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); |
![[Note]](../stylesheet/note.png) |
Note | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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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 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.
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
alignmentargument was not a power of two, or was not a multiple 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(),memalign(),
posix_memalign()
|
Thread safety | MT-Safe |
|
|
Thread safety | MT-Unsafe init |
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 <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).
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
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.
COLOPHON
This page is part of release 4.07 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/.
|
Copyright (c) 2001 by John Levon <mozcompsoc.man.ac.uk> Based in part on GNU libc documentation. %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END 2001-10-11, 2003-08-22, aeb, added some details 2012-03-23, Michael Kerrisk <mtk.manpagesmail.com> Document pvalloc() and aligned_alloc() |