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<h3 class="section">10.3 Traditional miscellany</h3>

<p>Here are some things to be aware of when using the traditional
preprocessor.

     <ul>
<li>Preprocessing directives are recognized only when their leading
&lsquo;<samp><span class="samp">#</span></samp>&rsquo; appears in the first column.  There can be no whitespace
between the beginning of the line and the &lsquo;<samp><span class="samp">#</span></samp>&rsquo;, but whitespace can
follow the &lsquo;<samp><span class="samp">#</span></samp>&rsquo;.

     <li>A true traditional C preprocessor does not recognize &lsquo;<samp><span class="samp">#error</span></samp>&rsquo; or
&lsquo;<samp><span class="samp">#pragma</span></samp>&rsquo;, and may not recognize &lsquo;<samp><span class="samp">#elif</span></samp>&rsquo;.  CPP supports all
the directives in traditional mode that it supports in ISO mode,
including extensions, with the exception that the effects of
&lsquo;<samp><span class="samp">#pragma GCC poison</span></samp>&rsquo; are undefined.

     <li>__STDC__ is not defined.

     <li>If you use digraphs the behavior is undefined.

     <li>If a line that looks like a directive appears within macro arguments,
the behavior is undefined.

   </ul>

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