POSIX_MEMALIGN
Section: Linux Programmer's Manual (3)
Updated: 2017-09-15
Index
Return to Main Contents
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 declaration 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 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
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.
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
|
valloc(),
pvalloc()
| 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.
SEE ALSO
brk(2),
getpagesize(2),
free(3),
malloc(3)
COLOPHON
This page is part of release 4.13 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- ATTRIBUTES
-
- CONFORMING TO
-
- Headers
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
|
|
- Running on 
-
:
: 