C Network Programming Tutorial

1. Introduction to Sockets

Sockets are used to establish communication between two machines over a network. A socket is an endpoint for sending or receiving data across a computer network.

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

int main() {
    int sockfd;
    struct sockaddr_in server_addr;
    
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) {
        perror("Socket creation failed");
        exit(EXIT_FAILURE);
    }
    
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(8080);
    server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");

    connect(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr));
    printf("Connected to the server.\n");

    return 0;
}
                

This code demonstrates how to create a socket and connect to a server at IP address `127.0.0.1` on port `8080`.

2. Server-Side Program

The server listens for incoming connections from clients and responds to requests. The server must bind to a specific port and wait for client connections.

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>

int main() {
    int sockfd, new_sock;
    struct sockaddr_in server_addr, client_addr;
    socklen_t addr_size;
    
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) {
        perror("Socket creation failed");
        exit(EXIT_FAILURE);
    }

    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(8080);
    server_addr.sin_addr.s_addr = INADDR_ANY;

    bind(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr));
    listen(sockfd, 5);
    addr_size = sizeof(client_addr);
    new_sock = accept(sockfd, (struct sockaddr*)&client_addr, &addr_size);
    
    printf("Connection established.\n");

    return 0;
}
                

This server code binds to port 8080 and listens for incoming client connections. When a connection is established, it accepts the client and prints a message.

3. Communication Between Client and Server

Once the connection is established, data can be exchanged between the client and server using the `send()` and `recv()` functions.

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>

int main() {
    int sockfd;
    struct sockaddr_in server_addr;
    char *message = "Hello, Server!";

    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) {
        perror("Socket creation failed");
        exit(EXIT_FAILURE);
    }

    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(8080);
    server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");

    connect(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr));
    
    send(sockfd, message, strlen(message), 0);
    printf("Message sent to server: %s\n", message);

    return 0;
}
                

This client code sends a message to the server using the `send()` function. The server can receive and process this message accordingly.

4. Conclusion

Network programming in C using sockets enables you to create client-server applications, allowing communication over the internet. By understanding how to create sockets, bind, listen, and accept connections, you can develop complex networked applications.