164 lines
7.9 KiB
HTML
164 lines
7.9 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- This file documents the GNU linker LD
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(Linaro_Binutils-2017.02)
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version 2.27.0.
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Copyright (C) 1991-2016 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, with no Front-Cover Texts, and with no
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Back-Cover Texts. A copy of the license is included in the
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section entitled "GNU Free Documentation License". -->
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>LD: Output Section Keywords</title>
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<meta name="description" content="LD: Output Section Keywords">
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<meta name="keywords" content="LD: Output Section Keywords">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="LD-Index.html#LD-Index" rel="index" title="LD Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="SECTIONS.html#SECTIONS" rel="up" title="SECTIONS">
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<link href="Output-Section-Discarding.html#Output-Section-Discarding" rel="next" title="Output Section Discarding">
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<link href="Output-Section-Data.html#Output-Section-Data" rel="prev" title="Output Section Data">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.indentedblock {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smallexample {margin-left: 3.2em}
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div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style:oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nocodebreak {white-space:nowrap}
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span.nolinebreak {white-space:nowrap}
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span.roman {font-family:serif; font-weight:normal}
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span.sansserif {font-family:sans-serif; font-weight:normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Output-Section-Keywords"></a>
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<div class="header">
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<p>
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Next: <a href="Output-Section-Discarding.html#Output-Section-Discarding" accesskey="n" rel="next">Output Section Discarding</a>, Previous: <a href="Output-Section-Data.html#Output-Section-Data" accesskey="p" rel="prev">Output Section Data</a>, Up: <a href="SECTIONS.html#SECTIONS" accesskey="u" rel="up">SECTIONS</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Output-Section-Keywords-1"></a>
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<h4 class="subsection">3.6.6 Output Section Keywords</h4>
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<p>There are a couple of keywords which can appear as output section
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commands.
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</p>
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<dl compact="compact">
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<dd><a name="index-CREATE_005fOBJECT_005fSYMBOLS"></a>
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<a name="index-input-filename-symbols"></a>
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<a name="index-filename-symbols"></a>
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</dd>
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<dt><code>CREATE_OBJECT_SYMBOLS</code></dt>
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<dd><p>The command tells the linker to create a symbol for each input file.
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The name of each symbol will be the name of the corresponding input
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file. The section of each symbol will be the output section in which
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the <code>CREATE_OBJECT_SYMBOLS</code> command appears.
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</p>
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<p>This is conventional for the a.out object file format. It is not
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normally used for any other object file format.
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</p>
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<a name="index-CONSTRUCTORS"></a>
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<a name="index-C_002b_002b-constructors_002c-arranging-in-link"></a>
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<a name="index-constructors_002c-arranging-in-link"></a>
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</dd>
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<dt><code>CONSTRUCTORS</code></dt>
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<dd><p>When linking using the a.out object file format, the linker uses an
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unusual set construct to support C++ global constructors and
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destructors. When linking object file formats which do not support
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arbitrary sections, such as ECOFF and XCOFF, the linker will
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automatically recognize C++ global constructors and destructors by name.
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For these object file formats, the <code>CONSTRUCTORS</code> command tells the
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linker to place constructor information in the output section where the
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<code>CONSTRUCTORS</code> command appears. The <code>CONSTRUCTORS</code> command is
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ignored for other object file formats.
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</p>
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<p>The symbol <code><span class="nolinebreak">__CTOR_LIST__</span></code><!-- /@w --> marks the start of the global
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constructors, and the symbol <code><span class="nolinebreak">__CTOR_END__</span></code><!-- /@w --> marks the end.
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Similarly, <code><span class="nolinebreak">__DTOR_LIST__</span></code><!-- /@w --> and <code><span class="nolinebreak">__DTOR_END__</span></code><!-- /@w --> mark
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the start and end of the global destructors. The
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first word in the list is the number of entries, followed by the address
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of each constructor or destructor, followed by a zero word. The
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compiler must arrange to actually run the code. For these object file
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formats <small>GNU</small> C++ normally calls constructors from a subroutine
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<code>__main</code>; a call to <code>__main</code> is automatically inserted into
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the startup code for <code>main</code>. <small>GNU</small> C++ normally runs
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destructors either by using <code>atexit</code>, or directly from the function
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<code>exit</code>.
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</p>
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<p>For object file formats such as <code>COFF</code> or <code>ELF</code> which support
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arbitrary section names, <small>GNU</small> C++ will normally arrange to put the
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addresses of global constructors and destructors into the <code>.ctors</code>
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and <code>.dtors</code> sections. Placing the following sequence into your
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linker script will build the sort of table which the <small>GNU</small> C++
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runtime code expects to see.
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</p>
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<div class="smallexample">
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<pre class="smallexample"> __CTOR_LIST__ = .;
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LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)
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*(.ctors)
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LONG(0)
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__CTOR_END__ = .;
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__DTOR_LIST__ = .;
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LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)
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*(.dtors)
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LONG(0)
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__DTOR_END__ = .;
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</pre></div>
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<p>If you are using the <small>GNU</small> C++ support for initialization priority,
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which provides some control over the order in which global constructors
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are run, you must sort the constructors at link time to ensure that they
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are executed in the correct order. When using the <code>CONSTRUCTORS</code>
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command, use ‘<samp>SORT_BY_NAME(CONSTRUCTORS)</samp>’ instead. When using the
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<code>.ctors</code> and <code>.dtors</code> sections, use ‘<samp>*(SORT_BY_NAME(.ctors))</samp>’ and
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‘<samp>*(SORT_BY_NAME(.dtors))</samp>’ instead of just ‘<samp>*(.ctors)</samp>’ and
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‘<samp>*(.dtors)</samp>’.
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</p>
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<p>Normally the compiler and linker will handle these issues automatically,
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and you will not need to concern yourself with them. However, you may
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need to consider this if you are using C++ and writing your own linker
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scripts.
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</p>
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</dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Output-Section-Discarding.html#Output-Section-Discarding" accesskey="n" rel="next">Output Section Discarding</a>, Previous: <a href="Output-Section-Data.html#Output-Section-Data" accesskey="p" rel="prev">Output Section Data</a>, Up: <a href="SECTIONS.html#SECTIONS" accesskey="u" rel="up">SECTIONS</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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