Question

 main() {
    unsigned int newfd;
    ...
    .....
    while (1) {        
      printf("Waiting for connection\n");
      addrlen = sizeof (clientaddr);
      if ((newfd = accept(listener, (struct sockaddr *) &clientaddr, &addrlen)) < 0) {
          perror("Server-accept() error lol!");
          break;
      }
      printf("New connection from %s on socket %u\n", inet_ntoa(clientaddr.sin_addr), newfd);
      pthread_create(&threads[i++], NULL, (void*)fileTransfer_Worker, &newfd); 
      sleep(1);
   }

}

void* fileTransfer_Worker(void *desc) {
     unsigned int sock = *(unsigned int *) desc;
     printf("Waiting for data in sock %d %u\n", sock, pthread_self());
}

Output

Waiting for connection
New connection from 192.168.4.57 on socket 4
Waiting for connection
New connection from 192.168.4.57 on socket 5
Waiting for connection
Waiting for data in sock 4 3076578160
Waiting for data in sock 5 3068189552
New connection from 192.168.4.57 on socket 6
Waiting for connection
Waiting for data in sock 6 3059800944
New connection from 192.168.4.57 on socket 7
Waiting for connection
New connection from 192.168.4.57 on socket 8
Waiting for connection
New connection from 192.168.4.57 on socket 9
Waiting for connection
Waiting for data in sock 8 3051412336
New connection from 192.168.4.57 on socket 10
Waiting for data in sock 9 3034635120
Waiting for data in sock 10 3043023728
Waiting for connection
New connection from 192.168.4.57 on socket 11
Waiting for connection
New connection from 192.168.4.57 on socket 12
Waiting for connection
New connection from 192.168.4.57 on socket 13
Waiting for connection
Waiting for data in sock 13 3001080688
Waiting for data in sock 13 3026246512
Waiting for data in sock 13 3017857904
Waiting for data in sock 13 3009469296

If you look in to the output, you can notice sock 13 was displayed 4 times which should actually be for socket 7, 11, 12 and 13.

The behavior changes every time when I invoke simultaneously more client connections.

If I add sleep(1) after pthread_create() then I was able to see the expected behavior. Is the sleep after pthread_create is MUST? or how can I resolve this issue without using sleep?

help me on this. Thanks in advance

Answer 1

pthread_create(&threads[i++], NULL, (void*)fileTransfer_Worker, &newfd);
                                                                ^^^^^^

The problem is that you’re passing the address of a local variable to pthread_create. By the time the thread starts using it, main may have already changed it. So if the descriptor is 10 when you call pthread_create, when the thread actually starts using main already had the chance of changing it.

You have at least 2 options to fix this:

• Pass a separate object for each thread (perhaps one which you malloc)
• Stuff the integer in the argument, as if it were a void *, then cast back to int in the thread function

Although the second option is widely used, it’s also non-portable.