Does realloc of memory allocated by C11 aligned_alloc keep the alignment?

Question

Consider the following (C11) code:

void *ptr = aligned_alloc(4096, 4096);
... // do something with 'ptr'
ptr = realloc(ptr, 6000);

Since the memory that ptr points to has a 4096-byte alignment from aligned_alloc, will it (read: is it guaranteed to) keep that alignment after a (successful) call to realloc? Or could the memory revert to the default alignment?

Answer 1

The alignment is not kept with the pointer. When you call realloc you can only rely on the alignment that realloc guarantees. You'll need to use aligned_alloc to perform any reallocations.