Competition Among processes for resources Mutual exclusion(互斥) Critical sections(临界区) Only one program at a time is allowed in its critical section(一次仅允许一个进程在临界区) Example only one process at a time is allowed to send command to the printer (critical resource (例如一次仅允许一个进程发打印命令) Deadlock(死锁) · Starvationcritical resource） （例如一次仅允许一个进程发打印命令）