msgrcv
int msgrcv(int, void *, size_t, long, int); /* XXX should return ssize_t */
struct msgrcv_args msgrcv;
if (msgrcv(sender_msqid, &m, sizeof(m), MTYPE_1_ACK, 0) != sizeof(m))
if (msgrcv(sender_msqid, &m, sizeof(m), MTYPE_2_ACK, 0) != sizeof(m))
if (msgrcv(msqid, &m, sizeof(m), MTYPE_1, 0) != sizeof(m))
if (msgrcv(msqid, &m, sizeof(m), MTYPE_2, 0) != sizeof(m))