Example: Sending messages between two processes using msgsnd() and msgrcv() Source: https://users.cs.cf.ac.uk/Dave.Marshall/C/node25.html

message_rec.c

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

	key = 2234;

	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);
	}

	printf("^%s\n", rbuf.mtext);
	exit(0);
}

message_send.c

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

	key = 2234;

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

	// We'll send message type 1 
	sbuf.mtype = 1;
	(void) fprintf(stderr, "msggeet: 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);
}