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 allocated memory. The memory address will be a multiple of
boundary, 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 memalign(sysconf(_SC_PAGESIZE),size).
On many systems there are alignment restrictions, e.g. 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 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 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.
Everybody agrees that posix_memalign() is declared in <stdlib.h>.
In order to declare it, glibc needs _GNU_SOURCE defined, or _XOPEN_SOURCE
defined to a value not less than 600.
Everybody agrees that memalign() is declared in <malloc.h>.
According to SUSv2, valloc() is declared in <stdlib.h>. Libc4,5 and
glibc declare it in <malloc.h> and perhaps also in <stdlib.h>
(namely, if _GNU_SOURCE is defined, or _BSD_SOURCE is defined, or, for glibc,
if _XOPEN_SOURCE_EXTENDED is defined, or, equivalently, _XOPEN_SOURCE is
defined to a value not less than 500).