If there is not enough memory available at the original location:
realloc compiler/OS dependent?
realloc attempts to extend your available memory range if sufficient memory is available behind it on the heap. If not then it is equivalent to
malloc a block of the new size,
memcpy your contents there,
free the old block. This is independent of both OS and compiler and depends on the implementation of
libc that you link against.
On a similar note:
mremap/MREMAP_MAYMOVE (available on modern Linux) will attempt to extend your virtual mapping by the requested size. If that is not possible then it will move your mapping to a new virtual address that has sufficient VM space behind it and then extend your mapping. This is very fast if you are frequently resizing large mappings since no physical copying is done.