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 || _XOPEN_SOURCE >= 600

       aligned_alloc(): _ISOC11_SOURCE

       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
               (The  (nonstandard)  header  file  <malloc.h> also exposes the declaration of val‐
               loc(); 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 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 suc‐
       cessfully passed to free(3).

       The obsolete function memalign() allocates size bytes and returns a pointer to  the  allo‐
       cated  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_PAGESIZE),size).

       The obsolete function pvalloc() is similar to valloc(), but rounds the size of the alloca‐
       tion 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 indeterminate after a call to posix_memalign().

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(),  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.

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 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.

       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 <malloc.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  feature  test  macros  are  defined  (see
       above).

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.  mema‐
       lign() 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  exam‐
       ple, memalign() would call malloc(3) and then align the obtained value).  The glibc imple‐
       mentation 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 3.74 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 http://www.kernel.org/doc/man-pages/.



GNU                                         2013-09-02                          POSIX_MEMALIGN(3)