message_queue_token_ring.c

// Compile: gcc -o mq_token_ring mq_token_ring.c
// Run: ./mq_token_ring N rounds
// Example: ./mq_token_ring 4 3

#include <stdio.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <unistd.h>
#include <sys/wait.h>

struct msgbuf {
    long mtype;
    int token;
};

int main(int argc,char **argv){
    if(argc!=3){ fprintf(stderr,"Usage: %s N rounds\n",argv[0]); return 1;}
    int N = atoi(argv[1]), rounds = atoi(argv[2]);
    key_t key = ftok(".", 'T');
    int msqid = msgget(key, IPC_CREAT | 0666);
    if(msqid < 0){ perror("msgget"); exit(1); }

    for(int i=0;i<N;i++){
        pid_t pid = fork();
        if(pid<0){ perror("fork"); exit(1); }
        if(pid==0){
            while(1){
                struct msgbuf mb;
                // receive messages of type = my id + 1 (msg types must be >0)
                if(msgrcv(msqid, &mb, sizeof(int), i+1, 0) < 0) break;
                printf("Proc %d (pid %d) got token=%d\n", i, getpid(), mb.token);
                fflush(stdout);
                mb.token++;
                // send to next: type = (i+1)%N +1
                mb.mtype = ( (i+1)%N ) + 1;
                if(msgsnd(msqid, &mb, sizeof(int), 0) < 0) break;
            }
            _exit(0);
        }
    }

    // parent starts token by sending type 1 (to process 0)
    struct msgbuf mb;
    mb.mtype = 1;
    mb.token = 0;
    if(msgsnd(msqid, &mb, sizeof(int), 0) < 0) perror("start msgsnd");

    // naive termination: allow some time then remove the queue
    sleep(1 + rounds * N / 10 + 1);
    // cleanup
    msgctl(msqid, IPC_RMID, NULL);
    for(int i=0;i<N;i++) wait(NULL);
    return 0;
}