从没代码到能跑起来的完整 TCP 网络程序，麻雀虽小五脏俱全。client-server 模型、goroutine 并发、消息广播，Go 网络编程最核心的东西都在里面了。

go
package main

import (
	"bufio"
	"fmt"
	"net"
	"sync"
)

var (
	clients = make(map[net.Conn]bool, 1000)
	mu      sync.Mutex
)

func main() {
	listener, err := net.Listen("tcp", ":9999")
	if err != nil {
		panic(err.Error())
	}
	defer listener.Close()

	for {
		conn, _ := listener.Accept()
		go handleconn(conn)
	}
}

func handleconn(conn net.Conn) {
	defer conn.Close()
	mu.Lock()
	clients[conn] = true
	mu.Unlock()
	broadcast(conn, fmt.Sprintf("New Friend Join,Now All People Num: %d", len(clients)))

	scanner := bufio.NewScanner(conn)
	for scanner.Scan() {
		msg := scanner.Text()
		if scanner.Err() != nil {
			fmt.Println(scanner.Err().Error())
			break
		}
		if msg == `\q` {
			break
		}
		broadcast(conn, msg)
	}
	broadcast(conn, "A Frend Left.")
}
func broadcast(sender net.Conn, msg string) {
	mu.Lock()
	defer mu.Unlock()
	for client := range clients {
		if client != sender {
			fmt.Fprintln(client, msg)
		}
	}
}


```go
package main

import (
	"bufio"
	"fmt"
	"net"
	"os"
	"strings"
)

func main() {
	conn, _ := net.Dial("tcp", ":9999")
	defer conn.Close()

	go func() {
		scanner := bufio.NewScanner(conn)
		for scanner.Scan() {
			if scanner.Err() != nil {
				fmt.Println(scanner.Err().Error())
				break
			}
			msg := scanner.Text()
			fmt.Printf("Message:%s\n", msg)
		}
	}()
	scanner := bufio.NewScanner(os.Stdin)
	for scanner.Scan() {
		if scanner.Err() != nil {
			fmt.Println(scanner.Err().Error())
			break
		}
		text := scanner.Text()
		if strings.TrimSpace(text) == `\q` {
			fmt.Println("Out!")
			break

		}
		_, err := fmt.Fprintln(conn, text)
		if err != nil {
			fmt.Println("send message error", err.Error())
			break

		}
	}

}