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
               declaration 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 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 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 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 cause of 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 There was insufficient memory to fulfill the allocation request.

VERSIONS
       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
       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
       <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.  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)

COLOPHON
       This page is part of release 5.09 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                                2020-08-13                  POSIX_MEMALIGN(3)