Server and Client in C using a Message Queue (no socket code) - c

Question

I am trying to implement threading, a binary semaphore, and message queues in a client and server program. Everything works, except for the reply in my server to send a response back to the client. When I uncomment the reply, the server loops infinitely after receiving a message.
They both use a message.h header file that is a struct containing:
contents (character), id(int), and mtype(int).
// server.c
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/msg.h>
#include <sys/ipc.h>
#include <errno.h>
#include <unistd.h>
#include <pthread.h>
#include <semaphore.h>
#include "message.h"
int sqid, cqid, pid;
msg m, n;
sem_t sem_var;
key_t key;
void requestSem();
void setSem();
void unlockSem();
void reply();
void readQueue();
void listener(void *ptr);
int main() {
key = ftok(".", 'z');
sqid = msgget(key, IPC_CREAT | 0600);
if(sqid < 0) {
perror("Error creating the message queue.\n");
}
pthread_t thread1;
pthread_t thread2;
pthread_t thread3;
char *msg1 = "Thread 1";
char *msg2 = "Thread 2";
char *msg3 = "Thread 3";
pthread_create(&thread1, NULL, (void *) &listener, (void *) msg1);
pthread_create(&thread2, NULL, (void *) &listener, (void *) msg2);
pthread_create(&thread3, NULL, (void *) &listener, (void *) msg3);
pthread_join(thread1, NULL);
pthread_join(thread2, NULL);
pthread_join(thread3, NULL);
}
// always listen for messages
void listener(void *ptr) {
while (1) {
requestSem();
setSem();
readQueue();
unlockSem();
//reply();
}
}
void readQueue()
{
// print the proccess ID and thread ID
printf("Process ID: %d, Thread ID: %d\n", getpid(), pthread_self());
// receive message
if(msgrcv(sqid,&m,sizeof(struct msg),0,0) < 0) {
perror("Error receiving a message.\n");
}
printf("Message was received...\n");
fprintf(stderr, "The contents are: %s\n", &m.contents);
fprintf(stderr, "The client's ID is: %d\n", (&m)->id);
fprintf(stderr, "The thread's ID is: %d\n", pthread_self());
}
void requestSem()
{
// request semaphore (lock)
sem_init(&sem_var, 1, 1);
printf("Semaphore requested.\n");
}
void setSem()
{
// lock semaphore
sem_wait(&sem_var);
printf("Semaphore set.\n");
}
void unlockSem()
{
// unlock semaphore
sem_post(&sem_var);
printf("Semaphore unlocked.\n");
}
void reply()
{
// send reply
printf("got here\n");
pid = (&m)->id;
cqid = msgget(pid, 0600);
n.contents = (&m)->contents;
n.mtype = 0;
n.id = getpid();
if(msgsnd(cqid, &n,sizeof(struct msg),0) < 0) {
perror("Msg send error");
}
msgctl(sqid,IPC_RMID,NULL);
}
// client.c
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/msg.h>
#include <sys/ipc.h>
#include <errno.h>
#include <pthread.h>
#include <semaphore.h>
#include "message.h"
int main() {
// initialize variables
int mqid, rqid;
msg m;
key_t key;
// initialize message
int pid;
pid = getpid();
m.id = pid;
m.mtype = 1;
// create message queue and response queue
key = ftok(".", 'z');
mqid = msgget(key, 0600);
rqid = msgget(pid, IPC_CREAT | 0600);
if(mqid < 0) {
perror("Error creating the message queue.\n");
}
if (rqid < 0)
{
perror("Error creating the message queue.\n");
}
// loop forever, can send as many messages as you want
while(1) {
fprintf(stderr, "The ID of the client is: %d\n", pid);
// prompt for user input
printf("Enter one character to send: \n");
scanf("%s", &m.contents);
// send message
if(msgsnd(mqid, &m,sizeof(struct msg), 1) < 0) {
perror("Error sending the message.");
}
//msgctl(mqid,IPC_RMID,NULL);
printf("Message was sent successfully!\n");
// receive response
printf("Waiting for a reply...\n");
if(msgrcv(rqid,&m,sizeof(msg), 0, 0) < 0)
{
perror("Error receiving a message.\n");
}
printf("Message was received.\n");
fprintf(stderr, "The contents are: %s\n", &m.contents);
}
}
Thank you in advance for any help. Everything functions as it's supposed to when reply is commented out within listener in the server. Uncommenting it creates an infinite loop.

Answer 1

the reason the server never exits is because each thread has a
while(1)
loop and no way to exit that loop