// TCPClient project main.go
package main

import (
    "fmt"
    "net"
    "os"
)

func main() {
    var buf [512]byte
    if len(os.Args) != 2 {
        fmt.Fprintf(os.Stderr, "Usage: %s host:port ", os.Args[0])
        os.Exit(1)
    }
    service := os.Args[1]
    tcpAddr, err := net.ResolveTCPAddr("tcp4", service)
    checkErr(err)
    conn, err := net.DialTCP("tcp", nil, tcpAddr)
    defer conn.Close()
    checkErr(err)
    rAddr := conn.RemoteAddr()
    n, err := conn.Write([]byte("Hello server!"))
    checkErr(err)
    n, err = conn.Read(buf[0:])
    checkErr(err)
    fmt.Println("Reply from server ", rAddr.String(), string(buf[0:n]))
    os.Exit(0)
}

func checkErr(err error) {
    if err != nil {
        fmt.Fprintf(os.Stderr, "Fatal error: %s", err.Error())
        os.Exit(1)
    }
}

// TCPServer project main.go
package main

import (
    "fmt"
    "net"
    "os"
)

func main() {
    service := ":5000"
    tcpAddr, err := net.ResolveTCPAddr("tcp4", service)
    checkErr(err)
    listener, err := net.ListenTCP("tcp", tcpAddr)
    checkErr(err)
    for {
        conn, err := listener.Accept()
        if err != nil {
            continue
        }
        go handleClient(conn)
    }
}

func checkErr(err error) {
    if err != nil {
        fmt.Fprintf(os.Stderr, "Fatal error: %s", err.Error())
        os.Exit(1)
    }
}

func handleClient(conn net.Conn) {
    defer conn.Close()
    var buf [512]byte
    for {
        n, err := conn.Read(buf[0:])
        if err != nil {
            return
        }
        rAddr := conn.RemoteAddr()
        fmt.Println("Receive from client", rAddr.String(), string(buf[0:n]))
        _, err2 := conn.Write([]byte("Welcome client!"))
        if err2 != nil {
            return
        }
    }
}

// UDPClient project main.go
package main

import (
    "fmt"
    "net"
    "os"
)

func main() {
    var buf [512]byte
    if len(os.Args) != 2 {
        fmt.Fprintf(os.Stderr, "Usage: %s host:port ", os.Args[0])
        os.Exit(1)
    }
    service := os.Args[1]
    udpAddr, err := net.ResolveUDPAddr("udp4", service)
    checkErr(err)
    conn, err := net.DialUDP("udp", nil, udpAddr)
    defer conn.Close()
    checkErr(err)
    rAddr := conn.RemoteAddr()
    n, err := conn.Write([]byte("Hello server!"))
    checkErr(err)
    n, err = conn.Read(buf[0:])
    checkErr(err)
    fmt.Println("Reply from server ", rAddr.String(), string(buf[0:n]))
    os.Exit(0)
}

func checkErr(err error) {
    if err != nil {
        fmt.Fprintf(os.Stderr, "Fatal error: %s", err.Error())
        os.Exit(1)
    }
}

// UDPServer project main.go
package main

import (
    "fmt"
    "net"
    "os"
)

func main() {
    service := ":5000"
    udpAddr, err := net.ResolveUDPAddr("udp4", service)
    checkErr(err)
    conn, err := net.ListenUDP("udp", udpAddr)
    checkErr(err)
    for {
        handleClient(conn)
    }
}

func checkErr(err error) {
    if err != nil {
        fmt.Fprintf(os.Stderr, "Fatal error: %s", err.Error())
        os.Exit(1)
    }
}

func handleClient(conn *net.UDPConn) {
    defer conn.Close()
    var buf [512]byte
    for {
        n, rAddr, err := conn.ReadFromUDP(buf[0:])
        if err != nil {
            return
        }
        fmt.Println("Receive from client", rAddr.String(), string(buf[0:n]))
        _, err2 := conn.WriteToUDP([]byte("Welcome client!"), rAddr)
        if err2 != nil {
            return
        }
    }
}