C++ Academy · Lesson

Building a TCP Echo Server

Walk through a complete TCP echo server implementation.

Building a TCP Echo Server is a free C++ Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C++ Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Goal

Build a minimal TCP server that accepts a connection, reads bytes, and writes them back. The "Hello World" of network programming.

Server Skeleton

The server flow: create socket, bind, listen, accept, handle. Loop forever.

int srv = socket(AF_INET, SOCK_STREAM, 0);
bind(srv, (sockaddr*)&addr, sizeof(addr));
listen(srv, 10);
while (true) {
    int client = accept(srv, nullptr, nullptr);
    handle(client);
    close(client);
}

Reuse Address Option

Set SO_REUSEADDR so the server can restart immediately without waiting for the OS to release the port.

int yes = 1;
setsockopt(srv, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes));

The Echo Loop

Read into a buffer; write the same bytes back. Stop when the client closes the connection.

void handle(int client) {
    char buf[1024];
    while (true) {
        ssize_t n = recv(client, buf, sizeof(buf), 0);
        if (n <= 0) break;          // closed or error
        send(client, buf, n, 0);
    }
}

Partial Reads and Writes

Both recv and send can transfer fewer bytes than requested. For a stream-based protocol, loop until done.

ssize_t total = 0;
while (total < n) {
    ssize_t s = send(client, buf + total, n - total, 0);
    if (s <= 0) break;
    total += s;
}

Handling Multiple Clients

The simplest approach: a new thread per connection. Works for hundreds of clients; struggles at thousands.

while (true) {
    int client = accept(srv, nullptr, nullptr);
    std::thread([client]{ handle(client); close(client); }).detach();
}

Better: Thread Pool

Spawning a thread per connection wastes resources at scale. A fixed thread pool with a work queue scales better.

Graceful Shutdown

Handle SIGTERM/SIGINT to close the listening socket cleanly. RAII helps ensure all client sockets are closed too.

Error Handling

Treat errors as expected: clients disconnect, networks hiccup, OS limits run out. Log and continue — do not crash on a bad client.

Logging Each Client

Log the remote address for observability.

sockaddr_in remote{};
socklen_t len = sizeof(remote);
int client = accept(srv, (sockaddr*)&remote, &len);
char ip[INET_ADDRSTRLEN];
inet_ntop(AF_INET, &remote.sin_addr, ip, sizeof(ip));
std::cout << "client " << ip << ":" << ntohs(remote.sin_port);

Testing with netcat

Test your echo server interactively with nc.

nc 127.0.0.1 8080
hello
hello       # echoed back

Beyond Echo

From echo you can extend to:

Line-based protocols (HTTP-like)
Framed binary protocols (length-prefix)
JSON or Protobuf messages

Quick Check

What is the purpose of the SO_REUSEADDR socket option?

Recap

A minimal TCP echo server: socket, setsockopt SO_REUSEADDR, bind, listen, accept-in-loop. For multiple clients, spawn threads or use a thread pool. For high scale, switch to async I/O (next lesson).

Frequently asked questions

Is the “Building a TCP Echo Server” lesson free?

Yes — the full text of “Building a TCP Echo Server” is free to read here on the web, and the C++ Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C++ Academy course, upgrade to CoddyKit PRO.

What will I learn in “Building a TCP Echo Server”?

Walk through a complete TCP echo server implementation. You practise C++ Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C++ Academy?

No prior experience is required. C++ Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Building a TCP Echo Server” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C++ Academy lesson?

Yes. Every C++ Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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