Sending messages between two processes

The full code listing for message_send.c is as follows:

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <stdio.h>
#include <string.h>

#define MSGSZ 128


/*
 * Declare the message structure.
 */

typedef struct msgbuf {
         long mtype;
         char mtext[MSGSZ];
         } message_buf;

main()
{
    int msqid;
    int msgflg = IPC_CREAT | 0666;
    key_t key;
    message_buf sbuf;
    size_t buf_length;

    /*
     * Get the message queue id for the
     * "name" 1234, which was created by
     * the server.
     */
    key = 1234;

(void) fprintf(stderr, "\nmsgget: Calling msgget(%#lx,\
%#o)\n",
key, msgflg);

    if ((msqid = msgget(key, msgflg )) < 0) {
        perror("msgget");
        exit(1);
    }
    else
     (void) fprintf(stderr,"msgget: msgget succeeded: msqid = %d\n", msqid);


    /*
     * We'll send message type 1
     */
     
    sbuf.mtype = 1;
    
    (void) fprintf(stderr,"msgget: msgget succeeded: msqid = %d\n", msqid);
    
    (void) strcpy(sbuf.mtext, "Did you get this?");
    
    (void) fprintf(stderr,"msgget: msgget succeeded: msqid = %d\n", msqid);
    
    buf_length = strlen(sbuf.mtext) + 1 ;
    
    

    /*
     * Send a message.
     */
    if (msgsnd(msqid, &sbuf, buf_length, IPC_NOWAIT) < 0) {
       printf ("%d, %d, %s, %d\n", msqid, sbuf.mtype, sbuf.mtext, buf_length);
        perror("msgsnd");
        exit(1);
    }

   else
      printf("Message: \"%s\" Sent\n", sbuf.mtext);
      
    exit(0);
}


The essential points to note here are:

• The Message queue is created with a basic key and message flag msgflg = IPC_CREAT | 0666 -- create queue and make it read and appendable by all.
• A message of type (sbuf.mtype) 1 is sent to the queue with the message ``Did you get this?''

The full code listing for message_send.c's companion process, message_rec.c is as follows:

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <stdio.h>

#define MSGSZ 128


/*
 * Declare the message structure.
 */

typedef struct msgbuf {
    long mtype;
    char mtext[MSGSZ];
} message_buf;


main()
{
    int msqid;
    key_t key;
    message_buf rbuf;

    /*
     * Get the message queue id for the
     * "name" 1234, which was created by
     * the server.
     */
    key = 1234;

    if ((msqid = msgget(key, 0666)) < 0) {
        perror("msgget");
        exit(1);
    }

    
    /*
     * Receive an answer of message type 1.
     */
    if (msgrcv(msqid, &rbuf, MSGSZ, 1, 0) < 0) {
        perror("msgrcv");
        exit(1);
    }

    /*
     * Print the answer.
     */
    printf("%s\n", rbuf.mtext);
    exit(0);
}



The essential points to note here are:

• The Message queue is opened with msgget (message flag 0666) and the same key as message_send.c.
• A message of the same type 1 is received from the queue with the message ``Did you get this?'' stored in rbuf.mtext.