POSIX_MEMALIGN(3)          Linux Programmer's Manual         POSIX_MEMALIGN(3)
NAME
       posix_memalign, memalign, valloc - Allocate aligned memory

SYNOPSIS
       #define _XOPEN_SOURCE 600
       #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);

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  parameter  was  not a power of two, or was not a
              multiple of sizeof(void *).

       ENOMEM There was insufficient memory to fulfill the allocation request.

NOTES
       On  many systems there are alignment restrictions, e.g. on buffers 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 parameter
       is correct.

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

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

AVAILABILITY
       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>. In
       order to declare it, glibc needs _GNU_SOURCE defined, or  _XOPEN_SOURCE
       defined to a value not less than 600.

       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).

SEE ALSO
       brk(2), getpagesize(2), free(3), malloc(3), feature_test_macros(7)

GNU                               2003-08-22                 POSIX_MEMALIGN(3)