Manual Reference Pages  - POSIX_MEMALIGN (3)


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




#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 || _XOPEN_SOURCE >= 600

aligned_alloc(): _ISOC11_SOURCE


Since glibc 2.12:
    (_XOPEN_SOURCE >= 500 ||
    !(_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600)

Before glibc 2.12:
(The (nonstandard) header file <malloc.h> also exposes the declaration of valloc(); no feature test macros are required.)


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 posix_memalign() returns 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 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.


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. Note that errno is not set.


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.


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.


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.1-2001.

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.

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 <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).


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 only pass to free(3) a pointer gotten 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 only needed if you require larger alignment values.


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


This page is part of release 3.44 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at

GNU POSIX_MEMALIGN (3) 2012-03-23
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