What's a compact way of casting a struct into a byte slice in Go?

Question

When writing network code we often find ourselves populating structs from byte slices to access the data in form of an object.

Let's take this struct

type PACKETHEAD struct {
    Type uint16
    Size uint16
    Hash uint32
}

and a byte slice that has been somehow populated with data

data := make([]byte, 1024)

My solution would be to

var pkthead PACKETHEAD
pktsiz := unsafe.Sizeof(pkthead)
pktbuf := bytes.NewReader(buf[:pktsiz])
err = binary.Read(pktbuf, binary.BigEndian, &pkthead)
if err != nil {
    // handle it
}

But

• It uses unsafe

• Requires ~7 lines of code for every cast (what if we had hundreds of different packets)

• Can't be trivially packed into a Cast(*struct, data) function

• No control over struct padding, what If go's compiler decides to add extra bytes in between members on one end of a network?

• binary.Read performs a data copy if I'm not mistaken (this isn't necessarily a con)

---

In C one would just #pragma pack(1) on both network ends, agree on one type of endianess

and finally PACKETHEAD* pkt = (PACKETHEAD*)dataptr;

How can we achieve the same thing with Go?

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Have a nice day, Kris

Answer 1

Shameless plug for gopack, a library I (and others) wrote to support bitpacking in Go. Note: it uses unsafe operations under the hood, if that's a problem.