function realloc(ptr,n) uses two arguments. the first argument ptr
is a pointer to a block of memory for which the size is to be
altered. The second argument n specifies the new size.
The size may be increased or decreased. If n is greater than the old
size and if sufficient space is not available subsequent to the old
region, the function realloc( ) may create a new region and all the
old data are moved to the new region.