XMIT_SIZE
if (CIRC_SPACE(head, tail, XMIT_SIZE) < n + 2) {
c = CIRC_SPACE_TO_END(head, tail, XMIT_SIZE);
c = CIRC_CNT_TO_END(iforce->xmit.head, iforce->xmit.tail, XMIT_SIZE);
#define XMIT_INC(var, n) (var)+=n; (var)&= XMIT_SIZE -1
unsigned char xmit_data[XMIT_SIZE];