POSIX_MEMALIGN(3) Linux Programmer's Manual POSIX_MEMALIGN(3)
posix_memalign, aligned_alloc, memalign, valloc, pvalloc - allocate aligned memory
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);
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
Since glibc 2.12:
(_XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED) &&
!(_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600)
Before glibc 2.12:
_BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
(The (nonstandard) header file <malloc.h> also exposes the declaration of val-
loc(); 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 the
value placed in *memptr is either NULL, or a unique pointer value that can later be suc-
cessfully passed to free(3).
The obsolete function memalign() allocates size bytes and returns a pointer to the allo-
cated memory. The memory address will be a multiple of alignment, which must be a power
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
The obsolete function pvalloc() is similar to valloc(), but rounds the size of the alloca-
tion 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. The value of errno is indeterminate after a call to posix_memalign().
EINVAL The alignment argument was not a power of two, or was not a multiple of
ENOMEM There was insufficient memory to fulfill the allocation request.
The functions memalign(), valloc(), and pvalloc() have been available in all Linux libc
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
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. mema-
lign() 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 exam-
ple, memalign() would call malloc(3) and then align the obtained value). The glibc imple-
mentation 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.
brk(2), getpagesize(2), free(3), malloc(3)
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GNU 2013-09-02 POSIX_MEMALIGN(3)