Simple TCP Client & Server in C

What is client & server?

What we will build (echo server)

• The server listens on port 12345 for incoming client connections.
• When the client sends a string, the server sends the same string back (echo).
• The client prints both the message it sent and the reply it received.

API flow (both sides)

Server program (echo_server.c)

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

#define PORT 12345
#define BUF_SIZE 1024

int main(void) {
    // 1. create socket
    int listen_fd = socket(AF_INET, SOCK_STREAM, 0);
    if (listen_fd < 0) { perror("socket"); return 1; }

    // (recommended) allow quick reuse of the port
    int yes = 1;
    setsockopt(listen_fd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes));

    // 2. prepare address to listen on
    struct sockaddr_in addr;
    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = htonl(INADDR_ANY);  // any local IP
    addr.sin_port = htons(PORT);

    // 3. bind — attach the socket to the port
    if (bind(listen_fd, (struct sockaddr*)&addr, sizeof(addr)) < 0) {
        perror("bind"); return 1;
    }

    // 4. listen — start accepting connections
    if (listen(listen_fd, 1) < 0) {
        perror("listen"); return 1;
    }
    printf("Listening on port %d...\n", PORT);

    // 5. accept one client
    struct sockaddr_in client_addr;
    socklen_t clen = sizeof(client_addr);
    int conn_fd = accept(listen_fd, (struct sockaddr*)&client_addr, &clen);
    if (conn_fd < 0) { perror("accept"); return 1; }
    printf("Client connected: %s\n", inet_ntoa(client_addr.sin_addr));

    // 6. echo loop — read what arrives, send it back
    char buf[BUF_SIZE];
    ssize_t n;
    while ((n = recv(conn_fd, buf, BUF_SIZE - 1, 0)) > 0) {
        buf[n] = '\0';
        printf("received: %s", buf);
        send(conn_fd, buf, n, 0);  // send the same bytes back
    }

    // 7. clean up
    close(conn_fd);
    close(listen_fd);
    printf("Connection closed\n");
    return 0;
}

Client program (echo_client.c)

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

#define PORT 12345
#define BUF_SIZE 1024

int main(void) {
    // 1. create socket
    int sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) { perror("socket"); return 1; }

    // 2. target address: localhost:12345
    struct sockaddr_in addr;
    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_port = htons(PORT);
    addr.sin_addr.s_addr = inet_addr("127.0.0.1");

    // 3. connect — reach the server
    if (connect(sockfd, (struct sockaddr*)&addr, sizeof(addr)) < 0) {
        perror("connect"); return 1;
    }
    printf("Connected. Type a message then Enter:\n");

    // 4. read one line from stdin and send it
    char msg[BUF_SIZE];
    if (fgets(msg, BUF_SIZE, stdin) == NULL) return 1;
    send(sockfd, msg, strlen(msg), 0);

    // 5. receive the reply
    char buf[BUF_SIZE];
    ssize_t n = recv(sockfd, buf, BUF_SIZE - 1, 0);
    if (n > 0) {
        buf[n] = '\0';
        printf("Server replied: %s", buf);
    }

    // 6. close
    close(sockfd);
    return 0;
}

Build & run

Terminal 1 (server)

Terminal 2 (client)

Challenges