POSIX_MEMALIGN
NAME
posix_memalign, memalign, valloc - Allocate aligned memory
SYNOPSIS
#define _XOPEN_SOURCE 600
#include <stdlib.h>
I int posix_memalign(void ** memptr , size_t alignment , size_t size );
#include <malloc.h>
I void *valloc(size_t size );
I void *memalign(size_t boundary , size_t size );
DESCRIPTION
The function
R posix_memalign ()
allocates
size
bytes and places the address of the allocated memory in
R *memptr .
The address of the allocated memory will be a multiple of
R alignment ,
which must be a power of two and a multiple of
R sizeof(void *).
The obsolete function
R memalign ()
allocates
size
bytes and returns a pointer to the allocated memory.
The memory address will be a multiple of
R boundary ,
which must be a power of two.
The obsolete function
R 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
R memalign(sysconf(_SC_PAGESIZE),size) .
For all three routines, the memory is not zeroed.
RETURN VALUE
R memalign ()
and
R valloc ()
return the pointer to the allocated memory, or NULL if the request fails.
R posix_memalign ()
returns zero on success, or one of the error values listed in the
next section on failure.
Note that
R errno
is not set.
ERRORS
EINVAL
The
R alignment
parameter was not a power of two, or was not a multiple of
R sizeof(void *) .
ENOMEM
There was insufficient memory to fulfill the allocation request.
VERSIONS
The functions
R memalign ()
and
R valloc ()
have been available in all Linux libc libraries.
The function
R posix_memalign ()
is available since glibc 2.1.91.
CONFORMING TO
The function
R 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
R memalign ()
appears in SunOS 4.1.3 but not in 4.4BSD.
The function
R posix_memalign ()
comes from POSIX.1d.
Headers
Everybody agrees that
R 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.
On some systems
R memalign ()
is declared in
<stdlib.h> instead of
<malloc.h>.
According to SUSv2,
R 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
R posix_memalign ()
to satisfy this requirement.
R posix_memalign ()
verifies that
R alignment
matches the requirements detailed above.
R memalign ()
may not check that the
R boundary
parameter is correct.
POSIX requires that memory obtained from
R posix_memalign ()
can be freed using
free(3).
Some systems provide no way to reclaim memory allocated with
R memalign ()
or
R valloc ()
(because one can only pass to
free(3)
a pointer gotten from
malloc(3),
while, for example,
R memalign ()
would call
malloc(3)
and then align the obtained value).
GNU libc allows memory obtained from any of these three routines to be
reclaimed with
free(3).
GNU libc
malloc(3)
always returns 8-byte aligned memory addresses, so these routines are only
needed if you require larger alignment values.
SEE ALSO
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