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<h3 class="section">7.8 <code>.comm </code><var>symbol</var><code> , </code><var>length</var> </h3>
<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
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–just one or more common symbols–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, the <code>.comm</code> directive takes an optional third argument.
This is the desired alignment of the symbol, specified as a byte boundary (for
example, an alignment of 16 means that the least significant 4 bits of the
address should be zero). 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, <span class="command">as</span> 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.
<p>The syntax for <code>.comm</code> differs slightly on the HPPA. The syntax is
<var>symbol</var><span class="samp"> .comm, </span><var>length</var>; <var>symbol</var> is optional.
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