概念描述

• 英文：semaphore 简称SEM，主要用来进行进程间同步
• 本质：内核维护的一个正整数，可对其进行各种+/-操作
• 分类：systemV 信号量、POSIX 有名信号量、POSIX 无名信号量
• 用途：用来标示系统中可用资源的的个数，协调各个进程有序的访问资源，防止发生冲突
• P操作：程序在进入临界区之前要对资源进行申请，对资源的引用计数要-1，当资源个数为0时进程p操作会发生阻塞
• V操作：程序离开临界区后要释放相应的资源，对资源对引用计数要+1

编程接口

以下接口编码过程中都可以通过man sem_open这种方式查看接口对具体使用方式，详细信息则不列举

sem_t *sem_open(const char *name, int oflag); //使用字符串创建一个信号量
sem_t *sem_open(const char *name, int oflag,mode_t mode, unsigned int value); //使用字符串创一个一个信号量，并初始化信号量的值
int sem_close(sem_t *sem);
int sem_post(sem_t *sem); //信号量的v操作 +1
int sem_wait(sem_t *sem); //信号量的p操作 -1
int sem_trywait(sem_t *sem); //信号量进行p操作，但是值已经为0，此时会立即返回错而非阻塞//信号量进行p操作，如信号量的值为0，则阻塞abs_timeout结构体中的时间内还是无法执行p操作，则返回错误
int sem_timedwait(sem_t *sem, const struct timespec *abs_timeout); 
int sem_unlink(const char *name);//删除名称为name的信号量
int sem_getvalue(sem_t *sem, int *sval);//获取信号量sem所代表的value数值

注意事项

• 以上编程接口包含头文件: #include
• 编译时需指定：-lpthread进行编译

编程案例

linux系统中创建的信号量存放路径为/dev/shm

信号量基本接口使用案例

sem_demo.c

#include 
#include 
#include 
#include 
#include int main (){ //创建信号量并初始化信号量的value值为4unsigned int sem_value = 4;sem_t *sem = sem_open("posix_sem", O_RDWR | O_CREAT,0777,sem_value);if (sem == SEM_FAILED) { printf("sem_open failed
");_exit(-1);}//获取信号量的value值if (sem_getvalue(sem,&sem_value) != -1) { printf("sem_getvalue is %d
",sem_value);}//对信号量进行p(-1)操作sem_wait(sem);sem_wait(sem);sem_wait(sem);sem_wait(sem);//sem_trywait(sem);//并不会发生阻塞，此时sem值已经为0，无法再进行p操作if(sem_getvalue(sem,&sem_value) != -1) { printf("sem_getvalue after sem_wait is %d
",sem_value);}//对信号量进行v(+1)操作sem_post(sem);sem_post(sem);sem_post(sem);sem_post(sem);if(sem_getvalue(sem,&sem_value) != -1) { printf("sem_getvalue after sem_post is %d
",sem_value);}if(sem_close(sem) != -1) { printf("sem_close success
");}printf("wait for unlink sem
");sleep(10);//删除名称为posix_sem信号量if(sem_unlink("posix_sem") != -1) { printf("sem_unlink success 
");}return 0;
}

编译gcc sem_demo.c -lpthread 输出如下：

sem_getvalue is 4
sem_getvalue after sem_wait is 0
sem_getvalue after sem_post is 4
sem_close success
wait for unlink sem
sem_unlink success 

同时在等待10秒删除期间我们查看/dev/shm目录下的sem文件存在，当10秒过后sem文件则被删除

信号量父子进程间通信

sem_comu.c

#include 
#include 
#include 
#include 
#include #define SEM_NAME "posix_sem_comu"int main (){ int i = 0, j = 0,sem_val = 0,ret;sem_t *sem;sem = sem_open(SEM_NAME, O_CREAT,0666,1);ret = fork();if (ret == -1) { printf("sem_open failed 
");sem_close(sem);sem_unlink(SEM_NAME);_exit(-1);}//创建的子进程，执行10次p操作，每次间隔1秒else if(ret == 0) { while (i++ <= 10) { sem_wait(sem);if (sem_getvalue(sem,&sem_val) != -1) { printf("child process :sem value is %d
",sem_val);sleep(1);}}_exit(1);}//父进程执行10次v操作，每次间隔两秒else { while (j++ <=10) { sem_post(sem);if (sem_getvalue(sem,&sem_val) != -1) { printf("prarent process :sem value is %d
",sem_val);sleep(2);}}}//最终删除sem信号量sem_close(sem);if (sem_unlink(SEM_NAME) != -1) { printf("sem_unlink success 
");}return 0;
}

编译：gcc sem_comu.c -o comu -lpthread

输出如下,可以很明显的看到子即使子进程的p操作频率快于父进程v操作，但是对同一信号量，他们仍然能够有序同步访问。因为子进程p操作频率较快，此时信号量的value会很快减为0，子进程此时再进行p操作会发生阻塞；直到父进程执行v操作将信号量的值变为1子进程才能继续访问

prarent process :sem value is 2
child process :sem value is 1
child process :sem value is 0
prarent process :sem value is 1
child process :sem value is 0
prarent process :sem value is 1
child process :sem value is 0
prarent process :sem value is 1
child process :sem value is 0
prarent process :sem value is 1
child process :sem value is 0
prarent process :sem value is 1
child process :sem value is 0
prarent process :sem value is 1
child process :sem value is 0
prarent process :sem value is 1
child process :sem value is 0
prarent process :sem value is 1
child process :sem value is 0
prarent process :sem value is 1
child process :sem value is 0
prarent process :sem value is 1
sem_unlink success 

信号量实现 两进程之间通信

两个进程，使用同一个信号量共同访问一个临界区，利用信号量的pv操作，实现同步访问

sem_post.c

#include 
#include 
#include 
#include 
#include int main(){ char *name = "/posix_sem";int j = 0, sem_value = 4;sem_t *sem;sem = sem_open(name, O_CREAT, 0666, sem_value);if (sem == SEM_FAILED) { printf("sem open failed 
");_exit(-1);}printf("sem_open %s success 
",name);//对信号量进行5次 v操作，每次间隔5秒while(j++ <= 10) { if(sem_post(sem) == -1) { printf("sem _posd failed
");_exit(-1);}if(sem_getvalue(sem,&sem_value) != -1) { printf("sem post success %d
",sem_value);}sleep(5);}sleep(10);if(sem_close(sem) != -1) { printf("sem_close success 
");}if (sem_unlink(name) != -1) { printf("sem_unlink success 
");}return 0;
}

sem_wait.c

#include 
#include 
#include 
#include 
#include int main(){ char *name = "/posix_sem";int j = 0, sem_value = 4;sem_t *sem;sem = sem_open(name, O_CREAT, 0666, sem_value);if (sem == SEM_FAILED) { printf("sem open failed 
");_exit(-1);}printf("sem_open %s success 
",name);//对信号量进行p操作，每次间隔5秒，总共进行10次while(j++ <= 10) { if(sem_post(sem) == -1) { printf("sem _posd failed
");_exit(-1);}if(sem_getvalue(sem,&sem_value) != -1) { printf("sem post success %d
",sem_value);}sleep(5);}//执行结束后关闭信号量，并删除信名称为name的信号量sleep(10);if(sem_close(sem) != -1) { printf("sem_close success 
");}if (sem_unlink(name) != -1) { printf("sem_unlink success 
");}return 0;
}

编译：

gcc sem_post.c -o post -lpthread

gcc sem_wait.c -o wait -lpthread

可以看到输出如下，两进程是能够正常进行信号量value值的访问同步