Trouble implementing multi-threading across sockets in C program

Question

I'm creating a client-server chat program in C using sockets and threads and having an issue implementing multi-threading. What I have currently got is a server that takes connections from clients using sockets that is working so far, the issue is when I try create multiple threads to handle sending and receiving.

When a client successfully connects to the server socket a thread is created in the client that handles receiving a message 'connected to server id: x', before it reaches the main while loop of the client program.

When a client reaches the server, a thread is created in the server to handle sending that data.

They seem to work okay but its the order of execution that is getting me. When I connect the client to the server what I thought would happen was the thread created to receive a message from the server would execute then the main while loop would. But instead it looks like the thread is waiting for main loop to take input.

// CURRENT CLIENT CODE - only including threading code

void * client_receive(void * sockID) {
    int network_socket = *((int *) sockID);

    while(1) {
       char data[1024];
       recv(network_socket, data, 1024, 0);
       printf("%s", data);
    }
}

int main() {
    pthread_t thread;
    pthread_create(&thread, NULL, client_receive, (void *) &network_socket);

    while(1) {
        printf("type a message: ");
        scanf("%s", message);
        send(network_socket, message, 1024, 0);
    }
}

________________________________________________________________________________
// CURRENT SERVER CODE
struct client {
    int index;
    int sockID;
    struct sockaddr_in clientAddr;
    int len;
};

void * client_interact(void * client_detail) {
    struct client* client_Detail = (struct client*) client_detail;
    int index = client_Detail -> index;
    int clientSocket = client_Detail -> sockID;
    index = index + 1;

    printf("Client connected");
    char message[1024] = "Welcome!";
    send(clientSocket, message, 1024, 0);

    while(1) {
        char receive[1024];
        recv(clientSocket, &receive, 1024, 0);
        printf("Client: %s", receive);
    }
}

struct client Client[1024];
pthread_t thread[1024];

int main() {

    while(1) {
        pthread_create(&thread[client_count], NULL, client_interact, (void *) &Client[client_count]);

        client_count++;
    }

    for (int i = 0; i < client_count; i++) {
        pthread_join(thread[i], NULL);
    }
    return 0;
}

Am I misunderstanding execution of threads? I thought that 'client_receive' would execute on the created thread before main printed 'type a message'

There's no synchronization between the client_receive and the main thread, so both will execute at the same time. So you might see them in either order. — Chris Dodd
Depends on what you're trying to do. If you want to fix the order, you could use some locks, but its probably easier to just not use threads. — Chris Dodd
Unfortunately its a requirement for this project. Although, the ordering at this point is annoying it doesn't explain the deadlock i'm getting — SkinnyBetas
The code you've shown currently has no apparent deadlock problems -- you need to create a minimal reproducible example that shows how you're getting a deadlock. — Chris Dodd

ashiswin ashiswin · Accepted Answer · 2019-10-25T01:58:00

It is not possible really to tell whether client_receive is being executed before the while loop in main; that is up to the scheduler to decide. However, client_receive will not print anything until some data is received from the socket, so unless your client sends your server some data, the client_receive function will block on the call to recv.

Hence, while it may seem like your program is waiting for an input before running client_receive, it could be that client_receive is blocking on the call to recv and your main thread is waiting for input.

In the case where your client is actually sending data over and your server doesn't seem to be responding to that data, it could be that there's some sort of buffering on your client's side. So make sure to flush your client's socket to force the data to be pushed to the server, causing recv to be called.

Side note: I'm assuming the code where you actually set up the socket connection and store the handle into network_socket has not been included for brevity, otherwise that could be an issue!

Trouble implementing multi-threading across sockets in C program

1 Answers