golang udp客户端无法从服务器接收消息

Question

i can send msg from client to server successfully. But when i try to reply message back to client, somehow client cant receive the message.

Client:

conn, _:= net.Dial("udp", serv_addr:port)
defer conn.close()
buf:= []byte("Hey, server")
conn.Write(buf)

recv:= make([]byte, 1024)
fmt.Println("Reading...
")
conn.Read(recv)

Server:

addr, _:= net.ResolveUDPAddr("udp", addr:port)
msg := make([]byte, 1024)

conn,_: net.ListenUDP("udp", addr)
_, ret_addr, _: conn.ReadFromUDP(msg)
//print msg
conn.WriteToUDP([]byte("got your msg", ret)

It's a very simple udp connection. server can printout the msg from client, but on client side, it keeps waiting before conn.read() while server finished its task.

I followed this article. my implementation is almost identical.

Answer 1

Here is a minimum code for you to play around with.

start a server part in one terminal:

go run main.go server :53033

start client in the other terminal:

go run main.go client :53033

now type messages at the client terminal and you should see both client and server exchanging messages. You can start multiple clients (in new terminal windows) and communicate with the same server.

package main

import (
    "bufio"
    "fmt"
    "io"
    "log"
    "net"
    "os"
)

func server(url string) {
    log.Printf("serving on %s
", url)
    addr, err := net.ResolveUDPAddr("udp", url)
    errcheck(err)

    conn, err := net.ListenUDP("udp", addr)
    errcheck(err)
    defer close(conn)

    msg := make([]byte, 1024)

    for {
        n, retAddr, err := conn.ReadFromUDP(msg)
        errcheck(err)

        log.Printf("received %v bytes, ret addr %v, msg %s", n, retAddr, string(msg[:n]))

        reply := []byte(fmt.Sprintf("received from you: %v bytes", n))
        n, err = conn.WriteToUDP(reply, retAddr)
        errcheck(err)

        log.Printf("sent reply %v bytes
", n)
    }
}

func client(url string) {
    log.Printf("client for server url: %s
", url)

    addr, err := net.ResolveUDPAddr("udp", url)
    errcheck(err)

    conn, err := net.DialUDP("udp", nil, addr)
    errcheck(err)
    defer close(conn)

    msg := make([]byte, 512)
    scanner := bufio.NewScanner(os.Stdin)
    for {
        if scanner.Scan() {
            line := scanner.Text()

            n, err := conn.Write([]byte(line))
            errcheck(err)
            log.Printf("sent %d bytes 
", n)

            n, err = conn.Read(msg)
            errcheck(err)
            log.Printf("server replied with: %s 
", string(msg[:n]))
        }
    }
}

func main() {
    args := os.Args[1:]

    if len(args) < 2 {
        log.Fatal("expecting 2 arguments client or server first followed by the address:port")
    }

    switch args[0] {
    case "server":
        server(args[1])

    case "client":
        client(args[1])

    default:
        log.Fatal("first argument must be server or client")
    }

}

func errcheck(err error) {
    if err != nil {
        log.Fatal(err)
    }
}

func close(c io.Closer) {
    err := c.Close()
    if err != nil {
        log.Fatal()
    }
}

Have fun!