Queue Reader Program

/* * queue_reader.c - a program that reads messages with a given identifier * off of a message queue. */
#include <stdio.h>        /* standard I/O functions. */
#include <stdlib.h>       /* malloc(), free() etc. */
#include <unistd.h>        /* sleep(), etc. */
#include <sys/types.h>    /* various type definitions. */
#include <sys/ipc.h>      /* general SysV IPC structures */
#include <sys/msg.h>      /* message queue functions and structs.*/

#define MAX_MSG_SIZE 200  /* size (in bytes) of largest message we'll send.   */
#define NUM_MESSAGES 50  /* number of messages the sender program will send. */
struct mymsgbuf{
  long int mtype;
  char mtext[128];
};
int main(int argc, char* argv[]) {
  int queue_id;            /* ID of the created queue. */
  struct mymsgbuf* msg;   /* structure used for received messages. */
  int rc;                 /* error code returned by system calls. */
  long int msg_type;     /* type of messages we want to receive. */
  /* read message type from command line */
  if (argc != 2) {
    fprintf(stderr, "Usage: %s <message type>\n", argv[0]);
    fprintf(stderr, " <message type> must be between 1 and 3.\n");
    exit(1);
  }
  msg_type = atol(argv[1]);
  if (msg_type < 1 || msg_type > 3) {
    fprintf(stderr, "Usage: %s <message type>\n", argv[0]);
    fprintf(stderr, " <message type> must be between 1 and 3.\n");
    exit(1);
  }
 /* access the public message queue that the sender program created. */
  queue_id = msgget((key_t)1234, 0666 | IPC_CREAT);
  if (queue_id == -1) {
    perror("main: msgget");
    exit(1);
  }
  printf("Message queue opened, queue id '%d'.\n", queue_id);
  msg = (struct mymsgbuf*)malloc((sizeof(struct mymsgbuf)));
 /* form a loop of receiving messages and printing them out. */
  while (1) {
    rc = msgrcv(queue_id, msg, MAX_MSG_SIZE+1, msg_type, IPC_NOWAIT);
    if (rc == -1) { perror("main: msgrcv"); exit(1); }
    printf("Read message: '%s' \n", msg->mtext);
/* slow down a little... */
    sleep(1);
  }
/* NOT REACHED */
}