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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
posix_memalign(): _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600
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 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.
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.
SEE ALSO
brk(2), getpagesize(2), free(3), malloc(3)
COLOPHON
This page is part of release 3.24 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/.