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POSIX_MEMALIGN(3) Linux Programmer's Manual POSIX_MEMALIGN(3)
posix_memalign, memalign, valloc - Allocate aligned memory
#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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
posix_memalign(): _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600
valloc():
Since glibc 2.12:
_BSD_SOURCE ||
(_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 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(3).
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.
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.
EINVAL The alignment argument was not a power of two, or was not a multiple of
sizeof(void *).
ENOMEM There was insufficient memory to fulfill the allocation request.
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 3.0BSD. 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.
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).
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.
brk(2), getpagesize(2), free(3), malloc(3)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
GNU 2010-09-20 POSIX_MEMALIGN(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface