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C语言多线程编程 semaphore 信号量（信号灯）是什么？如何使用？
semaphore 信号量、信号灯
定义
有时被称为信号灯，是在多线程环境下使用的一种设施，是可以用来保证两个或多个关键代码段不被并发调用。
目的
类似计数器，常用在多线程同步任务上，信号量可以在当前线程某个任务完成后，通知别的线程，再进行别的任务。
分类
二值信号量：信号量的值只有0和1，这和互斥量很类似，若资源被锁住，信号量的值为0，若资源可用，则信号量的值为1；
计数信号量：信号量的值在0到一个大于1的限制值之间，该计数表示可用的资源的个数。
信号量在创建时需要设置一个初始值，表示同时可以有几个任务可以访问该信号量保护的共享资源，初始值为1就变成互斥锁Mutex，即同时只能有一个任务可以访问信号量保护的共享资源
函数使用
首先需要
include这个库，没啥好说的，除非你自己实现内部函数。和互斥锁一样，也是四大金刚。（编译的时候需要加上-lpthread）sem_init 创建信号量
int sem_init(sem_t *sem, int pshared, unsigned int value); sem_post- 1
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第一个参数：指向的信号对象
第二个参数：控制信号量的类型，如果其值为0，就表示信号量是当前进程的局部信号量，否则信号量就可以在多个进程间共享
第三个参数：信号量sem的初始值
返回值：success为0，failure为-1
sem_post 信号量的值加1
int sem_post(sem_t *sem);- 1
第一个参数：信号量对象
返回值：success为0，failure为-1
sem_wait 信号量的值加-1
int sem_wait(sem_t *sem);- 1
第一个参数：信号量对象
返回值：success为0，failure为-1
sem_destroy 用完记得销毁哦~
int sem_destroy(sem_t *sem);- 1
第一个参数：信号量对象
返回值：success为0，failure为-1
示例程序1：进行三个下载任务，但是最多选择同时执行二个（创建两个线程）
（我对原作者的代码进行了一些改造）
#include#include #include //#include #define MAXNUM 2 sem_t semDownload; pthread_t a_thread, b_thread, c_thread; int g_phreadNum = 1; void InputInfo(void) { printf("****************************************\n"); printf("*** which task you want to download? ***\n"); printf("*** you can enter [1-3],[0] is done ***\n"); printf("****************************************\n"); } void *func1(void *arg) { int ret; //等待信号量的值>0 ret = sem_wait(&semDownload); if(ret != 0) { printf("error, sem_wait failed\n"); } printf("============== Downloading Task 1 ============== \n"); sleep(5); printf("============== Finished Task 1 ============== \n"); g_phreadNum--; //等待线程结束 //pthread_join(a_thread, NULL); } void *func2(void *arg) { int ret; ret = sem_wait(&semDownload); if(ret != 0) { printf("error, sem_wait failed\n"); } printf("============== Downloading Task 2 ============== \n"); sleep(3); printf("============== Finished Task 2 ============== \n"); g_phreadNum--; //pthread_join(b_thread, NULL); } void *func3(void *arg) { int ret; ret = sem_wait(&semDownload); if(ret != 0) { printf("error, sem_wait failed\n"); } printf("============== Downloading Task 3 ============== \n"); sleep(1); printf("============== Finished Task 3 ============== \n"); g_phreadNum--; //pthread_join(c_thread, NULL); } int main() { int ret; int taskNum; InputInfo(); //初始化信号量 ret = sem_init(&semDownload, 0, 0); if(ret != 0) { printf("error, sem_init failed\n"); } while (scanf("%d", &taskNum) != EOF) { //输入0,判断是否正常退出 if (taskNum == 0) { if (g_phreadNum <= 1) { break; } else { printf("Can not quit, casue count of threads is [%d]\n", g_phreadNum - 1); } } printf("your choose Downloading Task [%d]\n", taskNum); //线程数超过2个则不下载 if (g_phreadNum > MAXNUM) { printf("!!! You've reached a limit on the number of threads !!!\n"); continue; } //用户选择下载Task switch (taskNum) { case 1: //创建线程1 pthread_create(&a_thread, NULL, func1, NULL); //信号量+1，进而触发fun1的任务 ret = sem_post(&semDownload); if(ret != 0) { printf("error, sem_post failed\n"); } //总线程数+1 g_phreadNum++; break; case 2: pthread_create(&b_thread, NULL, func2, NULL); ret = sem_post(&semDownload); if(ret != 0) { printf("error, sem_post failed\n"); } g_phreadNum++; break; case 3: pthread_create(&c_thread, NULL, func3, NULL); ret = sem_post(&semDownload); if(ret != 0) { printf("error, sem_post failed\n"); } g_phreadNum++; break; default: printf("!!! eroor task [%d] !!!\n", taskNum); break; } } //销毁信号量 sem_destroy(&semDownload); if(ret != 0) { printf("error, sem_destroy failed\n"); } return 0; } - 1
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CentOS编译运行结果：
[root@localhost 20220816]# ./a.out **************************************** *** which task you want to download? *** *** you can enter [1-3],[0] is done *** **************************************** 1 your choose Downloading Task [1] ============== Downloading Task 1 ============== ============== Finished Task 1 ============== 2 your choose Downloading Task [2] ============== Downloading Task 2 ============== 3 your choose Downloading Task [3] ============== Downloading Task 3 ============== ============== Finished Task 2 ============== 4============== Finished Task 3 ============== your choose Downloading Task [4] !!! eroor task [4] !!! 5 your choose Downloading Task [5] !!! eroor task [5] !!! 1 your choose Downloading Task [1] ============== Downloading Task 1 ============== 2 your choose Downloading Task [2] ============== Downloading Task 2 ============== 3 your choose Downloading Task [3] !!! You've reached a limit on the number of threads !!! ============== Finished Task 1 ============== ============== Finished Task 2 ============== 4 your choose Downloading Task [4] !!! eroor task [4] !!!- 1
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示例程序：单生产者单消费者模型
简介：一个线程生产，另一个线程消费，线程A生产完后立马给线程B发信号，线程B收到后立即回应线程A，告诉它我收到了，发送100000次
（test_semaphore_1_1.c）
#include#include #include #include "unistd.h" #include #define printf(format, ...) \ do { \ char str[1024] = {0}; \ struct timeval tv; \ struct tm* t; \ gettimeofday(&tv, NULL); \ t = localtime(&tv.tv_sec); \ sprintf(str,"[%04d-%02d-%02d %02d:%02d:%02d.%03ld] ", \ 1900 + t->tm_year, 1 + t->tm_mon, t->tm_mday, \ t->tm_hour, t->tm_min, t->tm_sec, tv.tv_usec / 1000); \ printf("%s", str); \ printf("#%d "format, __LINE__, ##__VA_ARGS__); \ } while (0) #define LIM (100000) sem_t sem, sem_R; int count = 0; int count_R = 0; void* f_producer(void *arg) { sleep(1); int ret; while(1) { printf("[%s] produce start\n", __FUNCTION__); //信号量+1，进而触发fun1的任务 printf("[%s] produce finish, sem_post\n", __FUNCTION__); ret = sem_post(&sem); if(ret != 0) { printf("[%s] error, sem_post failed\n", __FUNCTION__); break; } count++; printf("\n[%s] count = %d\n", __FUNCTION__, count); if(count == LIM) { printf("[%s] break\n", __FUNCTION__); break; } printf("[%s] waiting for sem_R\n", __FUNCTION__); ret = sem_wait(&sem_R); if(ret != 0) { printf("[%s] error, sem_wait failed\n", __FUNCTION__); break; } printf("[%s] received sem_R\n", __FUNCTION__); } } void* f_consumer(void *arg) { sleep(1); int ret; while(1) { printf("[%s] waiting for sem\n", __FUNCTION__); ret = sem_wait(&sem); if(ret != 0) { printf("error, sem_wait failed\n"); break; } printf("[%s] received sem\n", __FUNCTION__); count_R++; printf("\n[%s] count_R = %d\n", __FUNCTION__, count_R); if(count_R == LIM) { printf("[%s] break\n", __FUNCTION__); break; } printf("[%s] return start\n", __FUNCTION__); ret = sem_post(&sem_R); if(ret != 0) { printf("[%s] error, sem_post failed\n", __FUNCTION__); break; } } } int main() { int ret; pthread_t consumer, producer; //初始化信号量 ret = sem_init(&sem, 0, 0); if(ret != 0) { printf("error, sem_init failed\n"); } //创建线程 pthread_create(&producer, NULL, f_producer, NULL); pthread_create(&consumer, NULL, f_consumer, NULL); ret = pthread_join(producer, NULL); if(ret != 0) { printf("error, join failed, ret = [%d]\n", ret); } ret = pthread_join(consumer, NULL); if(ret != 0) { printf("error, join failed, ret = [%d]\n", ret); } //销毁信号量 sem_destroy(&sem); if(ret != 0) { printf("error, sem_destroy failed\n"); } return 0; } - 1
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编译运行：
[root@localhost 20220817]# gcc test_semaphore_1_1.c -lpthread [root@localhost 20220817]# ./a.out- 1
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结果：
总共只花费了10秒钟就跑完了，且无信号丢失，还是很稳定的（比那个啥互斥锁+条件变量好用多了lll…）
示例程序：多生生产者多消费者模型
（略，后更。。。）
总结
这用来做线程间通知，不比锁加条件变量好用多了啊！！！！那个一大堆，还挺麻烦的，，，，，，
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- 原文地址：https://blog.csdn.net/Dontla/article/details/123066055
