Question

message queue. msgsend msgrcv. System V IPC system calls in C (Linux)

Asked by: ubuntuguy

Hi I'm working with some shared memory where different processes read and write data. I'm using a message queue to store messages of when data has changed between read and write operations.

/* struct that defines a message */
struct msgbuf{
            long mtype;                              /* must be positive */
        int childId;                        //ID of child sending message
            int bufferChanged;                  //Buffer at which value was changed
            int beforeValue;                  //Value before child sleeps
            int afterValue;                        //Value after child sleeps
};

So while reading and writing and checking for changes the processes store messages the following way

                  struct msgbuf msg = {BUFFER_CHANGED, id, position, read, bufferArr[position]};
                  if(msgsnd(msqid, &msg, sizeof(msg), 0)== -1){
                        perror("msgsnd in read.write");
                  }

This is working fine. Oh by the way here's how I create the message queue.
#define BUFFER_CHANGED 1

      qKey = ftok("./", 'A');
      
      msqid = msgget(qKey, (IPC_CREAT | 0666));
      
      /*Perform the following if the call is unsuccessful.*/
    if(msqid == -1){
          printf ("\nThe msgget call failed, error number = %d\n", errno);
    }
    /*Return the msqid upon successful completion.*/
    else{
          printf ("\nMessage queue successful. The msqid = %d\n", msqid);
          //exit(0);
      }

So my problem is that I'm not quite sure how to retrieve messages from the queue and display them on the screen.  I've been reading up on msgrcv system call, but it's not very clear to me.

rc = msgrcv(msqid, &msg, sizeof(msg), BUFFER_CHANGED, IPC_NOWAIT);

rc is an int since msgrcv retuns an int. How do i point this int to the actual message? How to I read contents from the message so I can display them? I'm assuming this should be done in some sort of loop.

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Asked On
2009-11-04 at 21:00:46ID24873497
Topics

C Programming Language

,

Linux

,

C++ Programming Language

Participating Experts
1
Points
500
Comments
1

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Answers

 

by: jkrPosted on 2009-11-05 at 05:31:48ID: 25749189

>>So my problem is that I'm not quite sure how to retrieve messages from the
>>queue and display them on the screen.

Take a look at http://www.ecst.csuchico.edu/~beej/guide/ipc/mq.html ("Message Queues") which illutrates that quite nicely and also comes with sample code. The idea (also with a loop) would be to

    #include <stdio.h>
    #include <stdlib.h>
    #include <errno.h>
    #include <sys/types.h>
    #include <sys/ipc.h>
    #include <sys/msg.h> 
    struct my_msgbuf {
        long mtype;
        char mtext[200];
    }; 
    int main(void)
    {
        struct my_msgbuf buf;
        int msqid;
        key_t key; 
        if ((key = ftok("kirk.c", 'B')) == -1) {  /* same key as kirk.c */
            perror("ftok");
            exit(1);
        } 
        if ((msqid = msgget(key, 0644)) == -1) { /* connect to the queue */
            perror("msgget");
            exit(1);
        }
        
        printf("spock: ready to receive messages, captain.\n"); 
        for(;;) { /* Spock never quits! */
            if (msgrcv(msqid, (struct msgbuf *)&buf, sizeof(buf), 0, 0) == -1) {
                perror("msgrcv");
                exit(1);
            }
            printf("spock: \"%s\"\n", buf.mtext);
        } 
        return 0;
    }
                                              
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