posix_memalign


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

       valloc():
           Since glibc 2.12:
               _BSD_SOURCE ||
                   (_XOPEN_SOURCE >= 500 ||
                       _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED) &&
                   !(_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600)
           Before glibc 2.12:
               _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
               _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED

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 *).  If size is 0, then posix_mema-
       lign() returns either NULL, or a unique pointer value that can later be
       successfully passed to free(3).

       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 *).

       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  <mal-
       loc.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 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 3.35 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://man7.org/linux/man-pages/.



GNU                               2010-09-20                 POSIX_MEMALIGN(3)
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