Man Linux: Main Page and Category List


       posix_memalign, memalign, valloc - Allocate aligned memory


       #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


       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

       For all three routines, the memory is not zeroed.


       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.


       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.


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


       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.

       Everybody agrees that posix_memalign() is declared in <stdlib.h>.

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

       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).


       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 satisfy this  requirement.

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

       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 routines are only needed if you require larger alignment  values.


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


       This  page  is  part of release 3.24 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at