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<h4 class="subsection">3.2.5 Automatic Storage with Variable Size</h4>

<p><a name="index-automatic-freeing-341"></a><a name="index-g_t_0040code_007balloca_007d-function-342"></a><a name="index-automatic-storage-with-variable-size-343"></a>
The function <code>alloca</code> supports a kind of half-dynamic allocation in
which blocks are allocated dynamically but freed automatically.

   <p>Allocating a block with <code>alloca</code> is an explicit action; you can
allocate as many blocks as you wish, and compute the size at run time.  But
all the blocks are freed when you exit the function that <code>alloca</code> was
called from, just as if they were automatic variables declared in that
function.  There is no way to free the space explicitly.

   <p>The prototype for <code>alloca</code> is in <samp><span class="file">stdlib.h</span></samp>.  This function is
a BSD extension. 
<a name="index-stdlib_002eh-344"></a>
<!-- stdlib.h -->
<!-- GNU, BSD -->

<div class="defun">
&mdash; Function: void * <b>alloca</b> (<var>size_t size</var>)<var>;<a name="index-alloca-345"></a></var><br>
<blockquote><p>The return value of <code>alloca</code> is the address of a block of <var>size</var>
bytes of memory, allocated in the stack frame of the calling function. 
</p></blockquote></div>

   <p>Do not use <code>alloca</code> inside the arguments of a function call&mdash;you
will get unpredictable results, because the stack space for the
<code>alloca</code> would appear on the stack in the middle of the space for
the function arguments.  An example of what to avoid is <code>foo (x,
alloca (4), y)</code>. 
<!-- This might get fixed in future versions of GCC, but that won't make -->
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<ul class="menu">
<li><a accesskey="1" href="Alloca-Example.html#Alloca-Example">Alloca Example</a>:               Example of using <code>alloca</code>. 
<li><a accesskey="2" href="Advantages-of-Alloca.html#Advantages-of-Alloca">Advantages of Alloca</a>:         Reasons to use <code>alloca</code>. 
<li><a accesskey="3" href="Disadvantages-of-Alloca.html#Disadvantages-of-Alloca">Disadvantages of Alloca</a>:      Reasons to avoid <code>alloca</code>. 
<li><a accesskey="4" href="GNU-C-Variable_002dSize-Arrays.html#GNU-C-Variable_002dSize-Arrays">GNU C Variable-Size Arrays</a>:   Only in GNU C, here is an alternative
				 method of allocating dynamically and
				 freeing automatically. 
</ul>

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