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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

       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 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(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_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_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 <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 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 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
       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

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       pages/.