std::aligned_alloc (3) - Linux Man Pages

std::aligned_alloc: std::aligned_alloc


std::aligned_alloc - std::aligned_alloc


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.


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.


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.


// 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));

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

Possible output:

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

See also

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