Uniquely identifying sockets

Question

I am writing a chatroom that can handle multiple clients. This is my first time programming with sockets and I'm having a bit of trouble understanding how to uniquely identify my sockets. Here is the relevant code:

//set up child processes and allocate shared memory
int numprocesses = 0;
pid_t pids[10];
int shmid;
key_t key = 5678;
int *shm;

if ((shmid = shmget(key, 5000, IPC_CREAT | 0666)) < 0)
{
    perror("shmget");
    exit(1);
}   

shm = (int *) shmat(shmid, NULL, 0); 
//initialize shared memory to 0
for (int i = 0; i < 100; i++)
{   
    *(shm + i) = 0;
}   

while(1)
{   
    //Accepting connections
    newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);
    if (newsockfd < 0)
    {   
        perror("Error on accept");
        exit(1);
    }    
    printf("socket: %d\n", newsockfd);

    //create a new process to handle each incoming connection
    pids[numprocesses] = fork();

    //child process
    if ((pids[numprocesses] == 0)) 
    {   
        *(shm + numprocesses) = newsockfd;
        close(sockfd);
        handle_client(newsockfd, shm);
        exit(0);
    }   
    //parent process
    {   
        numprocesses++;
        close(newsockfd);
    }   

}   

For some reason every new client that connects to the chatroom has a newsockfd of 4, so I have no idea how to differentiate them. Any help would be greatly appreciate thanks!

Answer 1

Socket descriptors get recycled on a per process base.

accept() gives you an sd, then you fork-off the client and close the sd in the parent and start over. As the sd had been closed it very well may get used again by the next successful call to accept().

As you fork-off a new process for each client, the process-id might be a way to identify a client, at least as long as it is connected and its process is existing.

To historise the connections this (the pid only) may not work as process-ids also might be recycled. So for this case you need to invent some really unique connection id, for example by combining the pid with the timestamp it was created at.