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：
#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 10
int 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 ...