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



NAME
       posix_memalign, memalign, valloc - Allocate aligned memory

SYNOPSIS
       #include <stdlib.h>

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

       #include <malloc.h>

       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 allocated memory will be a  multi-
       ple  of  alignment,  which must be a power of two and a multiple of sizeof(void *).
       If size is 0, then posix_memalign() returns either NULL, or a unique pointer  value
       that can later be successfully passed to free().

       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_PAGESIZE),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 multiple 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 <stdlib.h>.

       On some systems memalign() is declared in <stdlib.h> instead of <malloc.h>.

       According to SUSv2, valloc() is declared in <stdlib.h>.  Libc4,5 and glibc  declare
       it in <malloc.h> and perhaps also in <stdlib.h> (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 buffers used for
       direct block device I/O.   POSIX  specifies  the  pathconf(path,_PC_REC_XFER_ALIGN)
       call that tells what alignment is needed.  Now one can use posix_memalign() to sat-
       isfy 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  implementation 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  rou-
       tines 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 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-
       nel.org/doc/man-pages/.



GNU                               2009-03-30                 POSIX_MEMALIGN(3)

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