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diff --git a/share/doc/ldint.html b/share/doc/ldint.html new file mode 100644 index 0000000..8f8b2ee --- /dev/null +++ b/share/doc/ldint.html @@ -0,0 +1,1286 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- This file documents the internals of the GNU linker ld. + +Copyright (C) 1992-2023 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 1.3 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. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>Untitled Document</title> + +<meta name="description" content="Untitled Document"> +<meta name="keywords" content="Untitled Document"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="#Top" rel="start" title="Top"> +<link href="#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="dir.html#Top" rel="up" title="(dir)"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> + + + + +<a name="Top"></a> +<div class="header"> +<p> +Next: <a href="#README" accesskey="n" rel="next">README</a>, Up: <a href="dir.html#Top" accesskey="u" rel="up">(dir)</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p> +</div> +<a name="SEC_Top"></a> + +<p>This file documents the internals of the GNU linker <code>ld</code>. 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</code> as you discover it (or as you design changes to <code>ld</code>). +</p> +<p>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". +</p> +<table class="menu" border="0" cellspacing="0"> +<tr><td align="left" valign="top">• <a href="#README" accesskey="1">README</a>:</td><td> </td><td align="left" valign="top">The README File +</td></tr> +<tr><td align="left" valign="top">• <a href="#Emulations" accesskey="2">Emulations</a>:</td><td> </td><td align="left" valign="top">How linker emulations are generated +</td></tr> +<tr><td align="left" valign="top">• <a href="#Emulation-Walkthrough" accesskey="3">Emulation Walkthrough</a>:</td><td> </td><td align="left" valign="top">A Walkthrough of a Typical Emulation +</td></tr> +<tr><td align="left" valign="top">• <a href="#Architecture-Specific" accesskey="4">Architecture Specific</a>:</td><td> </td><td align="left" valign="top">Some Architecture Specific Notes +</td></tr> +<tr><td align="left" valign="top">• <a href="#GNU-Free-Documentation-License" accesskey="5">GNU Free Documentation License</a>:</td><td> </td><td align="left" valign="top">GNU Free Documentation License +</td></tr> +</table> + +<hr> +<a name="README"></a> +<div class="header"> +<p> +Next: <a href="#Emulations" accesskey="n" rel="next">Emulations</a>, Previous: <a href="#Top" accesskey="p" rel="previous">Top</a>, Up: <a href="#Top" accesskey="u" rel="up">Top</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p> +</div> +<a name="The-README-File"></a> +<h2 class="chapter">1 The <samp>README</samp> File</h2> + +<p>Check the <samp>README</samp> file; it often has useful information that does not +appear anywhere else in the directory. +</p> +<hr> +<a name="Emulations"></a> +<div class="header"> +<p> +Next: <a href="#Emulation-Walkthrough" accesskey="n" rel="next">Emulation Walkthrough</a>, Previous: <a href="#README" accesskey="p" rel="previous">README</a>, Up: <a href="#Top" accesskey="u" rel="up">Top</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p> +</div> +<a name="How-linker-emulations-are-generated"></a> +<h2 class="chapter">2 How linker emulations are generated</h2> + +<p>Each linker target has an <em>emulation</em>. The emulation includes the +default linker script, and certain emulations also modify certain types +of linker behaviour. +</p> +<p>Emulations are created during the build process by the shell script +<samp>genscripts.sh</samp>. +</p> +<p>The <samp>genscripts.sh</samp> script starts by reading a file in the +<samp>emulparams</samp> directory. This is a shell script which sets various +shell variables used by <samp>genscripts.sh</samp> and the other shell scripts +it invokes. +</p> +<p>The <samp>genscripts.sh</samp> script will invoke a shell script in the +<samp>scripttempl</samp> directory in order to create default linker scripts +written in the linker command language. The <samp>scripttempl</samp> 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. +</p> +<p>After creating the scripts, <samp>genscripts.sh</samp> will invoke yet another +shell script, this time in the <samp>emultempl</samp> 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 +<samp>emultempl/generic.em</samp>. +</p> +<p>To summarize, <samp>genscripts.sh</samp> reads three shell scripts: an +emulation parameters script in the <samp>emulparams</samp> directory, a linker +script generation script in the <samp>scripttempl</samp> directory, and an +emulation source file generation script in the <samp>emultempl</samp> +directory. +</p> +<p>For example, the Sun 4 linker sets up variables in +<samp>emulparams/sun4.sh</samp>, creates linker scripts using +<samp>scripttempl/aout.sc</samp>, and creates the emulation code using +<samp>emultempl/sunos.em</samp>. +</p> +<p>Note that the linker can support several emulations simultaneously, +depending upon how it is configured. An emulation can be selected with +the <code>-m</code> option. The <code>-V</code> option will list all supported +emulations. +</p> +<table class="menu" border="0" cellspacing="0"> +<tr><td align="left" valign="top">• <a href="#emulation-parameters" accesskey="1">emulation parameters</a>:</td><td> </td><td align="left" valign="top"><samp>emulparams</samp> scripts +</td></tr> +<tr><td align="left" valign="top">• <a href="#linker-scripts" accesskey="2">linker scripts</a>:</td><td> </td><td align="left" valign="top"><samp>scripttempl</samp> scripts +</td></tr> +<tr><td align="left" valign="top">• <a href="#linker-emulations" accesskey="3">linker emulations</a>:</td><td> </td><td align="left" valign="top"><samp>emultempl</samp> scripts +</td></tr> +</table> + +<hr> +<a name="emulation-parameters"></a> +<div class="header"> +<p> +Next: <a href="#linker-scripts" accesskey="n" rel="next">linker scripts</a>, Up: <a href="#Emulations" accesskey="u" rel="up">Emulations</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p> +</div> +<a name="emulparams-scripts"></a> +<h3 class="section">2.1 <samp>emulparams</samp> scripts</h3> + +<p>Each target selects a particular file in the <samp>emulparams</samp> directory +by setting the shell variable <code>targ_emul</code> in <samp>configure.tgt</samp>. +This shell variable is used by the <samp>configure</samp> script to control +building an emulation source file. +</p> +<p>Certain conventions are enforced. Suppose the <code>targ_emul</code> variable +is set to <var>emul</var> in <samp>configure.tgt</samp>. The name of the emulation +shell script will be <samp>emulparams/<var>emul</var>.sh</samp>. The +<samp>Makefile</samp> must have a target named <samp>e<var>emul</var>.c</samp>; this +target must depend upon <samp>emulparams/<var>emul</var>.sh</samp>, as well as the +appropriate scripts in the <samp>scripttempl</samp> and <samp>emultempl</samp> +directories. The <samp>Makefile</samp> target must invoke <code>GENSCRIPTS</code> +with two arguments: <var>emul</var>, and the value of the make variable +<code>tdir_<var>emul</var></code>. The value of the latter variable will be set by +the <samp>configure</samp> script, and is used to set the default target +directory to search. +</p> +<p>By convention, the <samp>emulparams/<var>emul</var>.sh</samp> shell script should +only set shell variables. It may set shell variables which are to be +interpreted by the <samp>scripttempl</samp> and the <samp>emultempl</samp> scripts. +Certain shell variables are interpreted directly by the +<samp>genscripts.sh</samp> script. +</p> +<p>Here is a list of shell variables interpreted by <samp>genscripts.sh</samp>, +as well as some conventional shell variables interpreted by the +<samp>scripttempl</samp> and <samp>emultempl</samp> scripts. +</p> +<dl compact="compact"> +<dt><code>SCRIPT_NAME</code></dt> +<dd><p>This is the name of the <samp>scripttempl</samp> script to use. If +<code>SCRIPT_NAME</code> is set to <var>script</var>, <samp>genscripts.sh</samp> will use +the script <samp>scripttempl/<var>script</var>.sc</samp>. +</p> +</dd> +<dt><code>TEMPLATE_NAME</code></dt> +<dd><p>This is the name of the <samp>emultempl</samp> script to use. If +<code>TEMPLATE_NAME</code> is set to <var>template</var>, <samp>genscripts.sh</samp> will +use the script <samp>emultempl/<var>template</var>.em</samp>. If this variable is +not set, the default value is ‘<samp>generic</samp>’. +</p> +</dd> +<dt><code>GENERATE_SHLIB_SCRIPT</code></dt> +<dd><p>If this is set to a nonempty string, <samp>genscripts.sh</samp> will invoke +the <samp>scripttempl</samp> script an extra time to create a shared library +script. <a href="#linker-scripts">linker scripts</a>. +</p> +</dd> +<dt><code>OUTPUT_FORMAT</code></dt> +<dd><p>This is normally set to indicate the BFD output format use (e.g., +‘<samp>"a.out-sunos-big"</samp>’. The <samp>scripttempl</samp> script will normally +use it in an <code>OUTPUT_FORMAT</code> expression in the linker script. +</p> +</dd> +<dt><code>ARCH</code></dt> +<dd><p>This is normally set to indicate the architecture to use (e.g., +‘<samp>sparc</samp>’). The <samp>scripttempl</samp> script will normally use it in an +<code>OUTPUT_ARCH</code> expression in the linker script. +</p> +</dd> +<dt><code>ENTRY</code></dt> +<dd><p>Some <samp>scripttempl</samp> scripts use this to set the entry address, in an +<code>ENTRY</code> expression in the linker script. +</p> +</dd> +<dt><code>TEXT_START_ADDR</code></dt> +<dd><p>Some <samp>scripttempl</samp> scripts use this to set the start address of the +‘<samp>.text</samp>’ section. +</p> +</dd> +<dt><code>SEGMENT_SIZE</code></dt> +<dd><p>The <samp>genscripts.sh</samp> script uses this to set the default value of +<code>DATA_ALIGNMENT</code> when running the <samp>scripttempl</samp> script. +</p> +</dd> +<dt><code>TARGET_PAGE_SIZE</code></dt> +<dd><p>If <code>SEGMENT_SIZE</code> is not defined, the <samp>genscripts.sh</samp> script +uses this to define it. +</p> +</dd> +<dt><code>ALIGNMENT</code></dt> +<dd><p>Some <samp>scripttempl</samp> scripts set this to a number to pass to +<code>ALIGN</code> to set the required alignment for the <code>end</code> symbol. +</p></dd> +</dl> + +<hr> +<a name="linker-scripts"></a> +<div class="header"> +<p> +Next: <a href="#linker-emulations" accesskey="n" rel="next">linker emulations</a>, Previous: <a href="#emulation-parameters" accesskey="p" rel="previous">emulation parameters</a>, Up: <a href="#Emulations" accesskey="u" rel="up">Emulations</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p> +</div> +<a name="scripttempl-scripts"></a> +<h3 class="section">2.2 <samp>scripttempl</samp> scripts</h3> + +<p>Each linker target uses a <samp>scripttempl</samp> script to generate the +default linker scripts. The name of the <samp>scripttempl</samp> script is +set by the <code>SCRIPT_NAME</code> variable in the <samp>emulparams</samp> script. +If <code>SCRIPT_NAME</code> is set to <var>script</var>, <code>genscripts.sh</code> will +invoke <samp>scripttempl/<var>script</var>.sc</samp>. +</p> +<p>The <samp>genscripts.sh</samp> script will invoke the <samp>scripttempl</samp> +script 5 to 9 times. Each time it will set the shell variable +<code>LD_FLAG</code> 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</code> emulation entry point; +this describes the conventional behaviour). +</p> +<p>The <samp>scripttempl</samp> script should just write a linker script, written +in the linker command language, to standard output. If the emulation +name–the name of the <samp>emulparams</samp> file without the <samp>.sc</samp> +extension–is <var>emul</var>, then the output will be directed to +<samp>ldscripts/<var>emul</var>.<var>extension</var></samp> in the build directory, +where <var>extension</var> changes each time the <samp>scripttempl</samp> script is +invoked. +</p> +<p>Here is the list of values assigned to <code>LD_FLAG</code>. +</p> +<dl compact="compact"> +<dt><code>(empty)</code></dt> +<dd><p>The script generated is used by default (when none of the following +cases apply). The output has an extension of <samp>.x</samp>. +</p></dd> +<dt><code>n</code></dt> +<dd><p>The script generated is used when the linker is invoked with the +<code>-n</code> option. The output has an extension of <samp>.xn</samp>. +</p></dd> +<dt><code>N</code></dt> +<dd><p>The script generated is used when the linker is invoked with the +<code>-N</code> option. The output has an extension of <samp>.xbn</samp>. +</p></dd> +<dt><code>r</code></dt> +<dd><p>The script generated is used when the linker is invoked with the +<code>-r</code> option. The output has an extension of <samp>.xr</samp>. +</p></dd> +<dt><code>u</code></dt> +<dd><p>The script generated is used when the linker is invoked with the +<code>-Ur</code> option. The output has an extension of <samp>.xu</samp>. +</p></dd> +<dt><code>shared</code></dt> +<dd><p>The <samp>scripttempl</samp> script is only invoked with <code>LD_FLAG</code> set to +this value if <code>GENERATE_SHLIB_SCRIPT</code> is defined in the +<samp>emulparams</samp> file. The <samp>emultempl</samp> script must arrange to use +this script at the appropriate time, normally when the linker is invoked +with the <code>-shared</code> option. The output has an extension of +<samp>.xs</samp>. +</p></dd> +<dt><code>c</code></dt> +<dd><p>The <samp>scripttempl</samp> script is only invoked with <code>LD_FLAG</code> set to +this value if <code>GENERATE_COMBRELOC_SCRIPT</code> is defined in the +<samp>emulparams</samp> file or if <code>SCRIPT_NAME</code> is <code>elf</code>. The +<samp>emultempl</samp> script must arrange to use this script at the appropriate +time, normally when the linker is invoked with the <code>-z combreloc</code> +option. The output has an extension of +<samp>.xc</samp>. +</p></dd> +<dt><code>cshared</code></dt> +<dd><p>The <samp>scripttempl</samp> script is only invoked with <code>LD_FLAG</code> set to +this value if <code>GENERATE_COMBRELOC_SCRIPT</code> is defined in the +<samp>emulparams</samp> file or if <code>SCRIPT_NAME</code> is <code>elf</code> and +<code>GENERATE_SHLIB_SCRIPT</code> is defined in the <samp>emulparams</samp> file. +The <samp>emultempl</samp> script must arrange to use this script at the +appropriate time, normally when the linker is invoked with the <code>-shared +-z combreloc</code> option. The output has an extension of <samp>.xsc</samp>. +</p></dd> +<dt><code>auto_import</code></dt> +<dd><p>The <samp>scripttempl</samp> script is only invoked with <code>LD_FLAG</code> set to +this value if <code>GENERATE_AUTO_IMPORT_SCRIPT</code> is defined in the +<samp>emulparams</samp> file. The <samp>emultempl</samp> script must arrange to +use this script at the appropriate time, normally when the linker is +invoked with the <code>--enable-auto-import</code> option. The output has +an extension of <samp>.xa</samp>. +</p></dd> +</dl> + +<p>Besides the shell variables set by the <samp>emulparams</samp> script, and the +<code>LD_FLAG</code> variable, the <samp>genscripts.sh</samp> script will set +certain variables for each run of the <samp>scripttempl</samp> script. +</p> +<dl compact="compact"> +<dt><code>RELOCATING</code></dt> +<dd><p>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</code> and <code>-Ur</code>). +</p> +</dd> +<dt><code>CONSTRUCTING</code></dt> +<dd><p>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</code>). +</p> +</dd> +<dt><code>DATA_ALIGNMENT</code></dt> +<dd><p>This will be set to an <code>ALIGN</code> expression when the output should be +page aligned, or to ‘<samp>.</samp>’ when generating the <code>-N</code> script. +</p> +</dd> +<dt><code>CREATE_SHLIB</code></dt> +<dd><p>This will be set to a non-empty string when generating a <code>-shared</code> +script. +</p> +</dd> +<dt><code>COMBRELOC</code></dt> +<dd><p>This will be set to a non-empty string when generating <code>-z combreloc</code> +scripts to a temporary file name which can be used during script generation. +</p></dd> +</dl> + +<p>The conventional way to write a <samp>scripttempl</samp> script is to first +set a few shell variables, and then write out a linker script using +<code>cat</code> with a here document. The linker script will use variable +substitutions, based on the above variables and those set in the +<samp>emulparams</samp> script, to control its behaviour. +</p> +<p>When there are parts of the <samp>scripttempl</samp> script which should only +be run when doing a final relocation, they should be enclosed within a +variable substitution based on <code>RELOCATING</code>. For example, on many +targets special symbols such as <code>_end</code> should be defined when doing +a final link. Naturally, those symbols should not be defined when doing +a relocatable link using <code>-r</code>. The <samp>scripttempl</samp> script +could use a construct like this to define those symbols: +</p><div class="smallexample"> +<pre class="smallexample"> ${RELOCATING+ _end = .;} +</pre></div> +<p>This will do the symbol assignment only if the <code>RELOCATING</code> +variable is defined. +</p> +<p>The basic job of the linker script is to put the sections in the correct +order, and at the correct memory addresses. For some targets, the +linker script may have to do some other operations. +</p> +<p>For example, on most MIPS platforms, the linker is responsible for +defining the special symbol <code>_gp</code>, used to initialize the +<code>$gp</code> register. It must be set to the start of the small data +section plus <code>0x8000</code>. Naturally, it should only be defined when +doing a final relocation. This will typically be done like this: +</p><div class="smallexample"> +<pre class="smallexample"> ${RELOCATING+ _gp = ALIGN(16) + 0x8000;} +</pre></div> +<p>This line would appear just before the sections which compose the small +data section (‘<samp>.sdata</samp>’, ‘<samp>.sbss</samp>’). All those sections would be +contiguous in memory. +</p> +<p>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</samp>’ section, and the address of each global destructor in +a ‘<samp>.dtor</samp>’ section (this is done by defining +<code>ASM_OUTPUT_CONSTRUCTOR</code> and <code>ASM_OUTPUT_DESTRUCTOR</code> in the +<code>gcc</code> configuration files). The <code>gcc</code> runtime support +routines expect the constructor table to be named <code>__CTOR_LIST__</code>. +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</code> behaviour has changed slightly over the years, it is safest +to provide both). Here is a typical way that might be handled in a +<samp>scripttempl</samp> file. +</p><div class="smallexample"> +<pre class="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__ = .;} +</pre></div> +<p>The use of <code>CONSTRUCTING</code> 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. +</p> +<p>Embedded systems often need to set a stack address. This is normally +best done by using the <code>PROVIDE</code> construct with a default stack +address. This permits the user to easily override the stack address +using the <code>--defsym</code> option. Here is an example: +</p><div class="smallexample"> +<pre class="smallexample"> ${RELOCATING+ PROVIDE (__stack = 0x80000000);} +</pre></div> +<p>The value of the symbol <code>__stack</code> would then be used in the startup +code to initialize the stack pointer. +</p> +<hr> +<a name="linker-emulations"></a> +<div class="header"> +<p> +Previous: <a href="#linker-scripts" accesskey="p" rel="previous">linker scripts</a>, Up: <a href="#Emulations" accesskey="u" rel="up">Emulations</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p> +</div> +<a name="emultempl-scripts"></a> +<h3 class="section">2.3 <samp>emultempl</samp> scripts</h3> + +<p>Each linker target uses an <samp>emultempl</samp> script to generate the +emulation code. The name of the <samp>emultempl</samp> script is set by the +<code>TEMPLATE_NAME</code> variable in the <samp>emulparams</samp> script. If the +<code>TEMPLATE_NAME</code> variable is not set, the default is +‘<samp>generic</samp>’. If the value of <code>TEMPLATE_NAME</code> is <var>template</var>, +<samp>genscripts.sh</samp> will use <samp>emultempl/<var>template</var>.em</samp>. +</p> +<p>Most targets use the generic <samp>emultempl</samp> script, +<samp>emultempl/generic.em</samp>. A different <samp>emultempl</samp> script is +only needed if the linker must support unusual actions, such as linking +against shared libraries. +</p> +<p>The <samp>emultempl</samp> script is normally written as a simple invocation +of <code>cat</code> with a here document. The document will use a few +variable substitutions. Typically each function names uses a +substitution involving <code>EMULATION_NAME</code>, for ease of debugging when +the linker supports multiple emulations. +</p> +<p>Every function and variable in the emitted file should be static. The +only globally visible object must be named +<code>ld_<var>EMULATION_NAME</var>_emulation</code>, where <var>EMULATION_NAME</var> is +the name of the emulation set in <samp>configure.tgt</samp> (this is also the +name of the <samp>emulparams</samp> file without the <samp>.sh</samp> extension). +The <samp>genscripts.sh</samp> script will set the shell variable +<code>EMULATION_NAME</code> before invoking the <samp>emultempl</samp> script. +</p> +<p>The <code>ld_<var>EMULATION_NAME</var>_emulation</code> variable must be a +<code>struct ld_emulation_xfer_struct</code>, as defined in <samp>ldemul.h</samp>. +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</code> and the default BFD target name (normally +set from the shell variable <code>OUTPUT_FORMAT</code> which is normally set +by the <samp>emulparams</samp> file). +</p> +<p>The <samp>genscripts.sh</samp> script will set the shell variable +<code>COMPILE_IN</code> when it invokes the <samp>emultempl</samp> script for the +default emulation. In this case, the <samp>emultempl</samp> script should +include the linker scripts directly, and return them from the +<code>get_scripts</code> entry point. When the emulation is not the default, +the <code>get_scripts</code> entry point should just return a file name. See +<samp>emultempl/generic.em</samp> for an example of how this is done. +</p> +<p>At some point, the linker emulation entry points should be documented. +</p> +<hr> +<a name="Emulation-Walkthrough"></a> +<div class="header"> +<p> +Next: <a href="#Architecture-Specific" accesskey="n" rel="next">Architecture Specific</a>, Previous: <a href="#Emulations" accesskey="p" rel="previous">Emulations</a>, Up: <a href="#Top" accesskey="u" rel="up">Top</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p> +</div> +<a name="A-Walkthrough-of-a-Typical-Emulation"></a> +<h2 class="chapter">3 A Walkthrough of a Typical Emulation</h2> + +<p>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. +</p> +<p>The starting point for the linker is in <samp>ldmain.c</samp> where +<code>main</code> is defined. The bulk of the code that’s emulation +specific will initially be in <code>emultempl/<var>emulation</var>.em</code> but +will end up in <code>e<var>emulation</var>.c</code> when the build is done. +Most of the work to select and interface with emulations is in +<code>ldemul.h</code> and <code>ldemul.c</code>. Specifically, <code>ldemul.h</code> +defines the <code>ld_emulation_xfer_struct</code> structure your emulation +exports. +</p> +<p>Your emulation file exports a symbol +<code>ld_<var>EMULATION_NAME</var>_emulation</code>. If your emulation is +selected (it usually is, since usually there’s only one), +<code>ldemul.c</code> sets the variable <var>ld_emulation</var> to point to it. +<code>ldemul.c</code> also defines a number of API functions that interface +to your emulation, like <code>ldemul_after_parse</code> which simply calls +your <code>ld_<var>EMULATION</var>_emulation.after_parse</code> function. For +the rest of this section, the functions will be mentioned, but you +should assume the indirect reference to your emulation also. +</p> +<p>We will also skip or gloss over parts of the link process that don’t +relate to emulations, like setting up internationalization. +</p> +<p>After initialization, <code>main</code> selects an emulation by pre-scanning +the command-line arguments. It calls <code>ldemul_choose_target</code> to +choose a target. If you set <code>choose_target</code> to +<code>ldemul_default_target</code>, it picks your <code>target_name</code> by +default. +</p> +<p><code>main</code> calls <code>ldemul_before_parse</code>, then <code>parse_args</code>. +<code>parse_args</code> calls <code>ldemul_parse_args</code> for each arg, which +must update the <code>getopt</code> globals if it recognizes the argument. +If the emulation doesn’t recognize it, then parse_args checks to see +if it recognizes it. +</p> +<p>Now that the emulation has had access to all its command-line options, +<code>main</code> calls <code>ldemul_set_symbols</code>. 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. +</p> +<p><code>main</code> now calls <code>ldemul_get_script</code> to get the emulation +script to use (based on arguments, no doubt, see <a href="#Emulations">Emulations</a>) and +runs it. While parsing, <code>ldgram.y</code> may call <code>ldemul_hll</code> or +<code>ldemul_syslib</code> to handle the <code>HLL</code> or <code>SYSLIB</code> +commands. It may call <code>ldemul_unrecognized_file</code> if you asked +the linker to link a file it doesn’t recognize. It will call +<code>ldemul_recognized_file</code> 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</code>) and symbols and stuff. After it’s +done reading the script, <code>main</code> calls <code>ldemul_after_parse</code>. +Use the after-parse hook to set up anything that depends on stuff the +script might have set up, like the entry point. +</p> +<p><code>main</code> next calls <code>lang_process</code> in <code>ldlang.c</code>. 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</code>, and calls +<code>ldemul_create_output_section_statements</code> 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. +</p> +<p>(*) 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. +</p> +<p>Next, the objects to be linked are opened and BFDs created for them, +and <code>ldemul_after_open</code> is called. At this point, you have all +the objects and symbols loaded, but none of the data has been placed +yet. +</p> +<p>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</code> will get called to figure out where it goes. +Next it figures out the offsets for each section, calling +<code>ldemul_before_allocation</code> before and +<code>ldemul_after_allocation</code> after deciding where each input section +ends up in the output sections. +</p> +<p>The last part of <code>lang_process</code> is to figure out all the symbols’ +values. After assigning final values to the symbols, +<code>ldemul_finish</code> is called, and after that, any undefined symbols +are turned into fatal errors. +</p> +<p>OK, back to <code>main</code>, which calls <code>ldwrite</code> in +<samp>ldwrite.c</samp>. <code>ldwrite</code> calls BFD’s final_link, which does +all the relocation fixups and writes the output bfd to disk, and we’re +done. +</p> +<p>In summary, +</p> +<ul> +<li> <code>main()</code> in <samp>ldmain.c</samp> +</li><li> <samp>emultempl/<var>EMULATION</var>.em</samp> has your code +</li><li> <code>ldemul_choose_target</code> (defaults to your <code>target_name</code>) +</li><li> <code>ldemul_before_parse</code> +</li><li> Parse argv, calls <code>ldemul_parse_args</code> for each +</li><li> <code>ldemul_set_symbols</code> +</li><li> <code>ldemul_get_script</code> +</li><li> parse script + +<ul> +<li> may call <code>ldemul_hll</code> or <code>ldemul_syslib</code> +</li><li> may call <code>ldemul_open_dynamic_archive</code> +</li></ul> + +</li><li> <code>ldemul_after_parse</code> +</li><li> <code>lang_process()</code> in <samp>ldlang.c</samp> + +<ul> +<li> create <code>output_bfd</code> +</li><li> <code>ldemul_set_output_arch</code> +</li><li> <code>ldemul_create_output_section_statements</code> +</li><li> read objects, create input bfds - all symbols exist, but have no values +</li><li> may call <code>ldemul_unrecognized_file</code> +</li><li> will call <code>ldemul_recognized_file</code> +</li><li> <code>ldemul_after_open</code> +</li><li> map input sections to output sections +</li><li> may call <code>ldemul_place_orphan</code> for remaining sections +</li><li> <code>ldemul_before_allocation</code> +</li><li> gives input sections offsets into output sections, places output sections +</li><li> <code>ldemul_after_allocation</code> - section addresses valid +</li><li> assigns values to symbols +</li><li> <code>ldemul_finish</code> - symbol values valid +</li></ul> + +</li><li> output bfd is written to disk + +</li></ul> + +<hr> +<a name="Architecture-Specific"></a> +<div class="header"> +<p> +Next: <a href="#GNU-Free-Documentation-License" accesskey="n" rel="next">GNU Free Documentation License</a>, Previous: <a href="#Emulation-Walkthrough" accesskey="p" rel="previous">Emulation Walkthrough</a>, Up: <a href="#Top" accesskey="u" rel="up">Top</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p> +</div> +<a name="Some-Architecture-Specific-Notes"></a> +<h2 class="chapter">4 Some Architecture Specific Notes</h2> + +<p>This is the place for notes on the behavior of <code>ld</code> on +specific platforms. Currently, only Intel x86 is documented (and +of that, only the auto-import behavior for DLLs). +</p> +<table class="menu" border="0" cellspacing="0"> +<tr><td align="left" valign="top">• <a href="#ix86" accesskey="1">ix86</a>:</td><td> </td><td align="left" valign="top">Intel x86 +</td></tr> +</table> + +<hr> +<a name="ix86"></a> +<div class="header"> +<p> +Up: <a href="#Architecture-Specific" accesskey="u" rel="up">Architecture Specific</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p> +</div> +<a name="Intel-x86"></a> +<h3 class="section">4.1 Intel x86</h3> + +<dl compact="compact"> +<dd><p><code>ld</code> 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. +</p> +</dd> +<dt><em>auto-import from DLLs</em></dt> +<dd><ol> +<li> 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</code> flag, or disabled via the +<code>--disable-auto-import</code> flag. Auto-import is disabled by default. + +</li><li> 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. + +</li><li> 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. + +</li><li> 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. +</li></ol> + +<p>Motivation +</p> +<p>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 +</p> +<p><code>mov dll_var,%eax,</code> +</p> +<p>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. +</p> +<p>Implementation +</p> +<p>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. +</p> +</dd> +</dl> + +<hr> +<a name="GNU-Free-Documentation-License"></a> +<div class="header"> +<p> +Previous: <a href="#Architecture-Specific" accesskey="p" rel="previous">Architecture Specific</a>, Up: <a href="#Top" accesskey="u" rel="up">Top</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p> +</div> +<a name="GNU-Free-Documentation-License-1"></a> +<h2 class="chapter">5 GNU Free Documentation License</h2> + +<div align="center">Version 1.3, 3 November 2008 +</div> + +<div class="display"> +<pre class="display">Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. +<a href="http://fsf.org/">http://fsf.org/</a> + +Everyone is permitted to copy and distribute verbatim copies +of this license document, but changing it is not allowed. +</pre></div> + +<ol> +<li> PREAMBLE + +<p>The purpose of this License is to make a manual, textbook, or other +functional and useful document <em>free</em> in the sense of freedom: to +assure everyone the effective freedom to copy and redistribute it, +with or without modifying it, either commercially or noncommercially. +Secondarily, this License preserves for the author and publisher a way +to get credit for their work, while not being considered responsible +for modifications made by others. +</p> +<p>This License is a kind of “copyleft”, which means that derivative +works of the document must themselves be free in the same sense. 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A copy of the license is included in the section entitled ``GNU + Free Documentation License''. +</pre></div> + +<p>If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, +replace the “with…Texts.” line with this: +</p> +<div class="smallexample"> +<pre class="smallexample"> with the Invariant Sections being <var>list their titles</var>, with + the Front-Cover Texts being <var>list</var>, and with the Back-Cover Texts + being <var>list</var>. +</pre></div> + +<p>If you have Invariant Sections without Cover Texts, or some other +combination of the three, merge those two alternatives to suit the +situation. +</p> +<p>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. +</p> + +<a name="SEC_Contents"></a> +<h2 class="contents-heading">Table of Contents</h2> + +<div class="contents"> + +<ul class="no-bullet"> + <li><a name="toc-The-README-File" href="#README">1 The <samp>README</samp> File</a></li> + <li><a name="toc-How-linker-emulations-are-generated" href="#Emulations">2 How linker emulations are generated</a> + <ul class="no-bullet"> + <li><a name="toc-emulparams-scripts" href="#emulation-parameters">2.1 <samp>emulparams</samp> scripts</a></li> + <li><a name="toc-scripttempl-scripts" href="#linker-scripts">2.2 <samp>scripttempl</samp> scripts</a></li> + <li><a name="toc-emultempl-scripts" href="#linker-emulations">2.3 <samp>emultempl</samp> scripts</a></li> + </ul></li> + <li><a name="toc-A-Walkthrough-of-a-Typical-Emulation" href="#Emulation-Walkthrough">3 A Walkthrough of a Typical Emulation</a></li> + <li><a name="toc-Some-Architecture-Specific-Notes" href="#Architecture-Specific">4 Some Architecture Specific Notes</a> + <ul class="no-bullet"> + <li><a name="toc-Intel-x86" href="#ix86">4.1 Intel x86</a></li> + </ul></li> + <li><a name="toc-GNU-Free-Documentation-License-1" href="#GNU-Free-Documentation-License">5 GNU Free Documentation License</a></li> +</ul> +</div> + +<hr> + + + +</body> +</html> |