Originální popis anglicky:
posix_memalign, memalign, valloc - Allocate aligned memory
Návod, kniha: Linux Programmer's Manual
#define _XOPEN_SOURCE 600
#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);
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).
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 parameter was not a power of two, or
was not a multiple of sizeof(void *).
- ENOMEM
- There was insufficient memory to fulfill the allocation
request.
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 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.
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).
brk(2),
getpagesize(2),
free(3),
malloc(3)
