#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():
(_XOPEN_SOURCE >= 500) && !(_POSIX_C_SOURCE >= 200112L)
|| /* Glibc since 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
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.
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.1-2016.
The function aligned_alloc() was added to glibc in version 2.16.
The function posix_memalign() is available since glibc 2.1.91.
| Interface | Attribute | Value |
|
aligned_alloc(),
memalign(), posix_memalign() | Thread safety | MT-Safe |
|
valloc(),
pvalloc() | Thread safety | MT-Unsafe init |
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.
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).
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.