PV读者写者问题

semaphore mutex = 1;semaphore db = 1;int readercount = 0;void Reader(){while(true){P(mutex);readercount ++;if(readercount == 1)P(db);V(mutex);//读数据P(mutex);   //读完了要走 readercount --;if(readercount == 0)V(db);V(mutex);}}void Writer(){P(db);//写数据 V(db);}