< c‎ | memory
Defined in header <stdlib.h>
void *aligned_alloc( size_t alignment, size_t size );
(since C11)

Allocate size bytes of uninitialized storage whose alignment is specified by alignment. The size parameter must be an integral multiple of alignment.

aligned_alloc is thread-safe: it behaves as though only accessing the memory locations visible through its argument, and not any static storage.

A previous call to free or realloc that deallocates a region of memory synchronizes-with a call to aligned_alloc that allocates the same or a part of the same region of memory. This synchronization occurs after any access to the memory by the deallocating function and before any access to the memory by aligned_alloc. There is a single total order of all allocation and deallocation functions operating on each particular region of memory.


[edit] Parameters

alignment - specifies the alignment. Must be a valid alignment supported by the implementation.
size - number of bytes to allocate. An integral multiple of alignment

[edit] Return value

On success, returns the pointer to the beginning of newly allocated memory. To avoid a memory leak, the returned pointer must be deallocated with free or realloc.

On failure, returns a null pointer.

[edit] Notes

Passing a size which is not an integral multiple of alignment or an alignment which is not valid or not supported by the implementation causes the function to fail and return a null pointer (C11, as published, specified undefined behavior in this case, this was corrected by DR460). Removal of size restrictions to make it possible to allocate small objects at restrictive alignment boundaries (similar to alignas) has been proposed by N2072.

As an example of the "supported by the implementation" requirement, POSIX function posix_memalign accepts any alignment that is a power of two and a multiple of sizeof(void *), and POSIX-based implementations of aligned_alloc inherit this requirements.

Fundamental alignments are always supported. If alignment is a power of two and not greater than _Alignof(max_align_t), aligned_alloc may simply call malloc.

Regular malloc aligns memory suitable for any object type with a fundamental alignment. The aligned_alloc is useful for over-aligned allocations, such as to SSE, cache line, or VM page boundary.

This function is not supported in Microsoft C Runtime library because its implementation of std::free is unable to handle aligned allocations of any kind. Instead, MS CRT provides _aligned_malloc (to be freed with _aligned_free).

[edit] Example

#include <stdio.h>
#include <stdlib.h>
int main(void)
    int *p1 = malloc(10*sizeof *p1);
    printf("default-aligned addr:   %p\n", (void*)p1);
    int *p2 = aligned_alloc(1024, 1024*sizeof *p2);
    printf("1024-byte aligned addr: %p\n", (void*)p2);

Possible output:

default-aligned addr:   0x1e40c20
1024-byte aligned addr: 0x1e41000

[edit] References

  • C23 standard (ISO/IEC 9899:2023):
  • The aligned_alloc function (p: TBD)
  • C17 standard (ISO/IEC 9899:2018):
  • The aligned_alloc function (p: 253)
  • C11 standard (ISO/IEC 9899:2011):
  • The aligned_alloc function (p: 347-348)

[edit] See also

C++ documentation for aligned_alloc