std::aligned_alloc

 

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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), 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.

(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. 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 (C++11)(deprecated in C++23)

defines the type suitable for use as uninitialized storage for types of given size (class template)