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<a name="Overriding-The-Default-Memory-Management"></a>
<div class="header">
<p>
Next: <a href="A-Note-About-yytext-And-Memory.html#A-Note-About-yytext-And-Memory" accesskey="n" rel="next">A Note About yytext And Memory</a>, Previous: <a href="The-Default-Memory-Management.html#The-Default-Memory-Management" accesskey="p" rel="prev">The Default Memory Management</a>, Up: <a href="Memory-Management.html#Memory-Management" accesskey="u" rel="up">Memory Management</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="Overriding-The-Default-Memory-Management-1"></a>
<h3 class="section">21.2 Overriding The Default Memory Management</h3>

<a name="index-yyalloc_002c-overriding"></a>
<a name="index-yyrealloc_002c-overriding"></a>
<a name="index-yyfree_002c-overriding"></a>

<p>Flex calls the functions <code>yyalloc</code>, <code>yyrealloc</code>, and <code>yyfree</code>
when it needs to allocate or free memory. By default, these functions are
wrappers around the standard C functions, <code>malloc</code>, <code>realloc</code>, and
<code>free</code>, respectively. You can override the default implementations by telling
flex that you will provide your own implementations.
</p>
<p>To override the default implementations, you must do two things:
</p>
<ol>
<li> Suppress the default implementations by specifying one or more of the
following options:

<ul>
<li> <a name="index-noyyalloc"></a>
<code>%option noyyalloc</code>
</li><li> <code>%option noyyrealloc</code>
</li><li> <code>%option noyyfree</code>.
</li></ul>

</li><li> Provide your own implementation of the following functions: <a name="DOCF3" href="#FOOT3"><sup>3</sup></a>

<div class="example">
<pre class="verbatim">// For a non-reentrant scanner
void * yyalloc (size_t bytes);
void * yyrealloc (void * ptr, size_t bytes);
void   yyfree (void * ptr);

// For a reentrant scanner
void * yyalloc (size_t bytes, void * yyscanner);
void * yyrealloc (void * ptr, size_t bytes, void * yyscanner);
void   yyfree (void * ptr, void * yyscanner);
</pre></div>

</li></ol>

<p>In the following example, we will override all three memory routines. We assume
that there is a custom allocator with garbage collection. In order to make this
example interesting, we will use a reentrant scanner, passing a pointer to the
custom allocator through <code>yyextra</code>.
</p>
<a name="index-overriding-the-memory-routines"></a>
<div class="example">
<pre class="verbatim">%{
#include &quot;some_allocator.h&quot;
%}

/* Suppress the default implementations. */
%option noyyalloc noyyrealloc noyyfree
%option reentrant

/* Initialize the allocator. */
%{
#define YY_EXTRA_TYPE  struct allocator*
#define YY_USER_INIT  yyextra = allocator_create();
%}

%%
.|\n   ;
%%

/* Provide our own implementations. */
void * yyalloc (size_t bytes, void* yyscanner) {
    return allocator_alloc (yyextra, bytes);
}

void * yyrealloc (void * ptr, size_t bytes, void* yyscanner) {
    return allocator_realloc (yyextra, bytes);
}

void yyfree (void * ptr, void * yyscanner) {      
    /* Do nothing -- we leave it to the garbage collector. */
}

</pre></div>


<div class="footnote">
<hr>
<h4 class="footnotes-heading">Footnotes</h4>

<h3><a name="FOOT3" href="#DOCF3">(3)</a></h3>
<p>It
is not necessary to override all (or any) of the memory management routines.
You may, for example, override <code>yyrealloc</code>, but not <code>yyfree</code> or
<code>yyalloc</code>.</p>
</div>
<hr>
<div class="header">
<p>
Next: <a href="A-Note-About-yytext-And-Memory.html#A-Note-About-yytext-And-Memory" accesskey="n" rel="next">A Note About yytext And Memory</a>, Previous: <a href="The-Default-Memory-Management.html#The-Default-Memory-Management" accesskey="p" rel="prev">The Default Memory Management</a>, Up: <a href="Memory-Management.html#Memory-Management" accesskey="u" rel="up">Memory Management</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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