memalign(3) -- Linux man page
NAME
posix_memalign, memalign, valloc - Allocate aligned memory
SYNOPSIS
#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);
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 *).
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
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.
NOTES
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.
AVAILABILITY
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 3.0 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.
HEADERS
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).
SEE ALSO
malloc(3),
free(3),
getpagesize(2),
brk(2)
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