posix_memalign(3) -- Linux man page



posix_memalign, memalign, valloc - Allocate aligned memory  


#define _XOPEN_SOURCE 600
#include <stdlib.h>

int posix_memalign(void **memptr, size_t alignment, size_t size);

#include <malloc.h>

void *valloc(size_t size);
void *memalign(size_t boundary, size_t size);


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 *).

The obsolete function memalign() allocates size bytes and returns a pointer to the allocated memory. The memory address will be a multiple of boundary, which must be a power of two.

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).

For all three routines, the memory is not zeroed.



memalign() and valloc() return the 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. Note that errno is not set.



The alignment parameter was not a power of two, or was not a multiple of sizeof(void *).
There was insufficient memory to fulfill the allocation request.



On many systems there are alignment restrictions, e.g. 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 boundary parameter is correct.

POSIX requires that memory obtained from posix_memalign() can be freed using free(). Some systems provide no way to reclaim memory allocated with memalign() or valloc() (because one can only pass to free() a pointer gotten from malloc(), while e.g. memalign() would call malloc() and then align the obtained value). GNU libc allows memory obtained from any of these three routines to be reclaimed with free().

GNU libc malloc() always returns 8-byte aligned memory addresses, so these routines are only needed if you require larger alignment values.



The functions memalign() and valloc() have been available in all Linux libc libraries. The function posix_memalign() is available since glibc 2.1.91.



The function valloc() appeared in 3.0 BSD. It is documented as being obsolete in BSD 4.3, and as legacy in SUSv2. It no longer occurs in SUSv3. The function memalign() appears in SunOS 4.1.3 but not in BSD 4.4. The function posix_memalign() comes from POSIX 1003.1d.



Everybody agrees that posix_memalign() is declared in <stdlib.h>. In order to declare it, glibc needs _GNU_SOURCE defined, or _XOPEN_SOURCE defined to a value not less than 600.

Everybody agrees that memalign() is declared in <malloc.h>.

According to SUSv2, valloc() is declared in <stdlib.h>. Libc4,5 and glibc declare it in <malloc.h> and perhaps also in <stdlib.h> (namely, if _GNU_SOURCE is defined, or _BSD_SOURCE is defined, or, for glibc, if _XOPEN_SOURCE_EXTENDED is defined, or, equivalently, _XOPEN_SOURCE is defined to a value not less than 500).



malloc(3), free(3), getpagesize(2), brk(2)