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IPC通信:Posix信号灯

  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# 

 

 

作者:PoleStar
来源链接:https://www.cnblogs.com/polestar/archive/2012/04/18/2455519.html

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