posix_memalign

POSIX_MEMALIGN(3)          Linux Programmer's Manual         POSIX_MEMALIGN(3)

NAME
       posix_memalign,  aligned_alloc,  memalign,  valloc,  pvalloc - allocate
       aligned memory

SYNOPSIS
       #include <stdlib.h>

       int posix_memalign(void **memptr, size_t alignment, size_t size);
       void *aligned_alloc(size_t alignment, size_t size);
       void *valloc(size_t size);

       #include <malloc.h>

       void *memalign(size_t alignment, size_t size);
       void *pvalloc(size_t size);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       posix_memalign(): _POSIX_C_SOURCE >= 200112L

       aligned_alloc(): _ISOC11_SOURCE

       valloc():
           Since glibc 2.12:
               (_XOPEN_SOURCE >= 500) && !(_POSIX_C_SOURCE >= 200112L)
                   || /* Glibc since 2.19: */ _DEFAULT_SOURCE
                   || /* Glibc versions <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
           Before glibc 2.12:
               _BSD_SOURCE || _XOPEN_SOURCE >= 500
               (The (nonstandard) header file <malloc.h> also exposes the dec-
               laration of valloc(); no feature test macros are required.)

DESCRIPTION
       The  function  posix_memalign() allocates size bytes and places the ad-
       dress 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 the value  placed
       in  *memptr is 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 alignment, which must be a power of two.

       The function aligned_alloc() is the same as memalign(), except for  the
       added restriction that size should be a multiple of alignment.

       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).

       The obsolete function pvalloc() is similar to valloc(), but rounds  the
       size of the allocation up to the next multiple of the system page size.

       For all of these functions, the memory is not zeroed.

RETURN VALUE
       aligned_alloc(),  memalign(),  valloc(), and pvalloc() return a pointer
       to the allocated memory on success.  On error, NULL  is  returned,  and
       errno is set to indicate the cause of the error.

       posix_memalign()  returns  zero  on success, or one of the error values
       listed in the next section on failure.  The value of errno is not  set.
       On  Linux  (and other systems), posix_memalign() does not modify memptr
       on failure.  A requirement standardizing this  behavior  was  added  in
       POSIX.1-2016.

ERRORS
       EINVAL The alignment argument was not a power of two, or was not a mul-
              tiple of sizeof(void *).

       ENOMEM There was insufficient memory to fulfill the allocation request.

VERSIONS
       The functions memalign(), valloc(), and pvalloc() have  been  available
       in all Linux libc libraries.

       The function aligned_alloc() was added to glibc in version 2.16.

       The function posix_memalign() is available since glibc 2.1.91.

ATTRIBUTES
       For  an  explanation  of  the  terms  used  in  this  section,  see at-
       tributes(7).

       +-----------------+---------------+----------------+
       |Interface        | Attribute     | Value          |
       +-----------------+---------------+----------------+
       |aligned_alloc(), | Thread safety | MT-Safe        |
       |memalign(),      |               |                |
       |posix_memalign() |               |                |
       +-----------------+---------------+----------------+
       |valloc(),        | Thread safety | MT-Unsafe init |
       |pvalloc()        |               |                |
       +-----------------+---------------+----------------+

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.

       The function pvalloc() is a GNU extension.

       The function memalign() appears in SunOS 4.1.3 but not in 4.4BSD.

       The function posix_memalign() comes from POSIX.1d and is  specified  in
       POSIX.1-2001 and POSIX.1-2008.

       The function aligned_alloc() is specified in the C11 standard.

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

       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 also in <stdlib.h> if suitable fea-
       ture test macros are defined (see above).

NOTES
       On many systems there are alignment restrictions, for example, on  buf-
       fers  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 de-
       tailed above.  memalign() may not check that the alignment 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 pass to free(3) only a
       pointer obtained from malloc(3), while, for example,  memalign()  would
       call malloc(3) and then align the obtained value).  The glibc implemen-
       tation allows memory obtained from any of these  functions  to  be  re-
       claimed with free(3).

       The  glibc malloc(3) always returns 8-byte aligned memory addresses, so
       these functions are needed only if you require larger alignment values.

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

COLOPHON
       This page is part of release 5.05 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.

GNU                               2019-05-09                 POSIX_MEMALIGN(3)