Networking | Advanced Topics

Introduction

Networking in C involves using sockets to create connections between two or more devices. This tutorial will guide you through the basics of networking in C, including setting up a server and a client to communicate with each other.

Understanding Sockets

A socket is an endpoint for communication between two machines. Sockets are created using the socket() function, which returns a socket descriptor. This socket descriptor is then used to connect, send, and receive data.

Example of creating a socket:

int sockfd;
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0) {
    perror("Error opening socket");
    exit(1);
}

Setting Up a Server

To set up a server, we need to create a socket, bind it to an address and port, and then listen for incoming connections. The server will use the accept() function to accept incoming client connections.

Example of setting up a server:

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

int main() {
    int sockfd, newsockfd, portno;
    socklen_t clilen;
    char buffer[256];
    struct sockaddr_in serv_addr, cli_addr;
    int n;

    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) {
        perror("Error opening socket");
        exit(1);
    }

    bzero((char *) &serv_addr, sizeof(serv_addr));
    portno = 5001;
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = INADDR_ANY;
    serv_addr.sin_port = htons(portno);

    if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) {
        perror("Error on binding");
        exit(1);
    }

    listen(sockfd, 5);
    clilen = sizeof(cli_addr);
    newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);
    if (newsockfd < 0) {
        perror("Error on accept");
        exit(1);
    }

    bzero(buffer, 256);
    n = read(newsockfd, buffer, 255);
    if (n < 0) {
        perror("Error reading from socket");
        exit(1);
    }

    printf("Here is the message: %s\n", buffer);

    n = write(newsockfd, "I got your message", 18);
    if (n < 0) {
        perror("Error writing to socket");
        exit(1);
    }

    close(newsockfd);
    close(sockfd);
    return 0;
}

Setting Up a Client

To set up a client, we need to create a socket and connect it to the server's address and port. The client can then send and receive data using the socket.

Example of setting up a client:

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

int main() {
    int sockfd, portno, n;
    struct sockaddr_in serv_addr;
    struct hostent *server;

    char buffer[256];
    portno = 5001;

    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) {
        perror("Error opening socket");
        exit(1);
    }

    server = gethostbyname("localhost");
    if (server == NULL) {
        fprintf(stderr, "Error, no such host\n");
        exit(0);
    }

    bzero((char *) &serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    bcopy((char *)server->h_addr, (char *)&serv_addr.sin_addr.s_addr, server->h_length);
    serv_addr.sin_port = htons(portno);

    if (connect(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) {
        perror("Error connecting");
        exit(1);
    }

    printf("Please enter the message: ");
    bzero(buffer, 256);
    fgets(buffer, 255, stdin);

    n = write(sockfd, buffer, strlen(buffer));
    if (n < 0) {
        perror("Error writing to socket");
        exit(1);
    }

    bzero(buffer, 256);
    n = read(sockfd, buffer, 255);
    if (n < 0) {
        perror("Error reading from socket");
        exit(1);
    }

    printf("%s\n", buffer);
    close(sockfd);
    return 0;
}

Compiling and Running

To compile and run the server and client programs, use the following commands:

gcc -o server server.c
gcc -o client client.c

./server
./client

Networking in C Language

Introduction

Understanding Sockets

Setting Up a Server

Setting Up a Client

Compiling and Running