IPC研究（6） -- 消息队列（message queue）-白红宇

IPC研究（6） -- 消息队列（message queue）

阅读量：6356 次

发布时间：2019-06-23

本文共 3579 字，大约阅读时间需要 11 分钟。

"In many ways, message queues are like named pipes, but without the complexity associated with opening and closing the pipe.

However, using messages doesn’t get you away from the problems that you have with named pipes,

such as blocking on full pipes.

Message queues provide a reasonably easy and efficient way of passing data between two unrelated

processes. They have the advantage over named pipes that the message queue exists independently of

both the sending and receiving processes, which removes some of the difficulties that occur in synchronizing the opening and closing of named pipes."

以上这段话摘自"Linux程序设计“。对于named pipe的开关同步，真心有点麻烦，如果处理不好，就会导致两个进程互锁。简单示例：

processA |-- named pipe1 --| processB

|-- named pipe2 --|

A打开pipe1来读(action1)，然后打开pipe2来写(action2)。

B打开pipe2来读(action3)，然后打开pipe1来写(action4)。

以上这种情况，死锁必然发生！（当然，如果你用nonblock方式打开就另说了。）

#include <sys/msg.h>

int msgctl(int msqid, int cmd, struct msqid_ds *buf);

int msgget(key_t key, int msgflg);

int msgrcv(int msqid, void *msg_ptr, size_t msg_sz, long int msgtype, int msgflg);

int msgsnd(int msqid, const void *msg_ptr, size_t msg_sz, int msgflg);

函数原型。和semaphore以及shared memory很像，没什么特别的。（不愧都是system V一族的。）

struct my_message {

long int message_type;

/* The data you wish to transfer */

}

message必须要以long int开头。可以用它来实现简单优先级队列。

If you simply want to retrieve messages in

the order in which they were sent, set msgtype to 0. If you want to retrieve only messages with a specific message type, set msgtype equal to that value. If you want to receive messages with a type of n or smaller, set msgtype to -n.

然后，无耻又无聊的照书上抄了一个例子：

/**recv**/#include 
    
     #include 
     
      #include 
      
       #include 
       
        #include 
        
         #include 
         
          struct my_msg_st{ long int my_msg_type; char sometext[BUFSIZ];};int main(){ int running = 1; int msgid; struct my_msg_st msg; long int msg_to_recv = 0; msgid = msgget( (key_t)1234, 0666 | IPC_CREAT); if (msgid < 0) { fprintf(stderr, "msgget failed with error: %d \n", errno); exit(EXIT_FAILURE); } while (running) { if (msgrcv(msgid, (void*)&msg, BUFSIZ, msg_to_recv, 0) == -1) { fprintf(stderr, "msgrcv failed with error %d \n", errno); exit(EXIT_FAILURE); } printf("You wrote: %s", msg.sometext); if (strncmp(msg.sometext, "end", 3) == 0) { running = 0; } } if (msgctl(msgid, IPC_RMID, 0) < 0) { fprintf(stderr, "msgctl(IPC_RMID) failed\n"); exit(EXIT_FAILURE); } exit(EXIT_SUCCESS);}/**send**/#include 
          
           #include 
           
            #include 
            
             #include 
             
              #include 
              
               #include 
               
                #define MAX_TEXT 512struct my_msg_st{ long int my_msg_type; char sometext[BUFSIZ];};int main(){ int running = 1; int msgid; struct my_msg_st msg; char buf[BUFSIZ]; msgid = msgget( (key_t)1234, 0666 | IPC_CREAT); if (msgid < 0) { fprintf(stderr, "msgget failed with error: %d \n", errno); exit(EXIT_FAILURE); } while (running) { printf("Enter some text: "); fgets(buf, BUFSIZ, stdin); /* fgets add \0 to the end of the buf */ msg.my_msg_type = 1; strcpy(msg.sometext, buf); if (msgsnd(msgid, (void *)&msg, MAX_TEXT, 0) < 0) { fprintf(stderr, "msgsnd failed\n"); exit(EXIT_FAILURE); } if (strncmp(buf, "end", 3)==0) { running = 0; } } exit(EXIT_SUCCESS);}

结果：

chenqi@chenqi-laptop ~/MyPro/CFiles/IPC/msg_queue $ ./recv &

[1] 19084

chenqi@chenqi-laptop ~/MyPro/CFiles/IPC/msg_queue $ ./send

Enter some text: hello

Enter some text: You wrote: hello

nice

Enter some text: You wrote: nice

Enter some text: You wrote: to

meet

Enter some text: You wrote: meet

you

Enter some text: You wrote: you

end

You wrote: end

[1]+ Done ./recv

转载于:https://my.oschina.net/u/158589/blog/56976

你可能感兴趣的文章

WebSocket详解（六）：刨根问底WebSocket与Socket的关系