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<h3 class="section">7.11 <code>.comm </code><var>symbol</var><code> , </code><var>length</var> </h3>

<p><a name="index-g_t_0040code_007bcomm_007d-directive-307"></a><a name="index-symbol_002c-common-308"></a><code>.comm</code> declares a common symbol named <var>symbol</var>.  When linking, a
common symbol in one object file may be merged with a defined or common symbol
of the same name in another object file.  If <code>ld</code> does not see a
definition for the symbol&ndash;just one or more common symbols&ndash;then it will
allocate <var>length</var> bytes of uninitialized memory.  <var>length</var> must be an
absolute expression.  If <code>ld</code> sees multiple common symbols with
the same name, and they do not all have the same size, it will allocate space
using the largest size.

   <p>When using ELF or (as a GNU extension) PE, the <code>.comm</code> directive takes
an optional third argument.  This is the desired alignment of the symbol,
specified for ELF as a byte boundary (for example, an alignment of 16 means
that the least significant 4 bits of the address should be zero), and for PE
as a power of two (for example, an alignment of 5 means aligned to a 32-byte
boundary).  The alignment must be an absolute expression, and it must be a
power of two.  If <code>ld</code> allocates uninitialized memory for the
common symbol, it will use the alignment when placing the symbol.  If no
alignment is specified, <samp><span class="command">as</span></samp> will set the alignment to the
largest power of two less than or equal to the size of the symbol, up to a
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>.

   <p>The syntax for <code>.comm</code> differs slightly on the HPPA.  The syntax is
&lsquo;<samp><var>symbol</var><span class="samp"> .comm, </span><var>length</var></samp>&rsquo;; <var>symbol</var> is optional.

   <div class="footnote">
<hr>
<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> This
is not the same as the executable image file alignment controlled by <code>ld</code>'s
&lsquo;<samp><span class="samp">--section-alignment</span></samp>&rsquo; option; image file sections in PE are aligned to
multiples of 4096, which is far too large an alignment for ordinary variables. 
It is rather the default alignment for (non-debug) sections within object
(&lsquo;<samp><span class="samp">*.o</span></samp>&rsquo;) files, which are less strictly aligned.</p>

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