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

NAME         top

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

SYNOPSIS         top

       #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

           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
               declaration   of   valloc();   no  feature  test  macros  are

DESCRIPTION         top

       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 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

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

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

RETURN VALUE         top

       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         top

       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

VERSIONS         top

       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         top

       For an explanation of the terms used in this section, see

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

CONFORMING TO         top

       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

       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.

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

       On some systems memalign() is declared in <stdlib.h> instead of

       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         top

       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

       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 implementation 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

SEE ALSO         top

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

COLOPHON         top

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GNU                              2016-03-15                POSIX_MEMALIGN(3)