syscall.Sockaddr Type Assertion

Question

I am opening a Linux packet socket and trying to read received packets into a struct:

type msg struct {
    n, oobn, flags int
    p, oob []byte
    from syscall.Sockaddr
}

socket, err := syscall.Socket(AF_PACKET, SOCK_RAW, ETH_P_ALL)

pkt := new(msg)
pkt.p = make([]byte, 1500)
pkt.oob = make([]byte, 1500)

pkt.n, pkt.oobn, pkt.flags, pkt.from, _ = syscall.Recvmsg(socket, pkt.p, pkt.oob, 0)

Per the documentation (http://golang.org/pkg/syscall/#Recvmsg) Recvmsg() returns the msghdr as a syscall.Sockaddr and the code pieces I've outlined above works.

Printing out the pkt.from struct member, I can see the values in the Sockaddr interface:

fmt.Printf("%+v\n", pkt.from)

>>> &{Protocol:8 Ifindex:3 Hatype:1 Pkttype:0 Halen:6 Addr:[0 0 36 205 126 213 0 0] raw:{Family:0 Protocol:0 Ifindex:0 Hatype:0 Pkttype:0 Halen:0 Addr:[0 0 0 0 0 0 0 0]}}

However, if I try to access them, I get an error:

fmt.Println(pkt.from.Ifindex)

>>> pkt.from.Ifindex undefined (type syscall.Sockaddr has no field or method Ifindex)

Via reflect.TypeOf(pkt.from) I found it was of type *syscall.SockaddrLinklayer. Trying to change my msg struct member to that type fails when Recvmsg tries to do the assignment since it not of type syscall.Sockaddr.

I was able to use a type assertion:

bar := pkt.from.(*syscall.SockaddrLinklayer)
fmt.Println(bar.Ifindex)

>>> 2

I'm very new to Go; it is my first statically typed language so I don't understand how the Recvmsg func is requiring a syscall.Sockaddr but returning a *syscall.SockaddrLinklayer? I'm clearly missing something very fundamental. Also, is using the type assertion the correct way to do this? It doesn't really feel right... but I'm not really qualified to make such judgments!

Answer 1

Sockaddr is an interface, so Recvmsg can return different types that fulfill that interface.

For more details about interfaces check Effective Go.

You should check if the assertion is valid or not so you wouldn't end up with a runtime error, for example:

bar, ok := pkt.from.(*syscall.SockaddrLinklayer) 
if !ok {
    //log error and break out of the loop
}
stuffWith(bar)