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<a name="malloc"></a>
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<a name="malloc_002c-realloc_002c-free_002d_002d_002dmanage-memory"></a>
<h3 class="section">2.24 <code>malloc</code>, <code>realloc</code>, <code>free</code>&mdash;manage memory</h3>
<a name="index-malloc"></a>
<a name="index-realloc"></a>
<a name="index-reallocf"></a>
<a name="index-free"></a>
<a name="index-memalign"></a>
<a name="index-malloc_005fusable_005fsize"></a>
<a name="index-_005fmalloc_005fr"></a>
<a name="index-_005frealloc_005fr"></a>
<a name="index-_005freallocf_005fr"></a>
<a name="index-_005ffree_005fr"></a>
<a name="index-_005fmemalign_005fr"></a>
<a name="index-_005fmalloc_005fusable_005fsize_005fr"></a>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">#include &lt;stdlib.h&gt;
void *malloc(size_t <var>nbytes</var>);
void *realloc(void *<var>aptr</var>, size_t <var>nbytes</var>);
void *reallocf(void *<var>aptr</var>, size_t <var>nbytes</var>);
void free(void *<var>aptr</var>);

void *memalign(size_t <var>align</var>, size_t <var>nbytes</var>);

size_t malloc_usable_size(void *<var>aptr</var>);

void *_malloc_r(void *<var>reent</var>, size_t <var>nbytes</var>);
void *_realloc_r(void *<var>reent</var>, 
    void *<var>aptr</var>, size_t <var>nbytes</var>);
void *_reallocf_r(void *<var>reent</var>, 
    void *<var>aptr</var>, size_t <var>nbytes</var>);
void _free_r(void *<var>reent</var>, void *<var>aptr</var>);

void *_memalign_r(void *<var>reent</var>,
    size_t <var>align</var>, size_t <var>nbytes</var>);

size_t _malloc_usable_size_r(void *<var>reent</var>, void *<var>aptr</var>);

</pre></div>
<p><strong>Description</strong><br>
These functions manage a pool of system memory.
</p>
<p>Use <code>malloc</code> to request allocation of an object with at least
<var>nbytes</var> bytes of storage available.  If the space is available,
<code>malloc</code> returns a pointer to a newly allocated block as its result.
</p>
<p>If you already have a block of storage allocated by <code>malloc</code>, but
you no longer need all the space allocated to it, you can make it
smaller by calling <code>realloc</code> with both the object pointer and the
new desired size as arguments.  <code>realloc</code> guarantees that the
contents of the smaller object match the beginning of the original object.
</p>
<p>Similarly, if you need more space for an object, use <code>realloc</code> to
request the larger size; again, <code>realloc</code> guarantees that the
beginning of the new, larger object matches the contents of the
original object.
</p>
<p>When you no longer need an object originally allocated by <code>malloc</code>
or <code>realloc</code> (or the related function <code>calloc</code>), return it to the
memory storage pool by calling <code>free</code> with the address of the object
as the argument.  You can also use <code>realloc</code> for this purpose by
calling it with <code>0</code> as the <var>nbytes</var> argument.
</p>
<p>The <code>reallocf</code> function behaves just like <code>realloc</code> except if the
function is required to allocate new storage and this fails.  In this
case <code>reallocf</code> will free the original object passed in whereas
<code>realloc</code> will not.
</p>
<p>The <code>memalign</code> function returns a block of size <var>nbytes</var> aligned
to a <var>align</var> boundary.  The <var>align</var> argument must be a power of
two.
</p>
<p>The <code>malloc_usable_size</code> function takes a pointer to a block
allocated by <code>malloc</code>.  It returns the amount of space that is
available in the block.  This may or may not be more than the size
requested from <code>malloc</code>, due to alignment or minimum size
constraints.
</p>
<p>The alternate functions <code>_malloc_r</code>, <code>_realloc_r</code>, <code>_reallocf_r</code>, 
<code>_free_r</code>, <code>_memalign_r</code>, and <code>_malloc_usable_size_r</code> are reentrant
versions.  The extra argument <var>reent</var> is a pointer to a reentrancy structure.
</p>
<p>If you have multiple threads of execution which may call any of these
routines, or if any of these routines may be called reentrantly, then
you must provide implementations of the <code>__malloc_lock</code> and
<code>__malloc_unlock</code> functions for your system.  See the documentation
for those functions.
</p>
<p>These functions operate by calling the function <code>_sbrk_r</code> or
<code>sbrk</code>, which allocates space.  You may need to provide one of these
functions for your system.  <code>_sbrk_r</code> is called with a positive
value to allocate more space, and with a negative value to release
previously allocated space if it is no longer required.
See <a href="Stubs.html#Stubs">Stubs</a>.
</p>
<br>
<p><strong>Returns</strong><br>
<code>malloc</code> returns a pointer to the newly allocated space, if
successful; otherwise it returns <code>NULL</code>.  If your application needs
to generate empty objects, you may use <code>malloc(0)</code> for this purpose.
</p>
<p><code>realloc</code> returns a pointer to the new block of memory, or <code>NULL</code>
if a new block could not be allocated.  <code>NULL</code> is also the result
when you use &lsquo;<code>realloc(<var>aptr</var>,0)</code>&rsquo; (which has the same effect as
&lsquo;<code>free(<var>aptr</var>)</code>&rsquo;).  You should always check the result of
<code>realloc</code>; successful reallocation is not guaranteed even when
you request a smaller object.
</p>
<p><code>free</code> does not return a result.
</p>
<p><code>memalign</code> returns a pointer to the newly allocated space.
</p>
<p><code>malloc_usable_size</code> returns the usable size.
</p>
<br>
<p><strong>Portability</strong><br>
<code>malloc</code>, <code>realloc</code>, and <code>free</code> are specified by the ANSI C
standard, but other conforming implementations of <code>malloc</code> may
behave differently when <var>nbytes</var> is zero.
</p>
<p><code>memalign</code> is part of SVR4.
</p>
<p><code>malloc_usable_size</code> is not portable.
</p>
<p>Supporting OS subroutines required: <code>sbrk</code>.  
</p><br>

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