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<a name="Options-for-Scanner-Speed-and-Size"></a>
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<p>
Next: <a href="Debugging-Options.html#Debugging-Options" accesskey="n" rel="next">Debugging Options</a>, Previous: <a href="Code_002dLevel-And-API-Options.html#Code_002dLevel-And-API-Options" accesskey="p" rel="prev">Code-Level And API Options</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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<hr>
<a name="Options-for-Scanner-Speed-and-Size-1"></a>
<h3 class="section">16.4 Options for Scanner Speed and Size</h3>

<dl compact="compact">
<dt>&lsquo;<samp>-C[aefFmr]</samp>&rsquo;</dt>
<dd><p>controls the degree of table compression and, more generally, trade-offs
between small scanners and fast scanners.
</p>
<dl compact="compact">
<dd><a name="index-_002dC"></a>
</dd>
<dt>&lsquo;<samp>-C</samp>&rsquo;</dt>
<dd><p>A lone &lsquo;<samp>-C</samp>&rsquo; specifies that the scanner tables should be compressed
but neither equivalence classes nor meta-equivalence classes should be
used.
</p>
<a name="option_002dalign"></a><a name="index-_002dCa"></a>
<a name="index-_002d_002d_002dalign"></a>
<a name="index-align"></a>
</dd>
<dt>&lsquo;<samp>-Ca, --align, <code>%option align</code></samp>&rsquo;</dt>
<dd><p>(&ldquo;align&rdquo;) instructs flex to trade off larger tables in the
generated scanner for faster performance because the elements of
the tables are better aligned for memory access and computation.  On some
RISC architectures, fetching and manipulating longwords is more efficient
than with smaller-sized units such as shortwords.  This option can
quadruple the size of the tables used by your scanner.
</p>
<a name="option_002decs"></a><a name="index-_002dCe"></a>
<a name="index-_002d_002d_002decs"></a>
<a name="index-ecs"></a>
</dd>
<dt>&lsquo;<samp>-Ce, --ecs, <code>%option ecs</code></samp>&rsquo;</dt>
<dd><p>directs <code>flex</code> to construct <em>equivalence classes</em>, i.e., sets
of characters which have identical lexical properties (for example, if
the only appearance of digits in the <code>flex</code> input is in the
character class &ldquo;[0-9]&rdquo; then the digits &rsquo;0&rsquo;, &rsquo;1&rsquo;, ..., &rsquo;9&rsquo; will all be
put in the same equivalence class).  Equivalence classes usually give
dramatic reductions in the final table/object file sizes (typically a
factor of 2-5) and are pretty cheap performance-wise (one array look-up
per character scanned).
</p>
<a name="index-_002dCf"></a>
</dd>
<dt>&lsquo;<samp>-Cf</samp>&rsquo;</dt>
<dd><p>specifies that the <em>full</em> scanner tables should be generated -
<code>flex</code> should not compress the tables by taking advantages of
similar transition functions for different states.
</p>
<a name="index-_002dCF"></a>
</dd>
<dt>&lsquo;<samp>-CF</samp>&rsquo;</dt>
<dd><p>specifies that the alternate fast scanner representation (described
above under the &lsquo;<samp>--fast</samp>&rsquo; flag) should be used.  This option cannot be
used with &lsquo;<samp>--c++</samp>&rsquo;.
</p>
<a name="option_002dmeta_002decs"></a><a name="index-_002dCm"></a>
<a name="index-_002d_002d_002dmeta_002decs"></a>
<a name="index-meta_002decs"></a>
</dd>
<dt>&lsquo;<samp>-Cm, --meta-ecs, <code>%option meta-ecs</code></samp>&rsquo;</dt>
<dd><p>directs
<code>flex</code>
to construct
<em>meta-equivalence classes</em>,
which are sets of equivalence classes (or characters, if equivalence
classes are not being used) that are commonly used together.  Meta-equivalence
classes are often a big win when using compressed tables, but they
have a moderate performance impact (one or two <code>if</code> tests and one
array look-up per character scanned).
</p>
<a name="option_002dread"></a><a name="index-_002dCr"></a>
<a name="index-_002d_002d_002dread"></a>
<a name="index-read"></a>
</dd>
<dt>&lsquo;<samp>-Cr, --read, <code>%option read</code></samp>&rsquo;</dt>
<dd><p>causes the generated scanner to <em>bypass</em> use of the standard I/O
library (<code>stdio</code>) for input.  Instead of calling <code>fread()</code> or
<code>getc()</code>, the scanner will use the <code>read()</code> system call,
resulting in a performance gain which varies from system to system, but
in general is probably negligible unless you are also using &lsquo;<samp>-Cf</samp>&rsquo;
or &lsquo;<samp>-CF</samp>&rsquo;.  Using &lsquo;<samp>-Cr</samp>&rsquo; can cause strange behavior if, for
example, you read from <samp>yyin</samp> using <code>stdio</code> prior to calling
the scanner (because the scanner will miss whatever text your previous
reads left in the <code>stdio</code> input buffer).  &lsquo;<samp>-Cr</samp>&rsquo; has no effect
if you define <code>YY_INPUT()</code> (see <a href="Generated-Scanner.html#Generated-Scanner">Generated Scanner</a>).
</p></dd>
</dl>

<p>The options &lsquo;<samp>-Cf</samp>&rsquo; or &lsquo;<samp>-CF</samp>&rsquo; and &lsquo;<samp>-Cm</samp>&rsquo; do not make sense
together - there is no opportunity for meta-equivalence classes if the
table is not being compressed.  Otherwise the options may be freely
mixed, and are cumulative.
</p>
<p>The default setting is &lsquo;<samp>-Cem</samp>&rsquo;, which specifies that <code>flex</code>
should generate equivalence classes and meta-equivalence classes.  This
setting provides the highest degree of table compression.  You can trade
off faster-executing scanners at the cost of larger tables with the
following generally being true:
</p>
<div class="example">
<pre class="verbatim">    slowest &amp; smallest
          -Cem
          -Cm
          -Ce
          -C
          -C{f,F}e
          -C{f,F}
          -C{f,F}a
    fastest &amp; largest
</pre></div>

<p>Note that scanners with the smallest tables are usually generated and
compiled the quickest, so during development you will usually want to
use the default, maximal compression.
</p>
<p>&lsquo;<samp>-Cfe</samp>&rsquo; is often a good compromise between speed and size for
production scanners.
</p>
<a name="option_002dfull"></a><a name="index-_002df"></a>
<a name="index-_002d_002d_002dfull"></a>
<a name="index-full"></a>
</dd>
<dt>&lsquo;<samp>-f, --full, <code>%option full</code></samp>&rsquo;</dt>
<dd><p>specifies
<em>fast scanner</em>.
No table compression is done and <code>stdio</code> is bypassed.
The result is large but fast.  This option is equivalent to
&lsquo;<samp>--Cfr</samp>&rsquo;
</p>

<a name="option_002dfast"></a><a name="index-_002dF"></a>
<a name="index-_002d_002d_002dfast"></a>
<a name="index-fast"></a>
</dd>
<dt>&lsquo;<samp>-F, --fast, <code>%option fast</code></samp>&rsquo;</dt>
<dd><p>specifies that the <em>fast</em> scanner table representation should be
used (and <code>stdio</code> bypassed).  This representation is about as fast
as the full table representation &lsquo;<samp>--full</samp>&rsquo;, and for some sets of
patterns will be considerably smaller (and for others, larger).  In
general, if the pattern set contains both <em>keywords</em> and a
catch-all, <em>identifier</em> rule, such as in the set:
</p>
<div class="example">
<pre class="verbatim">    &quot;case&quot;    return TOK_CASE;
    &quot;switch&quot;  return TOK_SWITCH;
    ...
    &quot;default&quot; return TOK_DEFAULT;
    [a-z]+    return TOK_ID;
</pre></div>

<p>then you&rsquo;re better off using the full table representation.  If only
the <em>identifier</em> rule is present and you then use a hash table or some such
to detect the keywords, you&rsquo;re better off using
&lsquo;<samp>--fast</samp>&rsquo;.
</p>
<p>This option is equivalent to &lsquo;<samp>-CFr</samp>&rsquo;.  It cannot be used
with &lsquo;<samp>--c++</samp>&rsquo;.
</p>
</dd>
</dl>

<hr>
<div class="header">
<p>
Next: <a href="Debugging-Options.html#Debugging-Options" accesskey="n" rel="next">Debugging Options</a>, Previous: <a href="Code_002dLevel-And-API-Options.html#Code_002dLevel-And-API-Options" accesskey="p" rel="prev">Code-Level And API Options</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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