Question

I’m trying to parse a string from WebSockets connection in Go language. I’m implementing both sides of the connection, so the specification of data format is depending only on me.

As this is a simple app (generally for learning purposes), I’ve come up with ActionId Data, where ActionId is a uint8. BackendHandler is a handler for every request in WebSocket Connection.

Platform information

kuba:~$ echo {$GOARCH,$GOOS,`6g -V`}
amd64 linux 6g version release.r60.3 9516

code:

const ( // Specifies ActionId's
  SabPause = iota
)

func BackendHandler(ws *websocket.Conn) {
  buf := make([]byte, 512)
  _, err := ws.Read(buf)
  if err != nil { panic(err.String()) }
  str := string(buf)
  tmp, _ := strconv.Atoi(str[:0])
  data := str[2:]
  fmt.Println(tmp, data)
  switch tmp {
    case SabPause:
      // Here I get `parsing "2": invalid argument`
      // when passing "0 2" to websocket connection
      minutes, ok := strconv.Atoui(data)
      if ok != nil {
        panic(ok.String())
      }
      PauseSab(uint8(minutes))
    default:
      panic("Unmatched input for BackendHandler")
  }
}

All the output: (note the Println that I used for inspecting)

0 2
panic: parsing "2": invalid argument [recovered]
    panic: runtime error: invalid memory address or nil pointer dereference

I couldn’t find the code from which this error is launch, only where the error code is defined (dependent on platform). I’d appreciate general ideas for improving my code, but mainly I just want to solve the conversion problem.

Is this related to my buffer -> string conversion and slice-manipulation(I didn’t want to use SplitAfter methods)?

Edit

This code reproduces the problem:

package main

import (
  "strconv"
  "io/ioutil"
)

func main() {
  buf , _ := ioutil.ReadFile("input")
  str := string(buf)
  _, ok := strconv.Atoui(str[2:])
  if ok != nil {
    panic(ok.String())
  }
}

The file input has to contain 0 2\r\n (depending on the file ending, it may look different on other OSes). This code can be fixed by adding the ending index for reslice, this way:

_, ok := strconv.Atoui(str[2:3])

Answer 1

You didn’t provide a small compilable and runnable program to illustrate your problem. Nor did you provide full and meaningful print diagnostic messages.

My best guess is that you have a C-style null-terminated string. For example, simplifying your code,

package main

import (
    "fmt"
    "strconv"
)

func main() {
    buf := make([]byte, 512)
    buf = []byte("0 2\x00") // test data
    str := string(buf)
    tmp, err := strconv.Atoi(str[:0])
    if err != nil {
        fmt.Println(err)
    }
    data := str[2:]
    fmt.Println("tmp:", tmp)
    fmt.Println("str:", len(str), ";", str, ";", []byte(str))
    fmt.Println("data", len(data), ";", data, ";", []byte(data))
    // Here I get `parsing "2": invalid argument`
    // when passing "0 2" to websocket connection
    minutes, ok := strconv.Atoui(data)
    if ok != nil {
        panic(ok.String())
    }
    _ = minutes
}

Output:

parsing "": invalid argument
tmp: 0
str: 4 ; 0 2 ; [48 32 50 0]
data 2 ; 2 ; [50 0]
panic: parsing "2": invalid argument

runtime.panic+0xac /home/peter/gor/src/pkg/runtime/proc.c:1254
    runtime.panic(0x4492c0, 0xf840002460)
main.main+0x603 /home/peter/gopath/src/so/temp.go:24
    main.main()
runtime.mainstart+0xf /home/peter/gor/src/pkg/runtime/amd64/asm.s:78
    runtime.mainstart()
runtime.goexit /home/peter/gor/src/pkg/runtime/proc.c:246
    runtime.goexit()
----- goroutine created by -----
_rt0_amd64+0xc9 /home/peter/gor/src/pkg/runtime/amd64/asm.s:65

If you add my print diagnostic statements to your code, what do you see?

Note that your tmp, _ := strconv.Atoi(str[:0]) statement is probably wrong, since str[:0] is equivalent to str[0:0], which is equivalent to the empty string "".

I suspect that your problem is that you are ignoring the n return value from ws.Read. For example (including diagnostic messages), I would expect,

buf := make([]byte, 512)
buf = buf[:cap(buf)]
n, err := ws.Read(buf)
if err != nil {
    panic(err.String())
}
fmt.Println(len(buf), n)
buf = buf[:n]
fmt.Println(len(buf), n)

Also, try using this code to set tmp,

tmp, err := strconv.Atoi(str[:1])
if err != nil {
    panic(err.String())
}