信号灯用来实现同步——用于多线程，多进程之间同步共享资源（临界资源）。信号灯分两种，一种是有名信号灯，一种是基于内存的信号灯。有名信号灯，是根据外部名字标识，通常指代文件系统中的某个文件。而基于内存的信号灯，它主要是把信号灯放入内存的，基于内存的信号灯，同步多线程时，可以放到该多线程所属进程空间里；如果是同步多进程，那就需要把信号灯放入到共享内存中（方便多个进程访问）。
有名信号灯和基于内存的信号灯，具体区别体现在创建和销毁两个函数。有名信号灯使用sem_open和sem_close函数。基于内存的信号灯使用sem_init和sem_destroy函数。sem_init的参数可以控制是同步多线程，还是多进程；且该函数只能调用1次，因为调用后信号灯就存在了（ 内存指针存在）。一般，使用基于内存的信号灯同步同进程多线程，使用有名信号灯同步多进程。
有名信号灯同步多线程：
1 1.sem_open函数。 2 功能:创建并初始化信号灯，如果存在就返回存在的信号灯。 3 头文件:#include <semaphore.h> 4 函数原型:sem_t * sem_open(const char * name,int oflag,mode_t mode,unsigned int value); 5 或者:sem_t * sem_open(const char * name,int oflag); 6 参数:name是给信号灯指定一个名字。oflag的值为O_CREAT,表示如果信号灯不存在，创建信号灯；为O_CREAT|O_EXCL,如果信号灯不存在报错。后面两个参数，只有新建信号灯时使用。mode为信号灯的权限（0644）,value为信号灯的值。 7 返回值:成功时，返回信号灯的指针，错误返回SEM_FAILED 8 9 2.sem_close函数。 10 功能:关闭引用信号灯，信号灯引用计数减1。 11 头文件:#include <semaphore.h> 12 函数原型:int sem_close(sem_t * sem) 13 参数:sem为信号灯的指针 14 返回值:成功时，返回0，失败，-1 15 注:每个信号灯有一个引用计数器记录当前打开次数.关闭一个信号灯并没有将它从系统中删除，而是信号灯引用计数减1 16 17 18 3.sem_unlink函数 19 功能:信号灯引用计数为0时，从系统中删除信号灯。 20 头文件:#include <semaphore.h> 21 函数原型:int sem_close(const char *name) 22 参数:name为信号灯的外部名字 23 返回值：成功时，返回0，失败，-1 24 25 4.sem_wait/sem_trywait函数 26 功能:等待共享资源，信号灯值为0就睡眠，信号灯值大于0，就使用共享资源，信号灯值减1。sem_trywait当信号灯值为0时，不睡眠，报错。 27 头文件:#include <semaphore.h> 28 函数原型:int sem_wait(sem_t *sem),int sem_trywait(sem_t *sem) 29 参数:sem为信号灯指针 30 返回值:成功时，返回0，失败，-1 31 32 5.sem_getvalue函数 33 功能:获得信号灯的值 34 头文件:#include <semaphore.h> 35 函数原型:int sem_getvalue(sem_t *sem,int *valp) 36 参数:sem为信号灯指针，valp为信号灯的值 37 返回值:成功时，返回0，失败，-1 38 39 6.sem_post函数 40 功能:使用完共享资源后，信号灯值加1，唤醒其他睡眠的。 41 头文件:#include <semaphore.h> 42 函数原型:int sem_post(sem_t *sem) 43 参数:sem为信号灯指针 44 返回值:成功时，返回0，失败，-1
示例代码：
1 /*semopen_pth.c*/
2 #include <stdio.h>
3 #include <semaphore.h>
4 #include <fcntl.h>
5 #include <pthread.h>
6
7 void print();
8 void * thread_function(void *arg);
9 sem_t * sem;
10
11 int main(int argc,char * argv[])
12 {
13 int n=0;
14 pthread_t tid;
15 if(argc != 2)
16 {
17 printf("Usage:%s name.\n",argv[0]);
18 exit(0);
19 }
20 //init semaphore
21 sem=sem_open(argv[1],O_CREAT,0644,3);
22 while(n++<5)
23 {
24 if((pthread_create(&tid,NULL,thread_function,NULL))!=0)
25 {
26 printf("can't create pthread.\n");
27 exit(0);
28 }
29 }
30 pthread_join(tid,NULL); //主线程需要等会其它线程
31 sem_close(sem);
32 sem_unlink(argv[1]);
33 return 0;
34 }
35
36 void * thread_function(void *arg)
37 {
38 sem_wait(sem);
39 print();
40 sleep(1); //因为共享段执行过快,不能达到同步效果,所以需要睡眠
41 sem_post(sem);
42 printf("finish, pthread_id is %d\n",pthread_self());
43 }
44
45 void print()
46 {
47 int value;
48 printf("pthread_id is %d, get the resource\n",pthread_self());
49 sem_getvalue(sem,&value);
50 printf("now,the semaphore value is %d\n",value);
51 }
编译并执行：
1 root@linux:/mnt/hgfs/C_libary# gcc -lpthread -o semopen_pth semopen_pth.c 2 semopen_pth.c: In function ‘main’: 3 semopen_pth.c:17: warning: incompatible implicit declaration of built-in function ‘exit’ 4 semopen_pth.c:26: warning: incompatible implicit declaration of built-in function ‘exit’ 5 semopen_pth.c: In function ‘thread_function’: 6 semopen_pth.c:41: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘pthread_t’ 7 semopen_pth.c: In function ‘print’: 8 semopen_pth.c:47: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘pthread_t’ 9 root@linux:/mnt/hgfs/C_libary# 10 注：产生了一些警告，先不管这些，看看是否能运行 11 root@linux:/mnt/hgfs/C_libary# ./semopen_pth sem 12 pthread_id is -1217336464, get the resource 13 now,the semaphore value is 2 14 pthread_id is -1225729168, get the resource 15 now,the semaphore value is 1 16 pthread_id is -1234121872, get the resource 17 now,the semaphore value is 0 18 finish, pthread_id is -1217336464 19 finish, pthread_id is -1225729168 20 finish, pthread_id is -1234121872 21 pthread_id is -1242514576, get the resource 22 now,the semaphore value is 2 23 pthread_id is -1250907280, get the resource 24 now,the semaphore value is 1 25 finish, pthread_id is -1242514576 26 finish, pthread_id is -1250907280 27 root@linux:/mnt/hgfs/C_libary#
有名信号灯同步多进程:
unix网络编程第二卷，如是说不同进程（不管是否彼此有无亲缘关系），他们都可以访问同一个信号灯，只是需要在sem_open的时候传入的名字是一样就行。在有亲缘关系时，Posix中的fork如是描述，在父进程中打开的任何信号灯，仍应在子进程中打开。
示例代码：
1 /*semopen_pro.c*/
2 #include <stdio.h>
3 #include <semaphore.h>
4 #include <fcntl.h>
5 #include <unistd.h>
6 #include <sys/types.h>
7 #include <sys/wait.h>
8
9 void print();
10 sem_t * sem;
11
12 int main(int argc,char * argv[])
13 {
14 int n=0,j;
15 pid_t pid;
16
17 if(argc != 2)
18 {
19 printf("Usage:%s name.\n",argv[0]);
20 exit(0);
21 }
22
23 //该信号灯不会因为不同进程而不同。
24 sem=sem_open(argv[1],O_CREAT,0644,3);
25 while(n++<5)
26 {
27 pid = fork();
28 if(pid == 0)
29 {
30 sem_wait(sem);
31 print();
32 sleep(1);
33 sem_post(sem);
34 printf("finish,the pid is %d\n",getpid());
35 exit(0);
36 }
37 }
38
39 j=0;
40
41 //等待所有子进程退出
42 while(j++<5)
43 wait(NULL);
44 sem_close(sem);
45 sem_unlink(argv[1]);
46 return 0;
47 }
48
49 void print()
50 {
51 int value;
52 printf("pid is %d, get the resource\n",getpid());
53 sem_getvalue(sem,&value);
54 printf("now,the semaphore value is %d\n",value);
55 }
编译并执行：
1 root@linux:/mnt/hgfs/C_libary# gcc -o semopen_pro semopen_pro.c 2 semopen_pro.c: In function ‘main’: 3 semopen_pro.c:19: warning: incompatible implicit declaration of built-in function ‘exit’ 4 semopen_pro.c:34: warning: incompatible implicit declaration of built-in function ‘exit’ 5 /tmp/ccUECdL7.o: In function `main': 6 semopen_pro.c:(.text+0x5d): undefined reference to `sem_open' 7 semopen_pro.c:(.text+0x81): undefined reference to `sem_wait' 8 semopen_pro.c:(.text+0x9f): undefined reference to `sem_post' 9 semopen_pro.c:(.text+0x107): undefined reference to `sem_close' 10 semopen_pro.c:(.text+0x117): undefined reference to `sem_unlink' 11 /tmp/ccUECdL7.o: In function `print': 12 semopen_pro.c:(.text+0x14e): undefined reference to `sem_getvalue' 13 collect2: ld returned 1 exit status 14 注：sem_XXX函数不是标准库函数,链接时需要指定库-lrt or -pthread. 15 root@linux:/mnt/hgfs/C_libary# gcc -lrt -o semopen_pro semopen_pro.c 16 semopen_pro.c: In function ‘main’: 17 semopen_pro.c:19: warning: incompatible implicit declaration of built-in function ‘exit’ 18 semopen_pro.c:34: warning: incompatible implicit declaration of built-in function ‘exit’ 19 root@linux:/mnt/hgfs/C_libary# ./semopen_pro sem 20 pid is 5262, get the resource 21 now,the semaphore value is 2 22 pid is 5263, get the resource 23 now,the semaphore value is 1 24 pid is 5264, get the resource 25 now,the semaphore value is 0 26 finish,the pid is 5262 27 pid is 5265, get the resource 28 now,the semaphore value is 0 29 finish,the pid is 5263 30 pid is 5266, get the resource 31 now,the semaphore value is 0 32 finish,the pid is 5264 33 finish,the pid is 5265 34 finish,the pid is 5266 35 root@linux:/mnt/hgfs/C_libary#
基于内存的信号灯同步多线程：
1 sem_init() 2 功能：初始化信号灯。 3 头文件：#include <semaphore.h> 4 函数原型：int sem_open(sem_t * sem,int shared,unsigned int value); 5 参数:sem为信号灯指针，shared是指同步多线程还是多进程（0:多线程,其他:多进程）,value为信号量值 6 返回值：成功时，返回0,失败时，返回-1 7 8 sem_destroy() 9 功能：关闭信号 10 头文件：#include <semaphore.h> 11 函数原型：int sem_destroy(sem_t * sem) 12 参数：sem为信号灯的指针 13 返回值：成功时，返回0，失败，-1
示例代码：
1 /*seminit_pth*/
2 #include <stdio.h>
3 #include <semaphore.h>
4 #include <fcntl.h>
5 #include <pthread.h>
6
7 void print();
8 void * thread_function(void *arg);
9 sem_t sem;
10
11 int main(int argc,char * argv[])
12 {
13 int n=0;
14 pthread_t tid;
15 //init semaphore
16 sem_init(&sem,0,3);
17 while(n++<5)
18 {
19 if((pthread_create(&tid,NULL,thread_function,NULL))!=0)
20 {
21 printf("can't create pthread.\n");
22 exit(0);
23 }
24 }
25 pthread_join(tid,NULL);
26 sem_destroy(&sem);
27 return 0;
28 }
29
30 void * thread_function(void *arg)
31 {
32 sem_wait(&sem);
33 print();
34 sleep(1); //
35 sem_post(&sem);
36 printf("finish, pthread_id is %d\n",pthread_self());
37 }
38
39 void print()
40 {
41 int value;
42 printf("pthread_id is %d, get the resource\n",pthread_self());
43 sem_getvalue(&sem,&value);
44 printf("now,the semaphore value is %d\n",value);
45 }
编译运行：
1 root@linux:/mnt/hgfs/C_libary# gcc -lrt -o seminit_pth seminit_pth.c 2 seminit_pth.c: In function ‘main’: 3 seminit_pth.c:21: warning: incompatible implicit declaration of built-in function ‘exit’ 4 seminit_pth.c: In function ‘thread_function’: 5 seminit_pth.c:35: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘pthread_t’ 6 seminit_pth.c: In function ‘print’: 7 seminit_pth.c:41: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘pthread_t’ 8 root@linux:/mnt/hgfs/C_libary# 9 root@linux:/mnt/hgfs/C_libary# ./seminit_pth 10 pthread_id is -1224959120, get the resource 11 now,the semaphore value is 2 12 pthread_id is -1233351824, get the resource 13 now,the semaphore value is 1 14 pthread_id is -1241744528, get the resource 15 now,the semaphore value is 0 16 finish, pthread_id is -1224959120 17 finish, pthread_id is -1233351824 18 finish, pthread_id is -1241744528 19 pthread_id is -1250137232, get the resource 20 now,the semaphore value is 2 21 pthread_id is -1216566416, get the resource 22 now,the semaphore value is 1 23 finish, pthread_id is -1250137232 24 finish, pthread_id is -1216566416 25 root@linux:/mnt/hgfs/C_libary#
来源：https://www.cnblogs.com/polestar/archive/2012/04/18/2455519.html