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
- 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.)
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 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
For all of these functions, the memory is not zeroed.
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.
alignment argument was not a power of two, or was not a multiple of
There was insufficient memory to fulfill the allocation request.
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.
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).
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 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.
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