Unable to send gob data over TCP in Go Programming

Question

I have a client server application, using TCP connection

Client:

type Q struct {
    sum int64
}

type P struct {
    M, N int64
}

func main() {
    ...
    //read M and N
    ...
    tcpAddr, err := net.ResolveTCPAddr("tcp4", service)
    ...
    var p P
    p.M = M
    p.N = N
    err = enc.Encode(p)
}

Server:

type Q struct {
    sum int64
}

type P struct {
    M, N int64
}

func main() {
    ...
    tcpAddr, err := net.ResolveTCPAddr("ip4", service)
    listener, err := net.ListenTCP("tcp", tcpAddr)
    ...
    var connB bytes.Buffer
    dec := gob.NewDecoder(&connB)
    var p P
    err = dec.Decode(p)
    fmt.Printf("{%d, %d}\n", p.M, p.N)
}

The result on serve is {0, 0} because I don't know how to obtain a bytes.Buffer variable from net.Conn.

Is there any way for sending gob variables over TCP ?

If true, how can this be done ? Or there are any alternative in sending numbers over TCP ?

Any help or sample code would really be appreciated.

Blount

Answer 1

Here's a complete example.

Server:

package main

import (
    "fmt"
    "net"
    "encoding/gob"
)

type P struct {
    M, N int64
}
func handleConnection(conn net.Conn) {
    dec := gob.NewDecoder(conn)
    p := &P{}
    dec.Decode(p)
    fmt.Printf("Received : %+v", p);
    conn.Close()
}

func main() {
    fmt.Println("start");
   ln, err := net.Listen("tcp", ":8080")
    if err != nil {
        // handle error
    }
    for {
        conn, err := ln.Accept() // this blocks until connection or error
        if err != nil {
            // handle error
            continue
        }
        go handleConnection(conn) // a goroutine handles conn so that the loop can accept other connections
    }
}

Client :

package main

import (
    "fmt"
    "log"
    "net"
    "encoding/gob"
)

type P struct {
    M, N int64
}

func main() {
    fmt.Println("start client");
    conn, err := net.Dial("tcp", "localhost:8080")
    if err != nil {
        log.Fatal("Connection error", err)
    }
    encoder := gob.NewEncoder(conn)
    p := &P{1, 2}
    encoder.Encode(p)
    conn.Close()
    fmt.Println("done");
}

Launch the server, then the client, and you see the server displaying the received P value.

A few observations to make it clear :

• When you listen on a socket, you should pass the open socket to a goroutine that will handle it.
• Conn implements the Reader and Writer interfaces, which makes it easy to use : you can give it to a Decoder or Encoder
• In a real application you would probably have the P struct definition in a package imported by both programs

Anticholinergic

Your example works great. Thanks. Only question I have is how can I send the result from the server back to the client? – Blount

A socket is bidirectionnal. Just write on it in the handleConnection function, exactly as you write in the client. – Pulchritudinous

This works great. Thanks for the example. I want to send a {map[string]string} to server. But it does not get decoded in server end. Any suggestion on this? – Sledge