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124 lines
6.4 KiB
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<title>Output Section Keywords - Untitled</title>
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<link title="Top" rel="start" href="index.html#Top">
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<!--
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This file documents the GNU linker LD
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(GNU Toolchain for the A-profile Architecture 9.2-2019.12 (arm-9.10))
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version 2.33.1.
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Copyright (C) 1991-2019 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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<div class="node">
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<a name="Output-Section-Keywords"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Output-Section-Discarding.html#Output-Section-Discarding">Output Section Discarding</a>,
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Previous: <a rel="previous" accesskey="p" href="Output-Section-Data.html#Output-Section-Data">Output Section Data</a>,
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Up: <a rel="up" accesskey="u" href="SECTIONS.html#SECTIONS">SECTIONS</a>
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<hr>
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</div>
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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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<a name="index-CREATE_005fOBJECT_005fSYMBOLS-486"></a>
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<a name="index-input-filename-symbols-487"></a>
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<a name="index-filename-symbols-488"></a>
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<dl><dt><code>CREATE_OBJECT_SYMBOLS</code><dd>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>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><a name="index-CONSTRUCTORS-489"></a><a name="index-C_002b_002b-constructors_002c-arranging-in-link-490"></a><a name="index-constructors_002c-arranging-in-link-491"></a><br><dt><code>CONSTRUCTORS</code><dd>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>The symbol <code>__CTOR_LIST__</code><!-- /@w --> marks the start of the global
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constructors, and the symbol <code>__CTOR_END__</code><!-- /@w --> marks the end.
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Similarly, <code>__DTOR_LIST__</code><!-- /@w --> and <code>__DTOR_END__</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 <span class="sc">gnu</span> 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>. <span class="sc">gnu</span> 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>For object file formats such as <code>COFF</code> or <code>ELF</code> which support
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arbitrary section names, <span class="sc">gnu</span> 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 <span class="sc">gnu</span> C++
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runtime code expects to see.
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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>
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<p>If you are using the <span class="sc">gnu</span> 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><span class="samp">SORT_BY_NAME(CONSTRUCTORS)</span></samp>’ instead. When using the
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<code>.ctors</code> and <code>.dtors</code> sections, use ‘<samp><span class="samp">*(SORT_BY_NAME(.ctors))</span></samp>’ and
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‘<samp><span class="samp">*(SORT_BY_NAME(.dtors))</span></samp>’ instead of just ‘<samp><span class="samp">*(.ctors)</span></samp>’ and
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‘<samp><span class="samp">*(.dtors)</span></samp>’.
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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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</dl>
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</body></html>
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