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POSIX_MEMALIGN(3) Linux Programmer’s Manual POSIX_MEMALIGN(3)
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 multi-
ple 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 mema-
lign() 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)
GNU 2003-08-22 POSIX_MEMALIGN(3)
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On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
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