posix_memalign, aligned_alloc, memalign, valloc, pvalloc - allocate aligned
memory
#include <stdlib.h>
int posix_memalign(void **memptr, size_t alignment, size_t size);
void *aligned_alloc(size_t alignment, size_t size);
void *valloc(size_t size);
#include <malloc.h>
void *memalign(size_t alignment, size_t size);
void *pvalloc(size_t size);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
posix_memalign(): _POSIX_C_SOURCE >= 200112L
aligned_alloc(): _ISOC11_SOURCE
valloc():
- Since glibc 2.12:
-
(_XOPEN_SOURCE >= 500) && !(_POSIX_C_SOURCE >= 200112L)
|| /* Glibc since 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
- Before glibc 2.12:
- _BSD_SOURCE || _XOPEN_SOURCE >= 500
(The (nonstandard) header file <malloc.h> also exposes the
declaration of valloc(); no feature test macros are required.)
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 the value placed in
*memptr is either NULL, or a unique pointer
value that can later be successfully passed to
free(3).
The obsolete function
memalign() allocates
size bytes and returns
a pointer to the allocated memory. The memory address will be a multiple of
alignment, which must be a power of two.
The function
aligned_alloc() is the same as
memalign(), except for
the added restriction that
size should be a multiple of
alignment.
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).
The obsolete function
pvalloc() is similar to
valloc(), but rounds
the size of the allocation up to the next multiple of the system page size.
For all of these functions, the memory is not zeroed.
aligned_alloc(),
memalign(),
valloc(), and
pvalloc()
return a pointer to the allocated memory on success. On error, NULL is
returned, and
errno is set to indicate the cause of the error.
posix_memalign() returns zero on success, or one of the error values
listed in the next section on failure. The value of
errno is not set.
On Linux (and other systems),
posix_memalign() does not modify
memptr on failure. A requirement standardizing this behavior was added
in POSIX.1-2016.
- 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(),
valloc(), and
pvalloc() have been
available since at least glibc 2.0.
The function
aligned_alloc() was added to glibc in version 2.16.
The function
posix_memalign() is available since glibc 2.1.91.
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
aligned_alloc (), memalign (), posix_memalign () |
Thread safety |
MT-Safe |
valloc (), pvalloc () |
Thread safety |
MT-Unsafe init |
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.
The function
pvalloc() is a GNU extension.
The function
memalign() appears in SunOS 4.1.3 but not in 4.4BSD.
The function
posix_memalign() comes from POSIX.1d and is specified in
POSIX.1-2001 and POSIX.1-2008.
The function
aligned_alloc() is specified in the C11 standard.
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 also in
<stdlib.h> if suitable feature test macros are defined (see
above).
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
alignment
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 pass to
free(3) only a pointer obtained 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 functions to be reclaimed with
free(3).
The glibc
malloc(3) always returns 8-byte aligned memory addresses, so
these functions are needed only if you require larger alignment values.
brk(2),
getpagesize(2),
free(3),
malloc(3)