POSIX_MEMALIGN(3) Linux Programmer's Manual POSIX_MEMALIGN(3)
NAME
posix_memalign, aligned_alloc, memalign, valloc, pvalloc - allocate
aligned memory
SYNOPSIS
#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 dec-
laration of valloc(); no feature test macros are required.)
DESCRIPTION
The function posix_memalign() allocates size bytes and places the
address of the allocated memory in *memptr. The address of the allo-
cated 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_PAGE-
SIZE),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.
RETURN VALUE
aligned_alloc(), memalign(), valloc(), and pvalloc() return a 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. 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.
ERRORS
EINVAL The alignment argument was not a power of two, or was not a mul-
tiple of sizeof(void *).
ENOMEM There was insufficient memory to fulfill the allocation request.
VERSIONS
The functions memalign(), valloc(), and pvalloc() have been available
in all Linux libc libraries.
The function aligned_alloc() was added to glibc in version 2.16.
The function posix_memalign() is available since glibc 2.1.91.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
+-----------------+---------------+----------------+
|Interface | Attribute | Value |
+-----------------+---------------+----------------+
|aligned_alloc(), | Thread safety | MT-Safe |
|memalign(), | | |
|posix_memalign() | | |
+-----------------+---------------+----------------+
|valloc(), | Thread safety | MT-Unsafe init |
|pvalloc() | | |
+-----------------+---------------+----------------+
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.
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.
Headers
Everybody agrees that posix_memalign() is declared in <stdlib.h>.
On some systems memalign() is declared in <stdlib.h> instead of <mal-
loc.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 fea-
ture test macros are defined (see above).
NOTES
On many systems there are alignment restrictions, for example, on buf-
fers 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 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 implemen-
tation 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.
SEE ALSO
brk(2), getpagesize(2), free(3), malloc(3)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
GNU 2017-09-15 POSIX_MEMALIGN(3)
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