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 multiple 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_mema-
       lign() 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 speci-
       fies the pathconf(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  mal-
       loc(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 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 3.25 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                                                        2009-03-30                                          POSIX_MEMALIGN(3)