72 lines
3.5 KiB
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72 lines
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HTML
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This file documents the GNU Assembler "as".
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Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001, 2002,
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2006, 2007 Free Software Foundation, Inc.
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<a name="Comm"></a>Next: <a rel="next" accesskey="n" href="CFI-directives.html#CFI-directives">CFI directives</a>,
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Previous: <a rel="previous" accesskey="p" href="Byte.html#Byte">Byte</a>,
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Up: <a rel="up" accesskey="u" href="Pseudo-Ops.html#Pseudo-Ops">Pseudo Ops</a>
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<h3 class="section">7.8 <code>.comm </code><var>symbol</var><code> , </code><var>length</var> </h3>
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<p><a name="index-_0040code_007bcomm_007d-directive-288"></a><a name="index-symbol_002c-common-289"></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, the <code>.comm</code> directive takes an optional third argument.
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This is the desired alignment of the symbol, specified as a byte boundary (for
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example, an alignment of 16 means that the least significant 4 bits of the
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address should be zero). The alignment must be an absolute expression, and it
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must be a power of two. If <code>ld</code> allocates uninitialized memory
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for the common symbol, it will use the alignment when placing the symbol. If
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no alignment is specified, <span class="command">as</span> 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.
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<p>The syntax for <code>.comm</code> differs slightly on the HPPA. The syntax is
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<var>symbol</var><span class="samp"> .comm, </span><var>length</var>; <var>symbol</var> is optional.
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