posix_memalign(3) — Linux manual page


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

           _POSIX_C_SOURCE >= 200112L


           Since glibc 2.12:
               (_XOPEN_SOURCE >= 500) && !(_POSIX_C_SOURCE >= 200112L)
                   || /* Glibc since 2.19: */ _DEFAULT_SOURCE
                   || /* Glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
           Before glibc 2.12:
               _BSD_SOURCE || _XOPEN_SOURCE >= 500

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 *).  This address can
       later be successfully passed to free(3).  If size is 0, then the
       value placed in *memptr is either NULL or a unique pointer value.

       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

       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 size.

       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 on success.  On error, NULL is
       returned, and errno is set to indicate 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.1-2008 TC2.

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 since at least glibc 2.0.

       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(), memalign(),   │ Thread safety │ MT-Safe        │
       │posix_memalign()               │               │                │
       │valloc(), pvalloc()            │ Thread safety │ MT-Unsafe init │

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 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.

       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>.  Glibc
       declares 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 values.

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.13 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

GNU                            2021-03-22              POSIX_MEMALIGN(3)

Pages that refer to this page: malloc(3)malloc_hook(3)mallopt(3)mtrace(3)pthread_attr_setstack(3)