The ISO C standards provide this typedef (defined in stddef.h). ptrdiff_t is a signed integer typedef which is guaranteed to be able to store the difference between two pointers. Note that you do not know that ptrdiff_t will be an int; for instance, on SGI's 64 bit memory model, ints are 4 bytes, while pointers are 8.
Even more perplexing is the fact that you cannot really rely on the sign of the difference! For instance, on a machine with 32 bit pointers and 32 bit ints, we might have
typedef int ptrdiff_t;
In such a situation, if (DANGER! Non-portable code ahead!)
char *p = (char*) 0x11111111;
char *q = (char*) 0x99999999;
will be a positive
ptrdiff_t is far from perfect. But used with care, it can help make some things portable which couldn't be without it.