NAME¶

std::aligned_alloc - std::aligned_alloc

Synopsis¶

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 (implicitly creating objects in the destination area). 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, std::free

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.

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. To avoid
a memory leak, the returned pointer must be deallocated with std::free or
std::realloc.

On failure, returns a null pointer.

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).

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(std::max_align_t), aligned_alloc may simply call std::malloc.

Regular std::malloc aligns memory suitable for any object type with a fundamental
alignment. This function 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).

Example¶

// Run this code

#include <cstdio>
#include <cstdlib>

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

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

Possible output:¶

default-aligned address: 0x2221c20
1024-byte aligned address: 0x2222400

See also¶

aligned_storage defines the type suitable for use as uninitialized
(C++11)(deprecated in C++23) storage for types of given size
(class template)
C documentation for
aligned_alloc