Semophere bridge=1;
Semophere mutexNS=1,mutexSN=1;//用于保护countNS,countSN 
int countNS=0,countSN=0; 
Semophere s1=1,s2=0;//用于交替通过 
StoN(){while(1){P(mutexSN);countSN++;//来车了v(mutexSN);p(mutexSN);if(countNS==0){//对面无车,则直接通过P(bridge);通过countSN--;V(bridge); }else{//对面有车，则依次通过 P(s1);//先让自己通过P(bridge);通过countSN--;//该方向车辆减一 V(bridge);V(s2); }V(mutexSN); }
}
NtoS(){while(1){P(mutexNS);countNS++;//来车了if(countSN==0){//对面无车,则直接通过P(bridge);通过countNS--;V(bridge); }else{//对面有车，则依次通过 P(s2);//先让自己通过P(bridge);通过countNS--;V(bridge);V(s1); }V(mutexNS);}
}