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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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	This file documents the GNU C library.

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	of `The GNU C Library Reference Manual', for version
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	Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002,
	2003, 2007, 2008, 2010 Free Software Foundation, Inc.

	Permission is granted to copy, distribute and/or modify this document
	under the terms of the GNU Free Documentation License, Version 1.3 or
	any later version published by the Free Software Foundation; with the
	Invariant Sections being ``Free Software Needs Free Documentation''
	and ``GNU Lesser General Public License'', the Front-Cover texts being
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	Next: <a rel="next" accesskey="n" href="Malloc-Tunable-Parameters.html#Malloc-Tunable-Parameters">Malloc Tunable Parameters</a>,
	Previous: <a rel="previous" accesskey="p" href="Efficiency-and-Malloc.html#Efficiency-and-Malloc">Efficiency and Malloc</a>,
	Up: <a rel="up" accesskey="u" href="Unconstrained-Allocation.html#Unconstrained-Allocation">Unconstrained Allocation</a>
	<hr>
	</div>

	<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—BSD does not provide any way to free
	such blocks.

	<!-- malloc.h -->
	<!-- BSD -->
	<div class="defun">
	— 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">
	— 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">
	— 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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