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:
- Queue ID
- Key
- Message structure:
- type
- text
Method calls:
- msgget()
- msgsnd()
- msgrcv()
Applications:
- VxWorks and QNX encourage the use of message queue for inter-process & inter-thread communication
- 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
