posix_memalign(3) - Linux manual page (original) (raw)

posixmemalign(3) Library Functions Manual posixmemalign(3)

NAME top

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

LIBRARY top

   Standard C library (_libc_, _-lc_)

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_**);**
   **[[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 top

   **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)](../man3/free.3.html).  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(SCPAGESIZE),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 top

   **aligned_alloc**(), **memalign**(), **valloc**(), and **pvalloc**() return a
   pointer to the allocated memory on success.  On error, NULL is
   returned, and _[errno](../man3/errno.3.html)_ 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](../man3/errno.3.html)_
   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** Out of memory.

ATTRIBUTES top

   For an explanation of the terms used in this section, see
   [attributes(7)](../man7/attributes.7.html).
   ┌───────────────────────────────┬───────────────┬────────────────┐
   │ **Interface** │ **Attribute** │ **Value** │
   ├───────────────────────────────┼───────────────┼────────────────┤
   │ **aligned_alloc**(), **memalign**(),  │ Thread safety │ MT-Safe        │
   │ **posix_memalign**()              │               │                │
   ├───────────────────────────────┼───────────────┼────────────────┤
   │ **valloc**(), **pvalloc**()           │ Thread safety │ MT-Unsafe init │
   └───────────────────────────────┴───────────────┴────────────────┘

STANDARDS top

   **aligned_alloc**()
          C11.

   **posix_memalign**()
          POSIX.1-2008.

   **memalign**()
   **valloc**()
          None.

   **pvalloc**()
          GNU.

HISTORY top

   **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 top

   On many systems there are alignment restrictions, for example, on
   buffers used for direct block device I/O.  POSIX specifies the
   _pathconf(path,PCRECXFERALIGN)_ 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)](../man3/free.3.html).  Some systems provide no way to reclaim
   memory allocated with **memalign**() or **valloc**() (because one can pass
   to [free(3)](../man3/free.3.html) only a pointer obtained from [malloc(3)](../man3/malloc.3.html), while, for
   example, **memalign**() would call [malloc(3)](../man3/malloc.3.html) and then align the
   obtained value).  The glibc implementation allows memory obtained
   from any of these functions to be reclaimed with [free(3)](../man3/free.3.html).

   The glibc [malloc(3)](../man3/malloc.3.html) always returns 8-byte aligned memory
   addresses, so these functions are needed only if you require
   larger alignment values.

SEE ALSO top

   [brk(2)](../man2/brk.2.html), [getpagesize(2)](../man2/getpagesize.2.html), [free(3)](../man3/free.3.html), [malloc(3)](../man3/malloc.3.html)

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Linux man-pages 6.10 2024-07-23 posixmemalign(3)

Pages that refer to this page:io_uring_register(2), io_uring_register_buf_ring(3), malloc(3), malloc_hook(3), mallopt(3), mtrace(3), pthread_attr_setstack(3)