IPC: Message queue with example

Message queue

Message queue is a type of inter process communication. It is used to transfer data between processes. It is asynchronous communication as in, sender can dump data in queue, and receiver can get the data out at its convenience.

Parameters of queue:

1. Queue ID
2. Key
3. Message structure:
• type
• text

Method calls:

1. msgget()
2. msgsnd()
3. msgrcv()

Applications:

1. VxWorks and QNX encourage the use of message queue for inter-process & inter-thread communication
2. It provides resilience functionality as the message dont get "lost" in communication in case of system failure.

Example:

Sender code - creates queue and enter message into it with a particular key

Receiver code - Access the queue, gets the message and prints the text. If while receiver process execution data is not available in queue, it waits at msgrcv(), and proceeds when it gets some data.

Shell command to view the queue IPC

$ipcs -q

*******************************

sender.c

*******************************

/*

 * sender.c

 *

 *  Created on: 19-Apr-2016

 *      Author: root

 */

//IPC_msgq_send.c

#include <sys/types.h>

#include <sys/ipc.h>

#include <sys/msg.h>

#include <stdio.h>

#include <string.h>

#include <stdlib.h>

#define MAXSIZE     128

void die(char *s)

{

  perror(s);

  exit(1);

}

struct msgbuf

{

    long    mtype;

    char    mtext[MAXSIZE];

};

main()

{

int count = 0;

//char data[3][MAXSIZE] = {"first", "second", "third"};

    int msqid;

    int msgflg = IPC_CREAT | 0666;

    key_t key;

    struct msgbuf sbuf;

    size_t buflen;

    key = 1234;

    if ((msqid = msgget(key, msgflg )) < 0)   //Get the message queue ID for the given key

      die("msgget");

    //Message Type

    sbuf.mtype = 1;

    printf("Enter a message to add to message queue : ");

    scanf("%[^\n]",sbuf.mtext);

    getchar();

    buflen = strlen(sbuf.mtext) + 1 ;

    while (count < 3){

    //*sbuf.mtext = data[count];

    //buflen = strlen(sbuf.mtext) + 1 ;

if (msgsnd(msqid, &sbuf, buflen, IPC_NOWAIT) < 0)

{

printf ("%d, %d, %s, %d\n", msqid, sbuf.mtype, sbuf.mtext, buflen);

die("msgsnd");

}

else

{

printf("Message Sent\n");

}

count++;

    }

    exit(0);

}

*******************************

receiver.c

*******************************

/*

 * receiver.c

 *

 *  Created on: 19-Apr-2016

 *      Author: root

 */

//IPC_msgq_rcv.c

#include <sys/types.h>

#include <sys/ipc.h>

#include <sys/msg.h>

#include <stdio.h>

#include <stdlib.h>

#define MAXSIZE     128

void die(char *s)

{

  perror(s);

  exit(1);

}

typedef struct msgbuf

{

    long    mtype;

    char    mtext[MAXSIZE];

} ;

main()

{

    int msqid;

    key_t key;

    struct msgbuf rcvbuffer;

    key = 1234;

    if ((msqid = msgget(key, 0666)) < 0)

      die("msgget()");

     //Receive an answer of message type 1.

    if (msgrcv(msqid, &rcvbuffer, MAXSIZE, 1, 0) < 0)

      die("msgrcv");

    printf("%s\n", rcvbuffer.mtext);

    exit(0);

}

--------------------------------------------------------------------------------------------------

OUTPUT:

$ ipcs -q

------ Message Queues --------

key        msqid      owner      perms      used-bytes   messages    

$./sender

Enter a message to add to message queue : text data to trasmit

Message Sent

Message Sent

Message Sent

$ ipcs -q

------ Message Queues --------

key        msqid      owner      perms      used-bytes   messages    

0x000004d2 0          root       666        63           3           

$./receiver

text data to trasmit

$ ipcs -q

------ Message Queues --------

key        msqid      owner      perms      used-bytes   messages    

0x000004d2 0          root       666        42           2           

$./receiver

text data to trasmit

$ ipcs -q

------ Message Queues --------

key        msqid      owner      perms      used-bytes   messages    

0x000004d2 0          root       666        21           1           

$./receiver

text data to trasmit

$ ipcs -q

------ Message Queues --------

key        msqid      owner      perms      used-bytes   messages    

0x000004d2 0          root       666        0            0