Failure in creation of simple TCP server that accepts variable message lengths - go

Question

I am trying to create a tcp server/client connection in golang. I am sending a message length so the server will accept multiple messages until the message length is reached. When I try to read the message header to determine the message length, there is no output. The code seems to gets stuck in the server's tcpreader function's for loop when "conn.read(b)" is called. What might be causing this failure?
My server code:
package main
import (
"bufio"
"bytes"
"encoding/binary"
"fmt"
"log"
"net"
"strings"
)
type hdr struct {
magicNum uint64
msgLen uint32
msgType uint16
padding uint16
}
func tcpReader(conn net.Conn) {
foundLength := false
var hdr struct {
magicNum uint64
msgLen uint32
msgType uint16
padding uint16
}
hdrSize := 16
fmt.Println("tcpReader() header size %v", hdrSize)
//localBuffer := new(bytes.Buffer)
defer conn.Close()
for {
if !foundLength {
fmt.Println("tcpReader() Getting the lenght of the message")
var b = make([]byte, hdrSize)
// where the code seems to get stuck
read, err := conn.Read(b)
if err != nil {
fmt.Println("Failed to read header size %v", hdrSize)
fmt.Println(err)
continue
}
fmt.Println("tcpReader() read header bytes %v %v x", read, string(b))
if read != hdrSize { //add logic check magic number etc
fmt.Println("invalid header")
continue
}
err = binary.Read(bytes.NewReader(b[:16]), binary.BigEndian, &hdr)
if err != nil {
log.Fatal(err)
}
fmt.Println("tcpReader() read header bytes %v", hdr.msgLen)
foundLength = true
// messageLength = int(binary.BigEndian.Uint64(b))
} else {
var message = make([]byte, hdr.msgLen)
read, err := conn.Read(message)
if err != nil {
fmt.Println(err)
continue
}
fmt.Println("Received bytes: %v", read)
if read != int(hdr.msgLen) {
//Add logic to append the read message and update the readSofarBytes etc
fmt.Println("invalid data")
continue
}
fmt.Println("Received:", string(message))
foundLength = false
}
}
}
func main() {
fmt.Println("Launching server...")
fmt.Println("Listen on port")
/* addr, err := net.ResolveTCPAddr("ip4", "127.0.0.1:8081")
if err != nil {
panic(err)
}*/
ln, err := net.Listen("tcp", "127.0.0.1:8081")
if err != nil {
log.Fatal(err)
}
defer ln.Close()
fmt.Println("Accept connection on port")
for {
conn, err := ln.Accept()
if err != nil {
log.Fatal(err)
}
fmt.Println("Calling handleConnection")
//go handleConnection(conn)
go tcpReader(conn)
}
}
My client code:
package main
import (
"bufio"
"bytes"
"encoding/binary"
"fmt"
"net"
)
type hdr struct {
magicNum uint64
msgLen int
msgType uint16
padding uint16
msg string
}
func main() {
var bin_buf bytes.Buffer
msg := hdr{magicNum: 123456789,
msgLen: 60,
msgType: 2,
padding: 0,
msg: "Hello world this is tcp message with big header"}
msg.msgLen = int(len(msg.msg))
fmt.Printf("message length %v\n", msg.msgLen)
binary.Write(&bin_buf, binary.BigEndian, msg)
addr, _ := net.ResolveTCPAddr("tcp", ":8081")
conn, err := net.DialTCP("tcp", nil, addr)
if err != nil {
panic(err.Error())
}
// conn.Write(bin_buf.Bytes())
conn.Write(bin_buf.Bytes())
//conn.Write([]byte("Hello from Clinet\n"))
message, _ := bufio.NewReader(conn).ReadString('\n')
fmt.Print(message)
conn.Close()
}

Answer 1

It is because you are not writing anything from the client side at all. You should never ignore error handling in golang.Change these lines and you'll see where the error is:
err := binary.Write(&bin_buf, binary.BigEndian, msg)
if err != nil {
panic(err.Error())
}
Client is writing 0 bytes to connection because there aren't any
panic: binary.Write: invalid type main.hdr

Answer 2

To complete Keser's answer, you can't use a struct whose fields have types without a fixed size. In this case your hdr struct has two fields that can't be written using binary.Write, the message field with type string and msgLen without specifying the int size. You will have to manually write the string message as it is up to the developer to figure out how to handle strings when writing it through a TCP connection. Additionally, all the fields of your struct are unexported, binary.Write will not be able to access them. Use an uppercase letter to export them.

Answer 3

If you try it, you'll know it.
func main() {
...
err := binary.Write(&bin_buf, binary.BigEndian, msg)
fmt.Println(err)
fmt.Println(string(bin_buf.Bytes()))
...
}