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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)
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