Or, in 'C',

buffer[idx++] = SBUF;
idx          &= RINGBUFMASK;

In 'C', arrays & pointers are very closely related - so the use of one doesn't necessarily preclude the use of the other.

Note that there is quite a widespread belief that pointers are inherently faster than indices; this is not necessarily so; eg, see:
http://www.keil.com/discuss/docs/thread1156.htm