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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of POSIX_MEMALIGN</TITLE>
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<H1>POSIX_MEMALIGN</H1>
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Section: Linux Programmer's Manual (3)<BR>Updated: 2019-05-09<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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posix_memalign, aligned_alloc, memalign, valloc, pvalloc - allocate aligned memory
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<PRE>
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<B>#include <<A HREF="file:///usr/include/stdlib.h">stdlib.h</A>></B>
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<B>int posix_memalign(void **</B><I>memptr</I><B>, size_t </B><I>alignment</I><B>, size_t </B><I>size</I><B>);</B>
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<B>void *aligned_alloc(size_t </B><I>alignment</I><B>, size_t </B><I>size</I><B>);</B>
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<B>void *valloc(size_t </B><I>size</I><B>);</B>
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<B>#include <<A HREF="file:///usr/include/malloc.h">malloc.h</A>></B>
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<B>void *memalign(size_t </B><I>alignment</I><B>, size_t </B><I>size</I><B>);</B>
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<B>void *pvalloc(size_t </B><I>size</I><B>);</B>
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</PRE>
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<P>
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Feature Test Macro Requirements for glibc (see
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<B><A HREF="/cgi-bin/man/man2html?7+feature_test_macros">feature_test_macros</A></B>(7)):
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<P>
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<B>posix_memalign</B>():
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_POSIX_C_SOURCE >= 200112L
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<P>
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<B>aligned_alloc</B>():
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_ISOC11_SOURCE
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<P>
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<B>valloc</B>():
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<BR>
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<DL COMPACT><DT id="1"><DD>
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<DL COMPACT>
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<DT id="2">Since glibc 2.12:<DD>
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<PRE>
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(_XOPEN_SOURCE >= 500) && !(_POSIX_C_SOURCE >= 200112L)
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|| /* Glibc since 2.19: */ _DEFAULT_SOURCE
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|| /* Glibc versions <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
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<BR>
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</PRE>
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<DT id="3">Before glibc 2.12:<DD>
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_BSD_SOURCE || _XOPEN_SOURCE >= 500
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<BR>
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(The (nonstandard) header file
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<I><<A HREF="file:///usr/include/malloc.h">malloc.h</A>></I>
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also exposes the declaration of
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<B>valloc</B>();
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no feature test macros are required.)
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</DL>
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</DL>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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The function
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<B>posix_memalign</B>()
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allocates
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<I>size</I>
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bytes and places the address of the allocated memory in
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<I>*memptr</I>.
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The address of the allocated memory will be a multiple of
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<I>alignment</I>,
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which must be a power of two and a multiple of
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<I>sizeof(void *)</I>.
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If
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<I>size</I>
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is 0, then
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the value placed in
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<I>*memptr</I>
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is either NULL,
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or a unique pointer value that can later be successfully passed to
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<B><A HREF="/cgi-bin/man/man2html?3+free">free</A></B>(3).
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<P>
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The obsolete function
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<B>memalign</B>()
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allocates
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<I>size</I>
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bytes and returns a pointer to the allocated memory.
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The memory address will be a multiple of
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<I>alignment</I>,
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which must be a power of two.
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<P>
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The function
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<B>aligned_alloc</B>()
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is the same as
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<B>memalign</B>(),
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except for the added restriction that
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<I>size</I>
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should be a multiple of
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<I>alignment</I>.
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<P>
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The obsolete function
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<B>valloc</B>()
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allocates
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<I>size</I>
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bytes and returns a pointer to the allocated memory.
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The memory address will be a multiple of the page size.
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It is equivalent to
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<I>memalign(sysconf(_SC_PAGESIZE),size)</I>.
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<P>
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The obsolete function
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<B>pvalloc</B>()
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is similar to
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<B>valloc</B>(),
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but rounds the size of the allocation up to
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the next multiple of the system page size.
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<P>
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For all of these functions, the memory is not zeroed.
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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<B>aligned_alloc</B>(),
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<B>memalign</B>(),
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<B>valloc</B>(),
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and
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<B>pvalloc</B>()
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return a pointer to the allocated memory on success.
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On error, NULL is returned, and <I>errno</I> is set
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to indicate the cause of the error.
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<P>
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<B>posix_memalign</B>()
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returns zero on success, or one of the error values listed in the
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next section on failure.
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The value of
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<I>errno</I>
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is not set.
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On Linux (and other systems),
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<B>posix_memalign</B>()
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does not modify
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<I>memptr</I>
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on failure.
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A requirement standardizing this behavior was added in POSIX.1-2016.
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<A NAME="lbAF"> </A>
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<H2>ERRORS</H2>
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<DL COMPACT>
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<DT id="4"><B>EINVAL</B>
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<DD>
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The
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<I>alignment</I>
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argument was not a power of two, or was not a multiple of
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<I>sizeof(void *)</I>.
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<DT id="5"><B>ENOMEM</B>
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<DD>
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There was insufficient memory to fulfill the allocation request.
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</DL>
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<A NAME="lbAG"> </A>
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<H2>VERSIONS</H2>
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The functions
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<B>memalign</B>(),
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<B>valloc</B>(),
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and
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<B>pvalloc</B>()
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have been available in all Linux libc libraries.
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<P>
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The function
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<B>aligned_alloc</B>()
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was added to glibc in version 2.16.
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<P>
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The function
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<B>posix_memalign</B>()
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is available since glibc 2.1.91.
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<A NAME="lbAH"> </A>
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<H2>ATTRIBUTES</H2>
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For an explanation of the terms used in this section, see
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<B><A HREF="/cgi-bin/man/man2html?7+attributes">attributes</A></B>(7).
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<TABLE BORDER>
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<TR VALIGN=top><TD><B>Interface</B></TD><TD><B>Attribute</B></TD><TD><B>Value</B><BR></TD></TR>
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<TR VALIGN=top><TD>
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<B>aligned_alloc</B>(),
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<BR>
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<B>memalign</B>(),
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<BR>
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<B>posix_memalign</B>()
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</TD><TD>Thread safety</TD><TD>MT-Safe<BR></TD></TR>
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<TR VALIGN=top><TD>
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<B>valloc</B>(),
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<BR>
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<B>pvalloc</B>()
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</TD><TD>Thread safety</TD><TD>MT-Unsafe init<BR></TD></TR>
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</TABLE>
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<P>
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<A NAME="lbAI"> </A>
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<H2>CONFORMING TO</H2>
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The function
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<B>valloc</B>()
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appeared in 3.0BSD.
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It is documented as being obsolete in 4.3BSD,
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and as legacy in SUSv2.
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It does not appear in POSIX.1.
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<P>
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The function
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<B>pvalloc</B>()
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is a GNU extension.
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<P>
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The function
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<B>memalign</B>()
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appears in SunOS 4.1.3 but not in 4.4BSD.
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<P>
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The function
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<B>posix_memalign</B>()
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comes from POSIX.1d and is specified in POSIX.1-2001 and POSIX.1-2008.
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<P>
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The function
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<B>aligned_alloc</B>()
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is specified in the C11 standard.
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<A NAME="lbAJ"> </A>
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<H3>Headers</H3>
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Everybody agrees that
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<B>posix_memalign</B>()
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is declared in <I><<A HREF="file:///usr/include/stdlib.h">stdlib.h</A>></I>.
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<P>
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On some systems
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<B>memalign</B>()
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is declared in <I><<A HREF="file:///usr/include/stdlib.h">stdlib.h</A>></I> instead of <I><<A HREF="file:///usr/include/malloc.h">malloc.h</A>></I>.
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<P>
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According to SUSv2,
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<B>valloc</B>()
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is declared in <I><<A HREF="file:///usr/include/stdlib.h">stdlib.h</A>></I>.
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Libc4,5 and glibc declare it in <I><<A HREF="file:///usr/include/malloc.h">malloc.h</A>></I>, and also in
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<I><<A HREF="file:///usr/include/stdlib.h">stdlib.h</A>></I>
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if suitable feature test macros are defined (see above).
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<A NAME="lbAK"> </A>
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<H2>NOTES</H2>
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On many systems there are alignment restrictions, for example, on buffers
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used for direct block device I/O.
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POSIX specifies the
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<I>pathconf(path,_PC_REC_XFER_ALIGN)</I>
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call that tells what alignment is needed.
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Now one can use
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<B>posix_memalign</B>()
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to satisfy this requirement.
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<P>
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<B>posix_memalign</B>()
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verifies that
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<I>alignment</I>
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matches the requirements detailed above.
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<B>memalign</B>()
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may not check that the
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<I>alignment</I>
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argument is correct.
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<P>
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POSIX requires that memory obtained from
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<B>posix_memalign</B>()
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can be freed using
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<B><A HREF="/cgi-bin/man/man2html?3+free">free</A></B>(3).
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Some systems provide no way to reclaim memory allocated with
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<B>memalign</B>()
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or
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<B>valloc</B>()
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(because one can pass to
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<B><A HREF="/cgi-bin/man/man2html?3+free">free</A></B>(3)
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only a pointer obtained from
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<B><A HREF="/cgi-bin/man/man2html?3+malloc">malloc</A></B>(3),
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while, for example,
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<B>memalign</B>()
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would call
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<B><A HREF="/cgi-bin/man/man2html?3+malloc">malloc</A></B>(3)
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and then align the obtained value).
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The glibc implementation
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allows memory obtained from any of these functions to be
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reclaimed with
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<B><A HREF="/cgi-bin/man/man2html?3+free">free</A></B>(3).
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<P>
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The glibc
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<B><A HREF="/cgi-bin/man/man2html?3+malloc">malloc</A></B>(3)
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always returns 8-byte aligned memory addresses, so these functions are
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needed only if you require larger alignment values.
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<A NAME="lbAL"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?2+brk">brk</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+getpagesize">getpagesize</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?3+free">free</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+malloc">malloc</A></B>(3)
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<A NAME="lbAM"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="6"><A HREF="#lbAB">NAME</A><DD>
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<DT id="7"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="8"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="9"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="10"><A HREF="#lbAF">ERRORS</A><DD>
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<DT id="11"><A HREF="#lbAG">VERSIONS</A><DD>
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<DT id="12"><A HREF="#lbAH">ATTRIBUTES</A><DD>
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<DT id="13"><A HREF="#lbAI">CONFORMING TO</A><DD>
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<DL>
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<DT id="14"><A HREF="#lbAJ">Headers</A><DD>
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</DL>
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<DT id="15"><A HREF="#lbAK">NOTES</A><DD>
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<DT id="16"><A HREF="#lbAL">SEE ALSO</A><DD>
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<DT id="17"><A HREF="#lbAM">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:52 GMT, March 31, 2021
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