posix_memalign(3)          Library Functions Manual          posix_memalign(3)

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

LIBRARY
       Standard C library (libc, -lc)

SYNOPSIS
       #include <stdlib.h>

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

       #include <malloc.h>

       [[deprecated]] void *memalign(size_t alignment, size_t size);
       [[deprecated]] 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 >= 2.19: */ _DEFAULT_SOURCE
                   || /* glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
           Before glibc 2.12:
               _BSD_SOURCE || _XOPEN_SOURCE >= 500

DESCRIPTION
       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.

       aligned_alloc() is the same as memalign(), except for the added
       restriction that alignment must be a power of two.

       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 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 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
       EINVAL The alignment argument was not a power of two, or was not a
              multiple of sizeof(void *).

       ENOMEM Out of memory.

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

       +-------------------------------------+---------------+----------------+
       |Interface                            | Attribute     | Value          |
       +-------------------------------------+---------------+----------------+
       |aligned_alloc (), memalign (),       | Thread safety | MT-Safe        |
       |posix_memalign ()                    |               |                |
       +-------------------------------------+---------------+----------------+
       |valloc (), pvalloc ()                | Thread safety | MT-Unsafe init |
       +-------------------------------------+---------------+----------------+
STANDARDS
       aligned_alloc()
              C11.

       posix_memalign()
              POSIX.1-2008.

       memalign()
       valloc()
              None.

       pvalloc()
              GNU.

HISTORY
       aligned_alloc()
              glibc 2.16.  C11.

       posix_memalign()
              glibc 2.1.91.  POSIX.1d, POSIX.1-2001.

       memalign()
              glibc 2.0.  SunOS 4.1.3.

       valloc()
              glibc 2.0.  3.0BSD.  Documented as obsolete in 4.3BSD, and as
              legacy in SUSv2.

       pvalloc()
              glibc 2.0.

   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>.  glibc declares
       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.  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
       brk(2), getpagesize(2), free(3), malloc(3)

Linux man-pages 6.7               2023-11-24                 posix_memalign(3)