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87 lines
4.4 KiB
HTML
87 lines
4.4 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Comm - Using as</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="Using as">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Pseudo-Ops.html#Pseudo-Ops" title="Pseudo Ops">
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<link rel="prev" href="CFI-directives.html#CFI-directives" title="CFI directives">
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<link rel="next" href="Data.html#Data" title="Data">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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This file documents the GNU Assembler "as".
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Copyright (C) 1991-2015 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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-->
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<style type="text/css"><!--
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span.sc { font-variant:small-caps }
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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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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Comm"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Data.html#Data">Data</a>,
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Previous: <a rel="previous" accesskey="p" href="CFI-directives.html#CFI-directives">CFI directives</a>,
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Up: <a rel="up" accesskey="u" href="Pseudo-Ops.html#Pseudo-Ops">Pseudo Ops</a>
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<hr>
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</div>
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<h3 class="section">7.11 <code>.comm </code><var>symbol</var><code> , </code><var>length</var> </h3>
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<p><a name="index-g_t_0040code_007bcomm_007d-directive-305"></a><a name="index-symbol_002c-common-306"></a><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>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, <samp><span class="command">as</span></samp> 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 rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>.
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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><span class="samp"> .comm, </span><var>length</var></samp>’; <var>symbol</var> is optional.
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<div class="footnote">
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<hr>
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<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> 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><span class="samp">--section-alignment</span></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><span class="samp">*.o</span></samp>’) files, which are less strictly aligned.</p>
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<hr></div>
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</body></html>
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