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POSIX_MEMALIGN(3)          Linux Programmer's Manual         POSIX_MEMALIGN(3)



NAME
       posix_memalign, memalign, valloc - Allocate aligned memory

SYNOPSIS
       #include <&lt;stdlib.h>&gt;

       int posix_memalign(void **memptr, size_t alignment, size_t size);

       #include <&lt;malloc.h>&gt;

       void *valloc(size_t size);
       void *memalign(size_t boundary, size_t size);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       posix_memalign(): _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600

DESCRIPTION
       The  function  posix_memalign()  allocates  size  bytes  and places the
       address of the allocated memory in *memptr.  The address of  the  allo-
       cated  memory will be a multiple of alignment, which must be a power of
       two and a multiple of sizeof(void *).

       The obsolete function memalign() allocates size  bytes  and  returns  a
       pointer to the allocated memory.  The memory address will be a multiple
       of boundary, which must be a power of two.

       The obsolete function valloc()  allocates  size  bytes  and  returns  a
       pointer to the allocated memory.  The memory address will be a multiple
       of the page  size.   It  is  equivalent  to  memalign(sysconf(_SC_PAGE-
       SIZE),size).

       For all three routines, the memory is not zeroed.

RETURN VALUE
       memalign()  and valloc() return the pointer to the allocated memory, or
       NULL if the request fails.

       posix_memalign() returns zero on success, or one of  the  error  values
       listed in the next section on failure.  Note that errno is not set.

ERRORS
       EINVAL The alignment argument was not a power of two, or was not a mul-
              tiple of sizeof(void *).

       ENOMEM There was insufficient memory to fulfill the allocation request.

VERSIONS
       The functions memalign() and valloc() have been available in all  Linux
       libc libraries.  The function posix_memalign() is available since glibc
       2.1.91.

CONFORMING TO
       The function valloc() appeared in 3.0BSD.  It is  documented  as  being
       obsolete  in  4.3BSD,  and  as  legacy in SUSv2.  It does not appear in
       POSIX.1-2001.  The function memalign() appears in SunOS 4.1.3  but  not
       in 4.4BSD.  The function posix_memalign() comes from POSIX.1d.

   Headers
       Everybody agrees that posix_memalign() is declared in &lt;stdlib.h&gt;.

       On  some  systems memalign() is declared in &lt;stdlib.h&gt; instead of &lt;mal-
       loc.h&gt;.

       According to SUSv2, valloc() is declared in  &lt;stdlib.h&gt;.   Libc4,5  and
       glibc  declare it in &lt;malloc.h&gt; and perhaps also in &lt;stdlib.h&gt; (namely,
       if _GNU_SOURCE is defined, or _BSD_SOURCE is defined, or, for glibc, if
       _XOPEN_SOURCE_EXTENDED  is  defined, or, equivalently, _XOPEN_SOURCE is
       defined to a value not less than 500).

NOTES
       On many systems there are alignment restrictions, for example, on  buf-
       fers  used  for  direct  block  device  I/O.  POSIX specifies the path-
       conf(path,_PC_REC_XFER_ALIGN) call that tells what alignment is needed.
       Now one can use posix_memalign() to satisfy this requirement.

       posix_memalign()  verifies  that  alignment  matches  the  requirements
       detailed above.  memalign() may not check that the boundary argument is
       correct.

       POSIX  requires that memory obtained from posix_memalign() can be freed
       using free(3).  Some systems provide no way to reclaim memory allocated
       with  memalign()  or  valloc()  (because one can only pass to free(3) a
       pointer gotten from malloc(3), while,  for  example,  memalign()  would
       call malloc(3) and then align the obtained value).  The glibc implemen-
       tation allows memory obtained from any of these three  routines  to  be
       reclaimed with free(3).

       The  glibc malloc(3) always returns 8-byte aligned memory addresses, so
       these routines are only needed if you require larger alignment values.

SEE ALSO
       brk(2), getpagesize(2), free(3), malloc(3)

COLOPHON
       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



GNU                               2007-07-26                 POSIX_MEMALIGN(3)