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@setfilename ldint.info

@c Copyright 1992, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001,

@c 2003, 2005, 2006, 2007

@c Free Software Foundation, Inc.

@ifinfo

@format

START-INFO-DIR-ENTRY

* Ld-Internals: (ldint).        The GNU linker internals.

END-INFO-DIR-ENTRY

@end format

@end ifinfo

@copying

This file documents the internals of the GNU linker ld.

Copyright @copyright{} 1992, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2007

Free Software Foundation, Inc.

Contributed by Cygnus Support.

Permission is granted to copy, distribute and/or modify this document

under the terms of the GNU Free Documentation License, Version 1.1 or

any later version published by the Free Software Foundation; with the

Invariant Sections being ``GNU General Public License'' and ``Funding

Free Software'', the Front-Cover texts being (a) (see below), and with

the Back-Cover Texts being (b) (see below).  A copy of the license is

included in the section entitled ``GNU Free Documentation License''.

(a) The FSF's Front-Cover Text is:

     A GNU Manual

(b) The FSF's Back-Cover Text is:

     You have freedom to copy and modify this GNU Manual, like GNU

     software.  Copies published by the Free Software Foundation raise

     funds for GNU development.

@end copying

@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{2.10.91}  % 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, 2000

Free Software Foundation, Inc.

      Permission is granted to copy, distribute and/or modify this document

      under the terms of the GNU Free Documentation License, Version 1.1

      or any later version published by the Free Software Foundation;

      with no Invariant Sections, with no Front-Cover Texts, and with no

      Back-Cover Texts.  A copy of the license is included in the

      section entitled "GNU Free Documentation License".

@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}).

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

@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{scripttempl/@var{script}.sc}.

@item TEMPLATE_NAME

This is the name of the @file{emultempl} 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 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 to 9 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}.

@item c

The @file{scripttempl} script is only invoked with @code{LD_FLAG} set to

this value if @code{GENERATE_COMBRELOC_SCRIPT} is defined in the

@file{emulparams} file or if @code{SCRIPT_NAME} is @code{elf}. The

@file{emultempl} script must arrange to use this script at the appropriate

time, normally when the linker is invoked with the @code{-z combreloc}

option.  The output has an extension of

@file{.xc}.

@item cshared

The @file{scripttempl} script is only invoked with @code{LD_FLAG} set to

this value if @code{GENERATE_COMBRELOC_SCRIPT} is defined in the

@file{emulparams} file or if @code{SCRIPT_NAME} is @code{elf} and

@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

-z combreloc} option.  The output has an extension of @file{.xsc}.

@item auto_import

The @file{scripttempl} script is only invoked with @code{LD_FLAG} set to

this value if @code{GENERATE_AUTO_IMPORT_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{--enable-auto-import} option.  The output has

an extension of @file{.xa}.

@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.

@item COMBRELOC

This will be set to a non-empty string when generating @code{-z combreloc}

scripts to a temporary file name which can be used during script generation.

@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 relocatable 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

@node Architecture Specific

@chapter Some Architecture Specific Notes

This is the place for notes on the behavior of @code{ld} on

specific platforms.  Currently, only Intel x86 is documented (and

of that, only the auto-import behavior for DLLs).

@menu

* ix86::                        Intel x86

@end menu

@node ix86

@section Intel x86

@table @emph

@code{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.

@item auto-import from DLLs

@enumerate

@item

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

@code{--enable-auto-import} flag, or disabled via the

@code{--disable-auto-import} flag.  Auto-import is disabled by default.

@item

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.

@item

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.

@item

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.

@end enumerate

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

@code{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.

@end table

@node GNU Free Documentation License

@chapter GNU Free Documentation License

                GNU Free Documentation License

                   Version 1.1, March 2000

 Copyright (C) 2000  Free Software Foundation, Inc.

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