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https://www.cnblogs.com/jiqingwu/p/linux_semaphore_example.html
信号量和互斥锁(mutex)的区别：互斥锁只允许一个线程进入临界区，而信号量允许多个线程同时进入临界区。
不多做解释，要使用信号量同步，需要包含头文件semaphore.h。
主要用到的函数：
int sem_init(sem_t *sem, int pshared, unsigned int value);，其中sem是要初始化的信号量，pshared表示此信号量是在进程间共享还是线程间共享，value是信号量的初始值。int sem_destroy(sem_t *sem);,其中sem是要销毁的信号量。只有用sem_init初始化的信号量才能用sem_destroy销毁。int sem_wait(sem_t *sem);等待信号量，如果信号量的值大于0,将信号量的值减1,立即返回。如果信号量的值为0,则线程阻塞。相当于P操作。成功返回0,失败返回-1。int sem_post(sem_t *sem); 释放信号量，让信号量的值加1。相当于V操作。下列的代码演示了如何用信号量同步，模拟一个窗口服务系统。
/* @purpose: 基于信号量的多线程同步，操作系统原理中的P,V操作 * @author: jollywing@foxmail.com * @create: 2015-03-20 Fri * */#include#include #include #include #include /* @Scene: 某行业营业厅同时只能服务两个顾客。 * 有多个顾客到来，每个顾客如果发现服务窗口已满，就等待， * 如果有可用的服务窗口，就接受服务。 *//* 将信号量定义为全局变量，方便多个线程共享 */sem_t sem;/* 每个线程要运行的例程 */void * get_service(void *thread_id){ /* 注意：立即保存thread_id的值，因为thread_id是对主线程中循环变量i的引用，它可能马上被修改 */ int customer_id = *((int *)thread_id); if(sem_wait(&sem) == 0) { usleep(100); /* service time: 100ms */ printf("customer %d receive service ...\n", customer_id); sem_post(&sem); }}#define CUSTOMER_NUM 10int main(int argc, char *argv[]){ /* 初始化信号量，初始值为2，表示有两个顾客可以同时接收服务 */ /* @prototype: int sem_init(sem_t *sem, int pshared, unsigned int value); */ /* pshared: if pshared == 0, the semaphore is shared among threads of a process * otherwise the semaphore is shared between processes. */ sem_init(&sem, 0, 2); /* 为每个顾客定义一个线程id, pthread_t 其实是unsigned long int */ pthread_t customers[CUSTOMER_NUM]; int i, ret; /* 为每个顾客生成一个线程 */ for(i = 0; i < CUSTOMER_NUM; i++){ int customer_id = i; ret = pthread_create(&customers[i], NULL, get_service, &customer_id); if(ret != 0){ perror("pthread_create"); exit(1); } else { printf("Customer %d arrived.\n", i); } usleep(10); } /* 等待所有顾客的线程结束 */ /* 注意：这地方不能再用i做循环变量，因为可能线程中正在访问i的值 */ int j; for(j = 0; j < CUSTOMER_NUM; j++) { pthread_join(customers[j], NULL); } /* Only a semaphore that has been initialized by sem_init(3) * should be destroyed using sem_destroy().*/ sem_destroy(&sem); return 0;}
编译：gcc main.c -lpthread。
运行结果（注意，每次运行都不相同）：
Customer 0 arrived.Customer 1 arrived.customer 0 receive service ...Customer 2 arrived.customer 1 receive service ...Customer 3 arrived.customer 2 receive service ...Customer 4 arrived.customer 3 receive service ...Customer 5 arrived.customer 4 receive service ...Customer 6 arrived.customer 5 receive service ...Customer 7 arrived.customer 6 receive service ...Customer 8 arrived.customer 7 receive service ...Customer 9 arrived.customer 8 receive service ...customer 9 receive service ...