# std::aligned_alloc

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< cpp‎ | memory‎ | c

Defined in header `<cstdlib>` |  |
void* aligned_alloc( std::size_t alignment, std::size_t size ); |  | (since C++17)

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

The following functions are required to be thread-safe:

- The library versions of `operator new` and `operator delete`
- User replacement versions of global `operator new` and `operator delete`
- std::calloc, std::malloc, std::realloc, std::aligned_alloc (since C++17)

Calls to these functions that allocate or deallocate a particular unit of storage occur in a single total order, and each such deallocation call happens-before the next allocation (if any) in this order.

(since C++11)

### 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`

### Return value

On success, returns the pointer to the beginning of newly allocated memory. The returned pointer must be deallocated with free() or `realloc()`.

On failure, returns a null pointer.

### Notes

Passing a `size` which is not an integral multiple of `alignment` or a `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 DR 460).

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.

Regular std::malloc aligns memory suitable for any object type (which, in practice, means that it is aligned to alignof(std::max_align_t)). This function is useful for over-aligned allocations, such as to SSE, cache line, or VM page boundary.

### Example

Run this code

    #include <cstdio>
    #include <cstdlib>

    int main()
    {
        int* p1 = std::malloc(10*sizeof *p1);
        std::printf("default-aligned addr:   %p\n", (void*)p1);
        std::free(p1);

        int* p2 = std::aligned_alloc(1024, 1024*sizeof *p2);
        std::printf("1024-byte aligned addr: %p\n", (void*)p2);
        std::free(p2);
    }

Possible output:

    default-aligned addr:   0x2221c20
    1024-byte aligned addr: 0x2222000

### See also

aligned_storage (C++11) | defines the type suitable for use as uninitialized storage for types of given size (class template)

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