR-ENTRY
+* Ld-Internals: (ldint).	The GNU linker internals.
+END-INFO-DIR-ENTRY
+@end format
+@end ifinfo
+
+@ifinfo
+This file documents the internals of the GNU linker ld.
+
+Copyright (C) 1992, 93, 94, 95, 96, 97, 1998 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.
+
+@ignore
+Permission is granted to process this file through Tex and print the
+results, provided the printed document carries copying permission
+notice identical to this one except for the removal of this paragraph
+(this paragraph not being relevant to the printed manual).
+
+@end ignore
+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).
+@end ifinfo
+
+@iftex
+@finalout
+@setchapternewpage off
+@settitle GNU Linker Internals
+@titlepage
+@title{A guide to the internals of the GNU linker}
+@author Per Bothner, Steve Chamberlain, Ian Lance Taylor, DJ Delorie
+@author Cygnus Support
+@page
+
+@tex
+\def\$#1${{#1}}  % Kluge: collect RCS revision info without $...$
+\xdef\manvers{\$Revision$}  % For use in headers, footers too
+{\parskip=0pt
+\hfill Cygnus Support\par
+\hfill \manvers\par
+\hfill \TeX{}info \texinfoversion\par
+}
+@end tex
+
+@vskip 0pt plus 1filll
+Copyright @copyright{} 1992, 93, 94, 95, 96, 97, 1998
+Free Software Foundation, Inc.
+
+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.
+
+@end titlepage
+@end iftex
+
+@node Top
+@top
+
+This file documents the internals of the GNU linker @code{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 @code{ld} as you discover it (or as you design changes to @code{ld}).
+
+@menu
+* README::			The README File
+* Emulations::			How linker emulations are generated
+* Emulation Walkthrough::	A Walkthrough of a Typical Emulation
+@end menu
+
+@node README
+@chapter The @file{README} File
+
+Check the @file{README} file; it often has useful information that does not
+appear anywhere else in the directory.
+
+@node Emulations
+@chapter How linker emulations are generated
+
+Each linker target has an @dfn{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
+@file{genscripts.sh}.
+
+The @file{genscripts.sh} script starts by reading a file in the
+@file{emulparams} directory.  This is a shell script which sets various
+shell variables used by @file{genscripts.sh} and the other shell scripts
+it invokes.
+
+The @file{genscripts.sh} script will invoke a shell script in the
+@file{scripttempl} directory in order to create default linker scripts
+written in the linker command language.  The @file{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, @file{genscripts.sh} will invoke yet another
+shell script, this time in the @file{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
+@file{emultempl/generic.em}.
+
+To summarize, @file{genscripts.sh} reads three shell scripts: an
+emulation parameters script in the @file{emulparams} directory, a linker
+script generation script in the @file{scripttempl} directory, and an
+emulation source file generation script in the @file{emultempl}
+directory.
+
+For example, the Sun 4 linker sets up variables in
+@file{emulparams/sun4.sh}, creates linker scripts using
+@file{scripttempl/aout.sc}, and creates the emulation code using
+@file{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 @code{-m} option.  The @code{-V} option will list all supported
+emulations.
+
+@menu
+* emulation parameters::        @file{emulparams} scripts
+* linker scripts::              @file{scripttempl} scripts
+* linker emulations::           @file{emultempl} scripts
+@end menu
+
+@node emulation parameters
+@section @file{emulparams} scripts
+
+Each target selects a particular file in the @file{emulparams} directory
+by setting the shell variable @code{targ_emul} in @file{configure.tgt}.
+This shell variable is used by the @file{configure} script to control
+building an emulation source file.
+
+Certain conventions are enforced.  Suppose the @code{targ_emul} variable
+is set to @var{emul} in @file{configure.tgt}.  The name of the emulation
+shell script will be @file{emulparams/@var{emul}.sh}.  The
+@file{Makefile} must have a target named @file{e@var{emul}.c}; this
+target must depend upon @file{emulparams/@var{emul}.sh}, as well as the
+appropriate scripts in the @file{scripttempl} and @file{emultempl}
+directories.  The @file{Makefile} target must invoke @code{GENSCRIPTS}
+with two arguments: @var{emul}, and the value of the make variable
+@code{tdir_@var{emul}}.  The value of the latter variable will be set by
+the @file{configure} script, and is used to set the default target
+directory to search.
+
+By convention, the @file{emulparams/@var{emul}.sh} shell script should
+only set shell variables.  It may set shell variables which are to be
+interpreted by the @file{scripttempl} and the @file{emultempl} scripts.
+Certain shell variables are interpreted directly by the
+@file{genscripts.sh} script.
+
+Here is a list of shell variables interpreted by @file{genscripts.sh},
+as well as some conventional shell variables interpreted by the
+@file{scripttempl} and @file{emultempl} scripts.
+
+@table @code
+@item SCRIPT_NAME
+This is the name of the @file{scripttempl} script to use.  If
+@code{SCRIPT_NAME} is set to @var{script}, @file{genscripts.sh} will use
+the script @file{scriptteml/@var{script}.sc}.
+
+@item TEMPLATE_NAME
+This is the name of the @file{emultemlp} script to use.  If
+@code{TEMPLATE_NAME} is set to @var{template}, @file{genscripts.sh} will
+use the script @file{emultempl/@var{template}.em}.  If this variable is
+not set, the default value is @samp{generic}.
+
+@item GENERATE_SHLIB_SCRIPT
+If this is set to a nonempty string, @file{genscripts.sh} will invoke
+the @file{scripttempl} script an extra time to create a shared library
+script.  @ref{linker scripts}.
+
+@item OUTPUT_FORMAT
+This is normally set to indicate the BFD output format use (e.g.,
+@samp{"a.out-sunos-big"}.  The @file{scripttempl} script will normally
+use it in an @code{OUTPUT_FORMAT} expression in the linker script.
+
+@item ARCH
+This is normally set to indicate the architecture to use (e.g.,
+@samp{sparc}).  The @file{scripttempl} script will normally use it in an
+@code{OUTPUT_ARCH} expression in the linker script.
+
+@item ENTRY
+Some @file{scripttempl} scripts use this to set the entry address, in an
+@code{ENTRY} expression in the linker script.
+
+@item TEXT_START_ADDR
+Some @file{scripttempl} scripts use this to set the start address of the
+@samp{.text} section.
+
+@item NONPAGED_TEXT_START_ADDR
+If this is defined, the @file{genscripts.sh} script sets
+@code{TEXT_START_ADDR} to its value before running the
+@file{scripttempl} script for the @code{-n} and @code{-N} options
+(@pxref{linker scripts}).
+
+@item SEGMENT_SIZE
+The @file{genscripts.sh} script uses this to set the default value of
+@code{DATA_ALIGNMENT} when running the @file{scripttempl} script.
+
+@item TARGET_PAGE_SIZE
+If @code{SEGMENT_SIZE} is not defined, the @file{genscripts.sh} script
+uses this to define it.
+
+@item ALIGNMENT
+Some @file{scripttempl} scripts set this to a number to pass to
+@code{ALIGN} to set the required alignment for the @code{end} symbol.
+@end table
+
+@node linker scripts
+@section @file{scripttempl} scripts
+
+Each linker target uses a @file{scripttempl} script to generate the
+default linker scripts.  The name of the @file{scripttempl} script is
+set by the @code{SCRIPT_NAME} variable in the @file{emulparams} script.
+If @code{SCRIPT_NAME} is set to @var{script}, @code{genscripts.sh} will
+invoke @file{scripttempl/@var{script}.sc}.
+
+The @file{genscripts.sh} script will invoke the @file{scripttempl}
+script 5 or 6 times.  Each time it will set the shell variable
+@code{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 @code{get_script} emulation entry point;
+this describes the conventional behaviour).
+
+The @file{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 @file{emulparams} file without the @file{.sc}
+extension--is @var{emul}, then the output will be directed to
+@file{ldscripts/@var{emul}.@var{extension}} in the build directory,
+where @var{extension} changes each time the @file{scripttempl} script is
+invoked.
+
+Here is the list of values assigned to @code{LD_FLAG}.
+
+@table @code
+@item (empty)
+The script generated is used by default (when none of the following
+cases apply).  The output has an extension of @file{.x}.
+@item n
+The script generated is used when the linker is invoked with the
+@code{-n} option.  The output has an extension of @file{.xn}.
+@item N
+The script generated is used when the linker is invoked with the
+@code{-N} option.  The output has an extension of @file{.xbn}.
+@item r
+The script generated is used when the linker is invoked with the
+@code{-r} option.  The output has an extension of @file{.xr}.
+@item u
+The script generated is used when the linker is invoked with the
+@code{-Ur} option.  The output has an extension of @file{.xu}.
+@item shared
+The @file{scripttempl} script is only invoked with @code{LD_FLAG} set to
+this value if @code{GENERATE_SHLIB_SCRIPT} is defined in the
+@file{emulparams} file.  The @file{emultempl} script must arrange to use
+this script at the appropriate time, normally when the linker is invoked
+with the @code{-shared} option.  The output has an extension of
+@file{.xs}.
+@end table
+
+Besides the shell variables set by the @file{emulparams} script, and the
+@code{LD_FLAG} variable, the @file{genscripts.sh} script will set
+certain variables for each run of the @file{scripttempl} script.
+
+@table @code
+@item RELOCATING
+This will be set to a non-empty string when the linker is doing a final
+relocation (e.g., all scripts other than @code{-r} and @code{-Ur}).
+
+@item 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
+@code{-r}).
+
+@item DATA_ALIGNMENT
+This will be set to an @code{ALIGN} expression when the output should be
+page aligned, or to @samp{.} when generating the @code{-N} script.
+
+@item CREATE_SHLIB
+This will be set to a non-empty string when generating a @code{-shared}
+script.
+@end table
+
+The conventional way to write a @file{scripttempl} script is to first
+set a few shell variables, and then write out a linker script using
+@code{cat} with a here document.  The linker script will use variable
+substitutions, based on the above variables and those set in the
+@file{emulparams} script, to control its behaviour.
+
+When there are parts of the @file{scripttempl} script which should only
+be run when doing a final relocation, they should be enclosed within a
+variable substitution based on @code{RELOCATING}.  For example, on many
+targets special symbols such as @code{_end} should be defined when doing
+a final link.  Naturally, those symbols should not be defined when doing
+a relocateable link using @code{-r}.  The @file{scripttempl} script
+could use a construct like this to define those symbols:
+@smallexample
+  $@{RELOCATING+ _end = .;@}
+@end smallexample
+This will do the symbol assignment only if the @code{RELOCATING}
+variable is defined.
+
+The basic job of the linker script is to put the sections in the correct
+order, and at the correct memory addresses.  For some targets, the
+linker script may have to do some other operations.
+
+For example, on most MIPS platforms, the linker is responsible for
+defining the special symbol @code{_gp}, used to initialize the
+@code{$gp} register.  It must be set to the start of the small data
+section plus @code{0x8000}.  Naturally, it should only be defined when
+doing a final relocation.  This will typically be done like this:
+@smallexample
+  $@{RELOCATING+ _gp = ALIGN(16) + 0x8000;@}
+@end smallexample
+This line would appear just before the sections which compose the small
+data section (@samp{.sdata}, @samp{.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 @samp{.ctor} section, and the address of each global destructor in
+a @samp{.dtor} section (this is done by defining
+@code{ASM_OUTPUT_CONSTRUCTOR} and @code{ASM_OUTPUT_DESTRUCTOR} in the
+@code{gcc} configuration files).  The @code{gcc} runtime support
+routines expect the constructor table to be named @code{__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
+@code{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
+@file{scripttempl} file.
+@smallexample
+    $@{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__ = .;@}
+@end smallexample
+The use of @code{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 @code{PROVIDE} construct with a default stack
+address.  This permits the user to easily override the stack address
+using the @code{--defsym} option.  Here is an example:
+@smallexample
+  $@{RELOCATING+ PROVIDE (__stack = 0x80000000);@}
+@end smallexample
+The value of the symbol @code{__stack} would then be used in the startup
+code to initialize the stack pointer.
+
+@node linker emulations
+@section @file{emultempl} scripts
+
+Each linker target uses an @file{emultempl} script to generate the
+emulation code.  The name of the @file{emultempl} script is set by the
+@code{TEMPLATE_NAME} variable in the @file{emulparams} script.  If the
+@code{TEMPLATE_NAME} variable is not set, the default is
+@samp{generic}.  If the value of @code{TEMPLATE_NAME} is @var{template},
+@file{genscripts.sh} will use @file{emultempl/@var{template}.em}.
+
+Most targets use the generic @file{emultempl} script,
+@file{emultempl/generic.em}.  A different @file{emultempl} script is
+only needed if the linker must support unusual actions, such as linking
+against shared libraries.
+
+The @file{emultempl} script is normally written as a simple invocation
+of @code{cat} with a here document.  The document will use a few
+variable substitutions.  Typically each function names uses a
+substitution involving @code{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
+@code{ld_@var{EMULATION_NAME}_emulation}, where @var{EMULATION_NAME} is
+the name of the emulation set in @file{configure.tgt} (this is also the
+name of the @file{emulparams} file without the @file{.sh} extension).
+The @file{genscripts.sh} script will set the shell variable
+@code{EMULATION_NAME} before invoking the @file{emultempl} script.
+
+The @code{ld_@var{EMULATION_NAME}_emulation} variable must be a
+@code{struct ld_emulation_xfer_struct}, as defined in @file{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 @code{EMULATION_NAME} and the default BFD target name (normally
+set from the shell variable @code{OUTPUT_FORMAT} which is normally set
+by the @file{emulparams} file).
+
+The @file{genscripts.sh} script will set the shell variable
+@code{COMPILE_IN} when it invokes the @file{emultempl} script for the
+default emulation.  In this case, the @file{emultempl} script should
+include the linker scripts directly, and return them from the
+@code{get_scripts} entry point.  When the emulation is not the default,
+the @code{get_scripts} entry point should just return a file name.  See
+@file{emultempl/generic.em} for an example of how this is done.
+
+At some point, the linker emulation entry points should be documented.
+
+@node Emulation Walkthrough
+@chapter 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 @file{ldmain.c} where
+@code{main} is defined.  The bulk of the code that's emulation
+specific will initially be in @code{emultempl/@var{emulation}.em} but
+will end up in @code{e@var{emulation}.c} when the build is done.
+Most of the work to select and interface with emulations is in
+@code{ldemul.h} and @code{ldemul.c}.  Specifically, @code{ldemul.h}
+defines the @code{ld_emulation_xfer_struct} structure your emulation
+exports.
+
+Your emulation file exports a symbol
+@code{ld_@var{EMULATION_NAME}_emulation}.  If your emulation is
+selected (it usually is, since usually there's only one),
+@code{ldemul.c} sets the variable @var{ld_emulation} to point to it.
+@code{ldemul.c} also defines a number of API functions that interface
+to your emulation, like @code{ldemul_after_parse} which simply calls
+your @code{ld_@var{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, @code{main} selects an emulation by pre-scanning
+the command line arguments.  It calls @code{ldemul_choose_target} to
+choose a target.  If you set @code{choose_target} to
+@code{ldemul_default_target}, it picks your @code{target_name} by
+default.
+
+@code{main} calls @code{ldemul_before_parse}, then @code{parse_args}.
+@code{parse_args} calls @code{ldemul_parse_args} for each arg, which
+must update the @code{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,
+@code{main} calls @code{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.
+
+@code{main} now calls @code{ldemul_get_script} to get the emulation
+script to use (based on arguments, no doubt, @pxref{Emulations}) and
+runs it.  While parsing, @code{ldgram.y} may call @code{ldemul_hll} or
+@code{ldemul_syslib} to handle the @code{HLL} or @code{SYSLIB}
+commands.  It may call @code{ldemul_unrecognized_file} if you asked
+the linker to link a file it doesn't recognize.  It will call
+@code{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
+@code{ldemul_open_dynamic_archive}) and symbols and stuff.  After it's
+done reading the script, @code{main} calls @code{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.
+
+@code{main} next calls @code{lang_process} in @code{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
+@code{ldemul_set_output_arch}, and calls
+@code{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 @code{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,
+@code{ldemul_place_orphan} will get called to figure out where it goes.
+Next it figures out the offsets for each section, calling
+@code{ldemul_before_allocation} before and
+@code{ldemul_after_allocation} after deciding where each input section
+ends up in the output sections.
+
+The last part of @code{lang_process} is to figure out all the symbols'
+values.  After assigning final values to the symbols,
+@code{ldemul_finish} is called, and after that, any undefined symbols
+are turned into fatal errors.
+
+OK, back to @code{main}, which calls @code{ldwrite} in
+@file{ldwrite.c}.  @code{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,
+
+@itemize @bullet
+
+@item @code{main()} in @file{ldmain.c}
+@item @file{emultempl/@var{EMULATION}.em} has your code
+@item @code{ldemul_choose_target} (defaults to your @code{target_name})
+@item @code{ldemul_before_parse}
+@item Parse argv, calls @code{ldemul_parse_args} for each
+@item @code{ldemul_set_symbols}
+@item @code{ldemul_get_script}
+@item parse script
+
+@itemize @bullet
+@item may call @code{ldemul_hll} or @code{ldemul_syslib}
+@item may call @code{ldemul_open_dynamic_archive}
+@end itemize
+
+@item @code{ldemul_after_parse}
+@item @code{lang_process()} in @file{ldlang.c}
+
+@itemize @bullet
+@item create @code{output_bfd}
+@item @code{ldemul_set_output_arch}
+@item @code{ldemul_create_output_section_statements}
+@item read objects, create input bfds - all symbols exist, but have no values
+@item may call @code{ldemul_unrecognized_file}
+@item will call @code{ldemul_recognized_file}
+@item @code{ldemul_after_open}
+@item map input sections to output sections
+@item may call @code{ldemul_place_orphan} for remaining sections
+@item @code{ldemul_before_allocation}
+@item gives input sections offsets into output sections, places output sections
+@item @code{ldemul_after_allocation} - section addresses valid
+@item assigns values to symbols
+@item @code{ldemul_finish} - symbol values valid
+@end itemize
+
+@item output bfd is written to disk
+
+@end itemize
+
+@contents
+@bye