TCP Socket Programming in Unix Using C Programming

This tutorial demonstrates a simple TCP echo client–server in C on a Unix-like system. The server listens on a port, accepts a client connection, reads a message, and sends it back. The client connects, sends text, and prints the echoed response.

Introduction

TCP (Transmission Control Protocol) is a connection-oriented transport protocol. It provides reliable, ordered delivery between applications. Socket programming uses the BSD socket API (socket, bind, listen, accept, connect, read/write) to exchange data over TCP/IP.

Concept

Role	Steps
Server	Create socket → bind to IP/port → listen → accept client → read/write → close
Client	Create socket → connect to server → write message → read response → close

TCP is stream-oriented: one read() may not return a full message. For production code, use length prefixes or delimiters. This example keeps messages small for clarity.

TCP Echo Server

Replace 127.0.0.1 and port 1012 with values suitable for your lab environment.

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

int main(void) {
    int server_fd, client_fd;
    struct sockaddr_in server_addr, client_addr;
    socklen_t client_len = sizeof(client_addr);
    char buffer[1024] = {0};

    server_fd = socket(AF_INET, SOCK_STREAM, 0);
    if (server_fd < 0) {
        perror("socket");
        return 1;
    }

    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(1012);
    inet_pton(AF_INET, "127.0.0.1", &server_addr.sin_addr);

    if (bind(server_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("bind");
        return 1;
    }

    if (listen(server_fd, 5) < 0) {
        perror("listen");
        return 1;
    }

    printf("Server listening on port 1012...\n");

    client_fd = accept(server_fd, (struct sockaddr *)&client_addr, &client_len);
    if (client_fd < 0) {
        perror("accept");
        return 1;
    }

    ssize_t n = read(client_fd, buffer, sizeof(buffer) - 1);
    if (n > 0) {
        buffer[n] = '\0';
        printf("Received: %s\n", buffer);
        write(client_fd, buffer, n);
    }

    close(client_fd);
    close(server_fd);
    return 0;
}

Server explanation

1. socket(AF_INET, SOCK_STREAM, 0) creates a TCP socket.
2. bind() attaches the socket to 127.0.0.1:1012.
3. listen() marks the socket as passive (accepting connections).
4. accept() blocks until a client connects and returns a new connected socket.
5. read() receives data; write() sends the same bytes back (echo).

Compile and run the server first:

gcc -o tcp_server tcp_server.c
./tcp_server

TCP Echo Client

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

int main(void) {
    int sock;
    struct sockaddr_in server_addr;
    char message[1024] = "Hello TCP Server";
    char response[1024] = {0};

    sock = socket(AF_INET, SOCK_STREAM, 0);
    if (sock < 0) {
        perror("socket");
        return 1;
    }

    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(1012);
    inet_pton(AF_INET, "127.0.0.1", &server_addr.sin_addr);

    if (connect(sock, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("connect");
        return 1;
    }

    write(sock, message, strlen(message));

    ssize_t n = read(sock, response, sizeof(response) - 1);
    if (n > 0) {
        response[n] = '\0';
        printf("Echo from server: %s\n", response);
    }

    close(sock);
    return 0;
}

Client explanation

1. Create a TCP socket with the same address family as the server.
2. connect() initiates the three-way handshake to the server.
3. write() sends the message; read() receives the echoed reply.

In a second terminal (while the server is running):

gcc -o tcp_client tcp_client.c
./tcp_client

Expected output on the client:

Echo from server: Hello TCP Server

Running Both Programs

1. Start the server in one terminal.
2. Start the client in another terminal on the same machine (or update the IP if remote).
3. Confirm the server prints the received message and the client prints the echo.

For multiple clients, loop accept() in the server and use fork(), threads, or select()/poll() — see TCP Chat Client/Server Programming.

Related Tutorials

• UDP Socket Programming in Unix Using C Programming
• TCP File Transfer Socket Programming
• Unix Basic Commands