POSIX_MEMALIGNSection: Linux Programmer's Manual (3)
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NAMEposix_memalign, memalign, valloc - Allocate aligned memory
#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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
DESCRIPTIONThe 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 posix_memalign() returns either NULL, or a unique pointer value that can later be successfully passed to free().
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).
RETURN VALUEmemalign() and valloc() return the pointer to the allocated memory, or NULL if the request fails.
- The alignment argument was not a power of two, or was not a multiple of sizeof(void *).
- There was insufficient memory to fulfill the allocation request.
VERSIONSThe functions memalign() and valloc() have been available in all Linux libc libraries. The function posix_memalign() is available since glibc 2.1.91.
CONFORMING TOThe 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.1-2001. The function memalign() appears in SunOS 4.1.3 but not in 4.4BSD. The function posix_memalign() comes from POSIX.1d.
HeadersEverybody agrees that posix_memalign() is declared in <stdlib.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).
NOTESOn 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 boundary 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 only pass to free(3) a pointer gotten 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 three routines to be reclaimed with free(3).
The glibc malloc(3) always returns 8-byte aligned memory addresses, so these routines are only needed if you require larger alignment values.
SEE ALSObrk(2), getpagesize(2), free(3), malloc(3)
COLOPHONThis page is part of release 3.21 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.
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