POSIX_MEMALIGN(3) Linux Programmer's Manual POSIX_MEMALIGN(3)
posix_memalign, memalign, valloc - Allocate aligned memory
int posix_memalign(void **memptr, size_t alignment, size_t size);
void *memalign(size_t boundary, size_t size);
void *valloc(size_t size);
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 *).
The obsolete function memalign() allocates size bytes and returns a pointer to the allo-
cated memory. The memory address will be a multiple of boundary, which must be a power of
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 three routines, the memory is not zeroed.
memalign() and valloc() return the pointer to the allocated memory, or NULL if the request
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 parameter was not a power of two, or was not a multiple of
ENOMEM There was insufficient memory to fulfill the allocation request.
posix_memalign() verifies that alignment matches the requirements detailed above. mema-
lign() may not check that the boundary parameter is correct.
POSIX requires that memory obtained from posix_memalign() can be freed using free(). Some
systems provide no way to reclaim memory allocated with memalign() or valloc() (because
one can only pass to free() a pointer gotten from malloc(), while e.g. memalign() would
call malloc() and then align the obtained value). GNU libc allows memory obtained from
any of these three routines to be reclaimed with free().
GNU libc malloc() always returns 8-byte aligned memory addresses, so these routines are
only needed if you require larger alignment values.
The functions memalign() and valloc() have been available in all Linux libc libraries.
The function posix_memalign() is available since glibc 2.1.91.
The function valloc() appeared in 3.0 BSD. It is documented as being obsolete in BSD 4.3,
and as legacy in SUSv2. It no longer occurs in SUSv3. The function memalign() appears in
SunOS 4.1.3 but not in BSD 4.4. The function posix_memalign() comes from POSIX 1003.1d.
malloc(3), free(3), getpagesize(2), brk(2)
GNU 2001-10-11 POSIX_MEMALIGN(3)