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<a name="Debugging-Options"></a>
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<p>
Next: <a href="Miscellaneous-Options.html#Miscellaneous-Options" accesskey="n" rel="next">Miscellaneous Options</a>, Previous: <a href="Options-for-Scanner-Speed-and-Size.html#Options-for-Scanner-Speed-and-Size" accesskey="p" rel="previous">Options for Scanner Speed and Size</a>, Up: <a href="Scanner-Options.html#Scanner-Options" accesskey="u" rel="up">Scanner Options</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Debugging-Options-1"></a>
<h3 class="section">16.5 Debugging Options</h3>

<dl compact="compact">
<dd>
<a name="option_002dbackup"></a><a name="index-_002db"></a>
<a name="index-_002d_002d_002dbackup"></a>
<a name="index-backup"></a>
</dd>
<dt>&lsquo;<samp>-b, --backup, <code>%option backup</code></samp>&rsquo;</dt>
<dd><p>Generate backing-up information to <samp>lex.backup</samp>.  This is a list of
scanner states which require backing up and the input characters on
which they do so.  By adding rules one can remove backing-up states.  If
<em>all</em> backing-up states are eliminated and &lsquo;<samp>-Cf</samp>&rsquo; or <code>-CF</code>
is used, the generated scanner will run faster (see the &lsquo;<samp>--perf-report</samp>&rsquo; flag).
Only users who wish to squeeze every last cycle out of their scanners
need worry about this option.  (see <a href="Performance.html#Performance">Performance</a>).
</p>


<a name="option_002ddebug"></a><a name="index-_002dd"></a>
<a name="index-_002d_002d_002ddebug"></a>
<a name="index-debug"></a>
</dd>
<dt>&lsquo;<samp>-d, --debug, <code>%option debug</code></samp>&rsquo;</dt>
<dd><p>makes the generated scanner run in <em>debug</em> mode.  Whenever a pattern
is recognized and the global variable <code>yy_flex_debug</code> is non-zero
(which is the default), the scanner will write to <samp>stderr</samp> a line
of the form:
</p>
<div class="example">
<pre class="verbatim">    -accepting rule at line 53 (&quot;the matched text&quot;)
</pre></div>

<p>The line number refers to the location of the rule in the file defining
the scanner (i.e., the file that was fed to flex).  Messages are also
generated when the scanner backs up, accepts the default rule, reaches
the end of its input buffer (or encounters a NUL; at this point, the two
look the same as far as the scanner&rsquo;s concerned), or reaches an
end-of-file.
</p>


<a name="option_002dperf_002dreport"></a><a name="index-_002dp"></a>
<a name="index-_002d_002d_002dperf_002dreport"></a>
<a name="index-perf_002dreport"></a>
</dd>
<dt>&lsquo;<samp>-p, --perf-report, <code>%option perf-report</code></samp>&rsquo;</dt>
<dd><p>generates a performance report to <samp>stderr</samp>.  The report consists of
comments regarding features of the <code>flex</code> input file which will
cause a serious loss of performance in the resulting scanner.  If you
give the flag twice, you will also get comments regarding features that
lead to minor performance losses.
</p>
<p>Note that the use of <code>REJECT</code>, and
variable trailing context (see <a href="Limitations.html#Limitations">Limitations</a>) entails a substantial
performance penalty; use of <code>yymore()</code>, the &lsquo;<samp>^</samp>&rsquo; operator, and
the &lsquo;<samp>--interactive</samp>&rsquo; flag entail minor performance penalties.
</p>


<a name="option_002dnodefault"></a><a name="index-_002ds"></a>
<a name="index-_002d_002d_002dnodefault"></a>
<a name="index-nodefault"></a>
</dd>
<dt>&lsquo;<samp>-s, --nodefault, <code>%option nodefault</code></samp>&rsquo;</dt>
<dd><p>causes the <em>default rule</em> (that unmatched scanner input is echoed
to <samp>stdout)</samp> to be suppressed.  If the scanner encounters input
that does not match any of its rules, it aborts with an error.  This
option is useful for finding holes in a scanner&rsquo;s rule set.
</p>


<a name="option_002dtrace"></a><a name="index-_002dT"></a>
<a name="index-_002d_002d_002dtrace"></a>
<a name="index-trace"></a>
</dd>
<dt>&lsquo;<samp>-T, --trace, <code>%option trace</code></samp>&rsquo;</dt>
<dd><p>makes <code>flex</code> run in <em>trace</em> mode.  It will generate a lot of
messages to <samp>stderr</samp> concerning the form of the input and the
resultant non-deterministic and deterministic finite automata.  This
option is mostly for use in maintaining <code>flex</code>.
</p>


<a name="option_002dnowarn"></a><a name="index-_002dw"></a>
<a name="index-_002d_002d_002dnowarn"></a>
<a name="index-nowarn"></a>
</dd>
<dt>&lsquo;<samp>-w, --nowarn, <code>%option nowarn</code></samp>&rsquo;</dt>
<dd><p>suppresses warning messages.
</p>


<a name="option_002dverbose"></a><a name="index-_002dv"></a>
<a name="index-_002d_002d_002dverbose"></a>
<a name="index-verbose"></a>
</dd>
<dt>&lsquo;<samp>-v, --verbose, <code>%option verbose</code></samp>&rsquo;</dt>
<dd><p>specifies that <code>flex</code> should write to <samp>stderr</samp> a summary of
statistics regarding the scanner it generates.  Most of the statistics
are meaningless to the casual <code>flex</code> user, but the first line
identifies the version of <code>flex</code> (same as reported by &lsquo;<samp>--version</samp>&rsquo;),
and the next line the flags used when generating the scanner, including
those that are on by default.
</p>


<a name="option_002dwarn"></a><a name="index-_002d_002d_002dwarn"></a>
<a name="index-warn"></a>
</dd>
<dt>&lsquo;<samp>--warn, <code>%option warn</code></samp>&rsquo;</dt>
<dd><p>warn about certain things. In particular, if the default rule can be
matched but no default rule has been given, the flex will warn you.
We recommend using this option always.
</p>
</dd>
</dl>

<hr>
<div class="header">
<p>
Next: <a href="Miscellaneous-Options.html#Miscellaneous-Options" accesskey="n" rel="next">Miscellaneous Options</a>, Previous: <a href="Options-for-Scanner-Speed-and-Size.html#Options-for-Scanner-Speed-and-Size" accesskey="p" rel="previous">Options for Scanner Speed and Size</a>, Up: <a href="Scanner-Options.html#Scanner-Options" accesskey="u" rel="up">Scanner Options</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p>
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