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<a name="Yacc"></a>
<div class="header">
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<a name="Interfacing-with-Yacc"></a>
<h2 class="chapter">15 Interfacing with Yacc</h2>

<a name="index-yacc_002c-interface"></a>

<a name="index-yylval_002c-with-yacc"></a>
<p>One of the main uses of <code>flex</code> is as a companion to the <code>yacc</code>
parser-generator.  <code>yacc</code> parsers expect to call a routine named
<code>yylex()</code> to find the next input token.  The routine is supposed to
return the type of the next token as well as putting any associated
value in the global <code>yylval</code>.  To use <code>flex</code> with <code>yacc</code>,
one specifies the &lsquo;<samp>-d</samp>&rsquo; option to <code>yacc</code> to instruct it to
generate the file <samp>y.tab.h</samp> containing definitions of all the
<code>%tokens</code> appearing in the <code>yacc</code> input.  This file is then
included in the <code>flex</code> scanner.  For example, if one of the tokens
is <code>TOK_NUMBER</code>, part of the scanner might look like:
</p>
<a name="index-yacc-interface"></a>
<div class="example">
<pre class="verbatim">    %{
    #include &quot;y.tab.h&quot;
    %}

    %%

    [0-9]+        yylval = atoi( yytext ); return TOK_NUMBER;
</pre></div>




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flex-doc 2.6.4-6 / usr / share / doc / flex-doc / html / Yacc.html
