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119 lines
5.7 KiB
HTML
119 lines
5.7 KiB
HTML
<!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 Assembler "as".
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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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Back-Cover Texts. A copy of the license is included in the
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Comm (Using as)</title>
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<meta name="description" content="Comm (Using as)">
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<meta name="keywords" content="Comm (Using as)">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="AS-Index.html#AS-Index" rel="index" title="AS Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Pseudo-Ops.html#Pseudo-Ops" rel="up" title="Pseudo Ops">
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<link href="Data.html#Data" rel="next" title="Data">
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<link href="CFI-directives.html#CFI-directives" rel="prev" title="CFI directives">
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</head>
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<body lang="en">
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<a name="Comm"></a>
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<div class="header">
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<p>
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Next: <a href="Data.html#Data" accesskey="n" rel="next">Data</a>, Previous: <a href="CFI-directives.html#CFI-directives" accesskey="p" rel="prev">CFI directives</a>, Up: <a href="Pseudo-Ops.html#Pseudo-Ops" accesskey="u" rel="up">Pseudo Ops</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="g_t_002ecomm-symbol-_002c-length-"></a>
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<h3 class="section">7.11 <code>.comm <var>symbol</var> , <var>length</var> </code></h3>
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<a name="index-comm-directive"></a>
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<a name="index-symbol_002c-common"></a>
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<p><code>.comm</code> declares a common symbol named <var>symbol</var>. When linking, a
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common symbol in one object file may be merged with a defined or common symbol
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of the same name in another object file. If <code>ld</code> does not see a
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definition for the symbol–just one or more common symbols–then it will
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allocate <var>length</var> bytes of uninitialized memory. <var>length</var> must be an
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absolute expression. If <code>ld</code> sees multiple common symbols with
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the same name, and they do not all have the same size, it will allocate space
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using the largest size.
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</p>
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<p>When using ELF or (as a GNU extension) PE, the <code>.comm</code> directive takes
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an optional third argument. This is the desired alignment of the symbol,
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specified for ELF as a byte boundary (for example, an alignment of 16 means
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that the least significant 4 bits of the address should be zero), and for PE
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as a power of two (for example, an alignment of 5 means aligned to a 32-byte
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boundary). The alignment must be an absolute expression, and it must be a
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power of two. If <code>ld</code> allocates uninitialized memory for the
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common symbol, it will use the alignment when placing the symbol. If no
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alignment is specified, <code>as</code> will set the alignment to the
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largest power of two less than or equal to the size of the symbol, up to a
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maximum of 16 on ELF, or the default section alignment of 4 on PE<a name="DOCF1" href="#FOOT1"><sup>1</sup></a>.
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</p>
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<p>The syntax for <code>.comm</code> differs slightly on the HPPA. The syntax is
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‘<samp><var>symbol</var> .comm, <var>length</var></samp>’; <var>symbol</var> is optional.
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</p>
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<div class="footnote">
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<hr>
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<h4 class="footnotes-heading">Footnotes</h4>
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<h3><a name="FOOT1" href="#DOCF1">(1)</a></h3>
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<p>This
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is not the same as the executable image file alignment controlled by <code>ld</code>’s
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‘<samp>--section-alignment</samp>’ option; image file sections in PE are aligned to
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multiples of 4096, which is far too large an alignment for ordinary variables.
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It is rather the default alignment for (non-debug) sections within object
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(‘<samp>*.o</samp>’) files, which are less strictly aligned.</p>
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</div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Data.html#Data" accesskey="n" rel="next">Data</a>, Previous: <a href="CFI-directives.html#CFI-directives" accesskey="p" rel="prev">CFI directives</a>, Up: <a href="Pseudo-Ops.html#Pseudo-Ops" accesskey="u" rel="up">Pseudo Ops</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-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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