POSIX_MEMALIGN
Section: Linux Programmer's Manual (3)
Updated: 2010-09-20
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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
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
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 3.27 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- CONFORMING TO
-
- Headers
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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Time: 19:49:24 GMT, April 27, 2011