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<h5 class="subsubsection">3.2.2.7 Allocating Aligned Memory Blocks</h5>

<p><a name="index-page-boundary-271"></a><a name="index-alignment-_0028with-_0040code_007bmalloc_007d_0029-272"></a><a name="index-stdlib_002eh-273"></a>The address of a block returned by <code>malloc</code> or <code>realloc</code> in
the GNU system is always a multiple of eight (or sixteen on 64-bit
systems).  If you need a block whose address is a multiple of a higher
power of two than that, use <code>memalign</code>, <code>posix_memalign</code>, or
<code>valloc</code>.  <code>memalign</code> is declared in <samp><span class="file">malloc.h</span></samp> and
<code>posix_memalign</code> is declared in <samp><span class="file">stdlib.h</span></samp>.

   <p>With the GNU library, you can use <code>free</code> to free the blocks that
<code>memalign</code>, <code>posix_memalign</code>, and <code>valloc</code> return.  That
does not work in BSD, however&mdash;BSD does not provide any way to free
such blocks.

<!-- malloc.h -->
<!-- BSD -->
<div class="defun">
&mdash; Function: void * <b>memalign</b> (<var>size_t boundary, size_t size</var>)<var><a name="index-memalign-274"></a></var><br>
<blockquote><p>The <code>memalign</code> function allocates a block of <var>size</var> bytes whose
address is a multiple of <var>boundary</var>.  The <var>boundary</var> must be a
power of two!  The function <code>memalign</code> works by allocating a
somewhat larger block, and then returning an address within the block
that is on the specified boundary. 
</p></blockquote></div>

<!-- stdlib.h -->
<!-- POSIX -->
<div class="defun">
&mdash; Function: int <b>posix_memalign</b> (<var>void **memptr, size_t alignment, size_t size</var>)<var><a name="index-posix_005fmemalign-275"></a></var><br>
<blockquote><p>The <code>posix_memalign</code> function is similar to the <code>memalign</code>
function in that it returns a buffer of <var>size</var> bytes aligned to a
multiple of <var>alignment</var>.  But it adds one requirement to the
parameter <var>alignment</var>: the value must be a power of two multiple of
<code>sizeof (void *)</code>.

        <p>If the function succeeds in allocation memory a pointer to the allocated
memory is returned in <code>*</code><var>memptr</var> and the return value is zero. 
Otherwise the function returns an error value indicating the problem.

        <p>This function was introduced in POSIX 1003.1d. 
</p></blockquote></div>

<!-- malloc.h stdlib.h -->
<!-- BSD -->
<div class="defun">
&mdash; Function: void * <b>valloc</b> (<var>size_t size</var>)<var><a name="index-valloc-276"></a></var><br>
<blockquote><p>Using <code>valloc</code> is like using <code>memalign</code> and passing the page size
as the value of the second argument.  It is implemented like this:

     <pre class="smallexample">          void *
          valloc (size_t size)
          {
            return memalign (getpagesize (), size);
          }
</pre>
        <p><a href="Query-Memory-Parameters.html#Query-Memory-Parameters">Query Memory Parameters</a> for more information about the memory
subsystem. 
</p></blockquote></div>

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