golang casting byte array to struct

Question

I am looking for clean way to cast byte array to struct for client-server application. I know most ppl turn to gob package for this solution however I do not control the encoding for the application. that being said, I only programmed the server application not the client, there is a mutual contract for the protocol that is being exchanged.

The best I could come out is the following.

type T struct {
    A int16
    B int8
    C []byte
}

func main() {
    // Create a struct and write it.
    t := T{A: 99, B: 10}
    buf := &bytes.Buffer{}

    buf1 := []byte{5, 100, 100}
    fmt.Println(buf1)

    buf.Write(buf1)

    //err := binary.Write(buf, binary.BigEndian, t)

    //if err != nil {
    //  panic(err)
    //}
    fmt.Println(buf)

    // Read into an empty struct.
    t = T{}
    err := binary.Read(buf, binary.BigEndian, &t)
    if err != nil {
        panic(err)
    }
    fmt.Printf("%d %d", t.A, t.B)
}

However, as soon as number bytes does not coincide with the size of the struct, then go will send a panic. How can I modify this to work without the panic if undersize or oversize

Go playground

you just control the server side, means you have already have the serial protocol. What protocol you have, is the key point of your question. golang have encoding package support for widely used protocols, like JSON, BSON or PROTOBUF. so find out the serial protocol and choose the encoding package. or if you have private protocol, implement a encoding yourself. — Jiang YD
– Jiang YD, Commented Jul 21, 2015 at 2:07

Liam · Accepted Answer · 2022-03-05 21:16:40Z

6

According to http://golang.org/pkg/encoding/binary/#Read :

Data must be a pointer to a fixed-size value or a slice of fixed-size values.

So you can't use slice []byte in your structure. But you can use fixed size array for it.
Like this:

package main

import (
    "bytes"
    "encoding/binary"
    "fmt"
)

type T struct {
    A int16
    B int8
    C [256]byte
}

func main() {
    // Create a struct and write it.
    t := T{A: 99, B: 10}
    buf := &bytes.Buffer{}
        
    err := binary.Write(buf, binary.BigEndian, t)
    
    if err != nil {
        panic(err)
    }
    fmt.Println(buf)

    // Read into an empty struct.
    t = T{}
    err = binary.Read(buf, binary.BigEndian, &t)
    if err != nil {
        panic(err)
    }
    fmt.Printf("%d %d", t.A, t.B)
}

Go playground

edited Mar 5, 2022 at 21:16

Liam

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answered Jul 21, 2015 at 6:15

RoninDev

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2 Comments

user642318 Over a year ago

Any idea how to cast to variable size

RoninDev Over a year ago

Depends on your data structure. You can split your source array onto 2 parts. First part will contains only fixed-size data (A and B in your case 3 bytes). Second part will contains array data. In that way you need to create new structure, without array part, like type T struct { A int16 B int8 }. And decode both parts separately

Sirui Zhuang · Accepted Answer · 2016-02-11 15:13:09Z

2

I think binpacker will be greate to handle this case:

package main

import (
    "bytes"
    "fmt"

    "github.com/zhuangsirui/binpacker"
)

type T struct {
    A uint16
    B string
    C []byte
}

func main() {
    field1 := uint16(1)
    field2 := "Hello World"
    field3 := []byte("Hello World")
    buffer := new(bytes.Buffer)
    binpacker.NewPacker(buffer).
        PushUint16(field1).
        PushUint16(uint16(len(field2))).PushString(field2).
        PushUint16(uint16(len(field3))).PushBytes(field3)

    t := new(T)
    unpacker := binpacker.NewUnpacker(buffer)
    unpacker.FetchUint16(&t.A).StringWithUint16Perfix(&t.B).BytesWithUint16Perfix(&t.C)
    fmt.Println(t)
}

edited Feb 11, 2016 at 15:13

answered Feb 11, 2016 at 15:07

Sirui Zhuang

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