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<a name="M4-Dependency"></a>
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<a name="M4-Dependency-1"></a>
<h3 class="section">A.3 M4 Dependency</h3>
<a name="index-m4"></a>
<p>The macro processor <code>m4</code><a name="DOCF8" href="#FOOT8"><sup>8</sup></a>
must be installed wherever flex is installed.
<code>flex</code> invokes &lsquo;<samp>m4</samp>&rsquo;, found by searching the directories in the
<code>PATH</code> environment variable. Any code you place in section 1 or in the
actions will be sent through m4. Please follow these rules to protect your
code from unwanted <code>m4</code> processing.
</p>
<ul>
<li> Do not use symbols that begin with, &lsquo;<samp>m4_</samp>&rsquo;, such as, &lsquo;<samp>m4_define</samp>&rsquo;,
or &lsquo;<samp>m4_include</samp>&rsquo;, since those are reserved for <code>m4</code> macro names. If for 
some reason you need m4_ as a prefix, use a preprocessor #define to get your
symbol past m4 unmangled.

</li><li> Do not use the strings &lsquo;<samp>[[</samp>&rsquo; or &lsquo;<samp>]]</samp>&rsquo; anywhere in your code. The
former is not valid in C, except within comments and strings, but the latter is valid in
code such as <code>x[y[z]]</code>. The solution is simple. To get the literal string 
<code>&quot;]]&quot;</code>, use <code>&quot;]&quot;&quot;]&quot;</code>. To get the array notation <code>x[y[z]]</code>,
use <code>x[y[z] ]</code>. Flex will attempt to detect these sequences in user code, and
escape them. However, it&rsquo;s best to avoid this complexity where possible, by
removing such sequences from your code.

</li></ul>

<p><code>m4</code> is only required at the time you run <code>flex</code>. The generated
scanner is ordinary C or C++, and does <em>not</em> require <code>m4</code>.
</p>
<div class="footnote">
<hr>
<h4 class="footnotes-heading">Footnotes</h4>

<h3><a name="FOOT8" href="#DOCF8">(8)</a></h3>
<p>The use of m4 is subject to change in
future revisions of flex. It is not part of the public API of flex. Do not depend on it.</p>
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flex-doc 2.6.4-6 / usr / share / doc / flex-doc / html / M4-Dependency.html
