Name
posix_memalign, memalign, valloc — Allocate aligned memory
Synopsis
#include <stdlib.h>
int posix_memalign( |
void **memptr, |
| size_t alignment, | |
size_t size); |
#include <malloc.h>
void *valloc( |
size_t size); |
void *memalign( |
size_t boundary, |
size_t size); |
![[Note]](../stylesheet/note.png) |
Note | ||
|---|---|---|---|
|
DESCRIPTION
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 posix_memalign() returns either NULL, or a
unique pointer value that can later be successfully passed to
free().
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).
For all three routines, the memory is not zeroed.
RETURN VALUE
memalign() and valloc() return the 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. Note that errno is not set.
ERRORS
- EINVAL
-
The
alignmentargument was not a power of two, or was not a multiple of sizeof(void *). - ENOMEM
-
There was insufficient memory to fulfill the allocation request.
VERSIONS
The functions memalign() and
valloc() have been available in
all Linux libc libraries. The function posix_memalign() is available since glibc
2.1.91.
CONFORMING TO
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 memalign() appears in SunOS 4.1.3 but not
in 4.4BSD. The function posix_memalign() comes from POSIX.1d.
Headers
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 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).
NOTES
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. memalign() may not check that the
boundary 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 only pass to
free(3) a pointer gotten
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 three routines to be reclaimed
with free(3).
The glibc malloc(3) always returns 8-byte aligned memory addresses, so these routines are only needed if you require larger alignment values.