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
       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 *).  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, 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.  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
       attributes(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
       detailed  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
       reclaimed 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 4.15 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                               2017-09-15                 POSIX_MEMALIGN(3)
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