POSIX_MEMALIGN(3) BSD Library Functions Manual POSIX_MEMALIGN(3)NAME
posix_memalign -- aligned memory allocation
posix_memalign(void **memptr, size_t alignment, size_t size);
The posix_memalign() function allocates size bytes of memory such that the allocation's base address is an exact multiple of alignment, and
returns the allocation in the value pointed to by memptr.
The requested alignment must be a power of 2 at least as large as sizeof(void *).
Memory that is allocated via posix_memalign() can be used as an argument in subsequent calls to realloc(3), reallocf(3), and free(3). (Note
however, that the allocation returned by realloc(3) or reallocf(3) is not guaranteed to preserve the original alignment).
posix_memalign() should be used judiciously as the algorithm that realizes the alignment constraint can incur significant memory overhead.
The posix_memalign() function returns the value 0 if successful; otherwise it returns an error value.
The posix_memalign() function will fail if:
[EINVAL] The alignment parameter is not a power of 2 at least as large as sizeof(void *).
[ENOMEM] Memory allocation error.
SEE ALSO free(3), malloc(3), realloc(3), reallocf(3), valloc(3), malloc_zone_memalign(3)STANDARDS
The posix_memalign() function conforms to IEEE Std 1003.1-2001 (``POSIX.1'').
BSD April 9, 2008 BSD
Check Out this Related Man Page
POSIX_MEMALIGN(3) Linux Programmer's Manual POSIX_MEMALIGN(3)NAME
posix_memalign, memalign, valloc - Allocate aligned memory
int posix_memalign(void **memptr, size_t alignment, size_t size);
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
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.
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 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.
The functions memalign() and valloc() have been available in all Linux libc libraries. The function posix_memalign() is available since
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.
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).
On many systems there are alignment restrictions, for example, on buffers used for direct block device I/O. POSIX specifies the path-
conf(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
POSIX requires that memory obtained from posix_memalign() can be freed using free(3). Some systems provide no way to reclaim memory allo-
cated 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 val-
SEE ALSO brk(2), getpagesize(2), free(3), malloc(3)COLOPHON
This page is part of release 3.25 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/.
GNU 2009-03-30 POSIX_MEMALIGN(3)