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posix_memalign - Man Page

allocate aligned memory


Standard C library (libc, -lc)


#include <stdlib.h>

int posix_memalign(void **memptr, size_t alignment, size_t size);
void *aligned_alloc(size_t alignment, size_t size);
[[deprecated]] void *valloc(size_t size);

#include <malloc.h>

[[deprecated]] void *memalign(size_t alignment, size_t size);
[[deprecated]] void *pvalloc(size_t size);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):


    _POSIX_C_SOURCE >= 200112L




    Since glibc 2.12:
        (_XOPEN_SOURCE >= 500) && !(_POSIX_C_SOURCE >= 200112L)
            || /* glibc >= 2.19: */ _DEFAULT_SOURCE
            || /* glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
    Before glibc 2.12:
        _BSD_SOURCE || _XOPEN_SOURCE >= 500


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 *). This address can later be successfully passed to free(3). If size is 0, then the value placed in *memptr is either NULL or a unique pointer value.

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.

aligned_alloc() is the same as memalign(), except for the added restriction that alignment 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).

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 on success. On error, NULL is returned, and errno is set to indicate the error.

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.1-2008 TC2.



The alignment argument was not a power of two, or was not a multiple of sizeof(void *).


Out of memory.


For an explanation of the terms used in this section, see attributes(7).

aligned_alloc(), memalign(), posix_memalign()Thread safetyMT-Safe
valloc(), pvalloc()Thread safetyMT-Unsafe init












glibc 2.16. C11.


glibc 2.1.91. POSIX.1d, POSIX.1-2001.


glibc 2.0. SunOS 4.1.3.


glibc 2.0. 3.0BSD. Documented as obsolete in 4.3BSD, and as legacy in SUSv2.


glibc 2.0.


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>. glibc declares it in <malloc.h>, and also in <stdlib.h> if suitable feature test macros are defined (see above).


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)

Referenced By

io_uring_register(2), io_uring_register_buf_ring(3), malloc(3), malloc_hook(3), mallopt(3), memkind_arena(3), mtrace(3), pthread_attr_setstack(3), reallocf.3bsd(3), stress-ng(1).

The man pages aligned_alloc(3), memalign(3), pvalloc(3) and valloc(3) are aliases of posix_memalign(3).

2024-05-02 Linux man-pages 6.9.1