Linux线程的信号量同步

叁歲伎倆 2022-06-02 04:14 200阅读 0赞

[https://www.cnblogs.com/jiqingwu/p/linux\_semaphore\_example.html][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 <pthread.h>
    #include <semaphore.h>
    #include <unistd.h>
    #include <stdio.h>
    #include <stdlib.h>
    
    
    /* @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 ...

分类: [C/C++][C_C], [Linux开发][Linux]

标签: [Linux][Linux 1], [thread][], [semaphore][]

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