IPC Message queue in C.

queue.c

// Example of message queue in C.
// For educational purposes only.
// Author: Vaclav Bohac

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <signal.h>
#include <stdarg.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>


#define PRIMES 4


int main ( void )
{
	int key, mask, msgid;

	key = getuid();
	mask = 0666;
	msgid = msgget(key, mask);	
	if (msgid == -1) {
		msgid = msgget(key, mask | IPC_CREAT);
		if (msgid == -1) {
			fprintf(stderr, "Could not create message queue.\n");
			exit(EXIT_FAILURE);
		}

		// Send messages ...
		int ret, prime, j, i = 3, msg[2] = {PRIMES, 0};
		while (1) {
			prime = 1;
			for (j = 2; j < i; j++) {
				if ((i % j) == 0) {
					prime = 0;
					break;
				}
			}	
			if (prime == 0) {
				i++;
				continue;
			}
			msg[1] = i;
			ret = msgsnd(msgid, msg, sizeof(int), IPC_NOWAIT);
			if (ret == -1) {
				if (errno != EAGAIN) {
					fprintf(stderr, "Message could not be sended.\n");
					exit(EXIT_FAILURE);
				}
				usleep(50000);
				if (msgsnd(msgid, msg, sizeof(int), 0) == -1) {
					fprintf(stderr, "Message could not be sended.\n");
					exit(EXIT_FAILURE);
				}
			}
			i++;
		}
	}
	
	int rcv[2];
	while (1) {
		if (msgrcv(msgid, &rcv, sizeof(int), 0, IPC_NOWAIT) == -1) {
			if (errno != ENOMSG) {
				fprintf(stderr, "Message could not be received.\n");
				exit(EXIT_FAILURE);
			}
			usleep(50000);
			if (msgrcv(msgid, &rcv, sizeof(int), 0, 0) == -1) {
				fprintf(stderr, "Message could not be received.\n");
				exit(EXIT_FAILURE);
			}
		}
		printf("%d, ", rcv[1]);
	}

	return EXIT_SUCCESS;
}

For me, msgid == -1. Why is this the case?

msgid == -1 in this example because the key argument passed to msgget must be of type key_t. The key_t is usually acquired by calling ftok. This example uses getuid to acquire a key, which is incorrect since getuid returns a uid_t type.