/usr/share/doc/flex-doc/html/How-do-I-track-the-byte-offset-for-lseek_0028_0029

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- 
The flex manual is placed under the same licensing conditions as the
rest of flex:

Copyright (C) 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2012
The Flex Project.

Copyright (C) 1990, 1997 The Regents of the University of California.
All rights reserved.

This code is derived from software contributed to Berkeley by
Vern Paxson.

The United States Government has rights in this work pursuant
to contract no. DE-AC03-76SF00098 between the United States
Department of Energy and the University of California.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:

1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.

2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.

Neither the name of the University nor the names of its contributors
may be used to endorse or promote products derived from this software
without specific prior written permission.

THIS SOFTWARE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR
IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE. -->
<!-- Created by GNU Texinfo 6.5, http://www.gnu.org/software/texinfo/ -->
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>How do I track the byte offset for lseek()? (Lexical Analysis With Flex, for Flex 2.6.4)</title>

<meta name="description" content="How do I track the byte offset for lseek()? (Lexical Analysis With Flex, for Flex 2.6.4)">
<meta name="keywords" content="How do I track the byte offset for lseek()? (Lexical Analysis With Flex, for Flex 2.6.4)">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<link href="index.html#Top" rel="start" title="Top">
<link href="Indices.html#Indices" rel="index" title="Indices">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="FAQ.html#FAQ" rel="up" title="FAQ">
<link href="How-do-I-use-my-own-I_002fO-classes-in-a-C_002b_002b-scanner_003f.html#How-do-I-use-my-own-I_002fO-classes-in-a-C_002b_002b-scanner_003f" rel="next" title="How do I use my own I/O classes in a C++ scanner?">
<link href="Memory-leak-_002d-16386-bytes-allocated-by-malloc_002e.html#Memory-leak-_002d-16386-bytes-allocated-by-malloc_002e" rel="prev" title="Memory leak - 16386 bytes allocated by malloc.">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.indentedblock {margin-right: 0em}
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
blockquote.smallquotation {font-size: smaller}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
div.lisp {margin-left: 3.2em}
div.smalldisplay {margin-left: 3.2em}
div.smallexample {margin-left: 3.2em}
div.smalllisp {margin-left: 3.2em}
kbd {font-style: oblique}
pre.display {font-family: inherit}
pre.format {font-family: inherit}
pre.menu-comment {font-family: serif}
pre.menu-preformatted {font-family: serif}
pre.smalldisplay {font-family: inherit; font-size: smaller}
pre.smallexample {font-size: smaller}
pre.smallformat {font-family: inherit; font-size: smaller}
pre.smalllisp {font-size: smaller}
span.nolinebreak {white-space: nowrap}
span.roman {font-family: initial; font-weight: normal}
span.sansserif {font-family: sans-serif; font-weight: normal}
ul.no-bullet {list-style: none}
-->
</style>


</head>

<body lang="en">
<a name="How-do-I-track-the-byte-offset-for-lseek_0028_0029_003f"></a>
<div class="header">
<p>
Next: <a href="How-do-I-use-my-own-I_002fO-classes-in-a-C_002b_002b-scanner_003f.html#How-do-I-use-my-own-I_002fO-classes-in-a-C_002b_002b-scanner_003f" accesskey="n" rel="next">How do I use my own I/O classes in a C++ scanner?</a>, Previous: <a href="Memory-leak-_002d-16386-bytes-allocated-by-malloc_002e.html#Memory-leak-_002d-16386-bytes-allocated-by-malloc_002e" accesskey="p" rel="prev">Memory leak - 16386 bytes allocated by malloc.</a>, Up: <a href="FAQ.html#FAQ" accesskey="u" rel="up">FAQ</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="How-do-I-track-the-byte-offset-for-lseek_0028_0029_003f-1"></a>
<h3 class="unnumberedsec">How do I track the byte offset for lseek()?</h3>

<div class="example">
<pre class="verbatim">&gt;   We thought that it would be possible to have this number through the
&gt;   evaluation of the following expression:
&gt;
&gt;   seek_position = (no_buffers)*YY_READ_BUF_SIZE + yy_c_buf_p - YY_CURRENT_BUFFER-&gt;yy_ch_buf
</pre></div>

<p>While this is the right idea, it has two problems.  The first is that
it&rsquo;s possible that <code>flex</code> will request less than <code>YY_READ_BUF_SIZE</code> during
an invocation of <code>YY_INPUT</code> (or that your input source will return less
even though <code>YY_READ_BUF_SIZE</code> bytes were requested).  The second problem
is that when refilling its internal buffer, <code>flex</code> keeps some characters
from the previous buffer (because usually it&rsquo;s in the middle of a match,
and needs those characters to construct <code>yytext</code> for the match once it&rsquo;s
done).  Because of this, <code>yy_c_buf_p - YY_CURRENT_BUFFER-&gt;yy_ch_buf</code> won&rsquo;t
be exactly the number of characters already read from the current buffer.
</p>
<p>An alternative solution is to count the number of characters you&rsquo;ve matched
since starting to scan.  This can be done by using <code>YY_USER_ACTION</code>.  For
example,
</p>
<div class="example">
<pre class="verbatim">#define YY_USER_ACTION num_chars += yyleng;
</pre></div>

<p>(You need to be careful to update your bookkeeping if you use <code>yymore(</code>),
<code>yyless()</code>, <code>unput()</code>, or <code>input()</code>.)
</p>



</body>
</html>
flex-doc 2.6.4-6 / usr / share / doc / flex-doc / html / How-do-I-track-the-byte-offset-for-lseek_0028_0029_003f.html
