How to write a TCP chat server in 55 lines of Golang

The go net package lets you write a TCP server. Here’s a chat server, where every byte sent by a client is copied to every other client (including the sender).

package main
import "net"
func main() {
  newConns := make(chan net.Conn, 128)
  deadConns := make(chan net.Conn, 128)
  publishes := make(chan []byte, 128)
  conns := make(map[net.Conn]bool)
	listener, err := net.Listen("tcp", ":8080")
	if err != nil { panic(err) }
	go func() {
		for {
			conn, err := listener.Accept()
			if err != nil { panic(err) }
			newConns <- conn
		}
	}()
	for {
		select {
		case conn := <-newConns:
			conns[conn] = true
			go func() {
				buf := make([]byte, 1024)
				for {
					nbyte, err := conn.Read(buf)
					if err != nil {
						deadConns <- conn
						break
					} else {
						fragment := make([]byte, nbyte)
						copy(fragment, buf[:nbyte])
						publishes <- fragment
					}
				}
			}()
		case deadConn := <-deadConns:
			_ = deadConn.Close()
			delete(conns, deadConn)
		case publish := <-publishes:
			for conn, _ := range conns {
				go func(conn net.Conn) {
					totalWritten := 0
					for totalWritten < len(publish) {
						writtenThisCall, err := conn.Write(publish[totalWritten:])
						if err != nil {
							deadConns <- conn
							break
						}
						totalWritten += writtenThisCall
					}
				}(conn)
			}
		}
	}
	listener.Close()
}

Compare this to C:

• net.Listen does the socket, bind, and listen syscalls.
• Instead of raw file descriptors, we get net.Listener and net.Conn objects.
• The go runtime manages multiple goroutines, each making blocking syscalls like conn.Read, and multiplexes them onto a single process, ensuring that those goroutines don’t block each other. dtruss suggests Go is using select and kqueue for this on my machine.

This page copyright James Fisher 2017. Content is not associated with my employer. Found an error? Edit this page.

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