TOD Service – Time of the Day

Writing a basic Time-of-the-Day (TOD) server is a good experiment for learning basic network programming concepts. A TOD server listens for incoming network connections and responds with the current date and time.

Steps to Implement a TOD Service on a Server

1. Create a Socket: Set up a TCP socket to listen for incoming connections.
2. Bind the Socket: Bind the socket to a specific IP address and port.
3. Listen for Connections: Set the socket to listen for incoming connection requests.
4. Accept Connections: Accept incoming client connections and handle them.
5. Send the Time: Send the current date and time to the connected client.
6. Close the Connection: Close the connection after sending the data.

Code snippet

complete example of a simple ToD service written in C can be found at below

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <time.h>

#define PORT 45000  // The port number on which the ToD service will listen
#define BUFFER_SIZE 128  // variable size buffer

int main() {
    int server_fd, client_fd;
    struct sockaddr_in server_addr, client_addr;
    socklen_t client_addr_len = sizeof(client_addr);
    char buffer[BUFFER_SIZE];
    time_t current_time;
    struct tm *local_time;
    int bytes_sent;

    // Create a socket
    if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
        perror("socket");
        exit(EXIT_FAILURE);
    }

    // Prepare the sockaddr_in structure
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = INADDR_ANY;  // Bind to all available interfaces including lo interface 
    server_addr.sin_port = htons(PORT);  // Port number in network byte order

    // Bind the socket to the address and port
    if (bind(server_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) == -1) {
        perror("bind");
        close(server_fd);
        exit(EXIT_FAILURE);
    }

    // Listen for incoming connections
    if (listen(server_fd, 5) == -1) {
        perror("listen");
        close(server_fd);
        exit(EXIT_FAILURE);
    }

    printf("Server listening for CLIENT request on port %d...\n", PORT);

    while (1) {
        // Accept an incoming connection
        if ((client_fd = accept(server_fd, (struct sockaddr *)&client_addr, &client_addr_len)) == -1) {
            perror("accept");
            close(server_fd);
            exit(EXIT_FAILURE);
        }

        // Get the current time
        current_time = time(NULL);
        local_time = localtime(&current_time);

        // copy the formatted time into buffer
        snprintf(buffer, BUFFER_SIZE, "Current time: %s", asctime(local_time));

        // Send copied buffer to the client
        bytes_sent = send(client_fd, buffer, strlen(buffer), 0);
        if (bytes_sent == -1) {
            perror("send");
        }

        // current client requested connection closed
        close(client_fd);
    }

    // server socket is closed
    close(server_fd);
    return 0;
}

Tick Tick Tick …

simplify above code

1. Socket Creation: server_fd = socket(AF_INET, SOCK_STREAM, 0); Creates a TCP socket.
   Wondering why we didn’t choose UDP sockets ?
2. Binding: server_addr.sin_family = AF_INET;
server_addr.sin_addr.s_addr = INADDR_ANY;
server_addr.sin_port = htons(PORT);

bind(server_fd, (struct sockaddr *)&server_addr, sizeof(server_addr));
   Binds the socket to all available interfaces on the specified port.
3. Listening: listen(server_fd, 5); Sets the socket to listen for incoming connections, with a backlog queue size of 5
   use below command on Linux to know more about listen system call
   man listen
4. Accepting Connections: client_fd = accept(server_fd, (struct sockaddr *)&client_addr, &client_addr_len);
   Accepts an incoming connection from a client.
5. Sending the Time: current_time = time(NULL);
local_time = localtime(&current_time);
snprintf(buffer, BUFFER_SIZE, "Current time: %s", asctime(local_time)); send(client_fd, buffer, strlen(buffer), 0);
   Retrieves the current time, formats it, and sends it to the client.
6. Closing Connections: close(client_fd); close(server_fd);
   Closes the client connection and server socket.

run services on server

1. Compile the Code gcc -o tod_daemon tod_server.c
2. Run the Server: ./tod_daemon use “netstat -tulpen” command to check/verify whether process is listening on the port or not
3. Connect to the Server: Send client request to fetch the ToD and You should see the current date and time displayed by the server.

Please write your own TOD client in C language to test the ToD service.