If ptr is the NULL pointer, the behavior of the realloc function
    is identical to the malloc function.

    The contents of the area are unchanged up to the lesser of the
    old and new sizes. The ANSI C Standard states that, "If the
    new size is larger than the old size, the value of the newly
    allocated portion of memory is indeterminate." For compatibility
    with old implementations, Compaq C initializes the newly
    allocated memory to 0.

    For efficiency, the previous actual allocation could have been
    larger than the requested size. If it was allocated with malloc,
    the value of the portion of memory between the previous requested
    allocation and the actual allocation is indeterminate. If it was
    allocated with calloc, that same memory was initialized to 0. If
    your application relies on realloc initializing memory to 0, then
    use calloc instead of malloc to perform the initial allocation.
    The maximum amount of memory allocated at once is limited to
    0xFFFFD000.

    See also free, cfree, calloc, and malloc.