linux多线程极大方便了程序编写，但同时必须考虑线程同步、互斥问题。 示例代码给出了利用信号量实现P、V操作，从而保证多线程同步、互斥。-linux multithread programming is very convenient, but we must pay attention to synchronization and mutual exclusion. The sample code shows us how to ensure multi-thread synch