sysv message queue send and receive example.

message_queue.c

// Playing around with sysv message queue
// This program will
// 1. Create a sysv message queue
// 2. Send messages
// 3. Receive messages
// How to run
// touch /tmp/c.txt <- this file is necessary for ftok to run.
// gcc -o mq message_queue.c
// ./mq send <- sends random long each time it is executed
// ./mq receive <- receives the values in the queue and displays them one at a time, oldest first.

#include "sys/msg.h"                                                                                                                                                                                                                            
#include "sys/ipc.h"                                                                                                                                                                                                                            
#include "stdio.h"                                                                                                                                                                                                                              
#include "stdlib.h"                                                                                                                                                                                                                             
#include "string.h"                                                                                                                                                                                                                             
#include "time.h"                                                                                                                                                                                                                               
                                                                                                                                                                                                                                                
struct databag {                                                                                                                                                                                                                                
  long mtype;                                                                                                                                                                                                                                   
  double salary;                                                                                                                                                                                                                                
  char name[5];                                                                                                                                                                                                                                 
};

int msgsz = sizeof(struct databag) - sizeof(long);
int mtype = 10;

int get_qid(key_t keyval) {
  int qid;
  qid = msgget(keyval, IPC_CREAT | 0660);
  if (qid == -1) {
    return -1;
  } else {
    return qid;
  }
}

int send_message(int qid, struct databag *msg) {
  printf("Sending message\n");
  int ret = msgsnd(qid, msg, msgsz, 0);
  if (ret == -1) {
    perror("msgsnd error");
    exit(1);
  }
  return ret;
}

int receive_message(int qid, struct databag *msg) {
  printf("Receiving message\n");
  int ret = msgrcv(qid, msg, msgsz, mtype, IPC_NOWAIT);
  if (ret == -1) {
    perror("msgrcv error");
    exit(1);
  }
  return ret;
}

int main(int argc, char* argv[]) {
  char file_path[] = "/tmp/c.txt";
  key_t keyval = ftok(file_path, 'A');

  printf("keyval in hex=%x\n", keyval);
  int qid = get_qid(keyval);
  if (qid == -1) {
    perror("msgget failed");
    exit(1);
  }
  printf("msgget success: queue qid is %d \n", qid);
  
  if (argc > 1 && !strcmp(argv[1], "receive")) {
    struct databag rcvmsg;
    receive_message(qid, &rcvmsg);
    printf("Name %s has salary is %f\n", rcvmsg.name, rcvmsg.salary);
  }

  if (argc > 1 && ~strcmp(argv[1], "send")) {
    srand(time(0));
    struct databag sndmsg;
    sndmsg.mtype = mtype;
    sndmsg.salary = rand();
    strcpy(sndmsg.name, "JACK");

    printf("new salary is %f\n", sndmsg.salary);
    send_message(qid, &sndmsg);
  }
}