Manpage of POSIX_MEMALIGN

POSIX_MEMALIGN

Section: Linux Programmer's Manual (3)
Updated: 2016-03-15
Index
 

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 sizebytes 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 sizeis 0, then the value placed in *memptris either NULL, or a unique pointer value that can later be successfully passed to free(3).

The obsolete function memalign() allocates sizebytes 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 sizeshould be a multiple of alignment.

The obsolete function valloc() allocates sizebytes 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 errnois not set. On Linux (and other systems), posix_memalign() does not modify memptron 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).
InterfaceAttributeValue
aligned_alloc(),
memalign(),
posix_memalign()
Thread safetyMT-Safe
valloc(),
pvalloc()
Thread safetyMT-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 alignmentmatches the requirements detailed above. memalign() may not check that the alignmentargument 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)


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
VERSIONS
ATTRIBUTES
CONFORMING TO
Headers
NOTES
SEE ALSO

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Time: 22:27:58 GMT, June 20, 2016