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. 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, (since C++11)
std::aligned_alloc
(since C++17), 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 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 = 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)