syscall: Expose cmsgData

[vincentbernat]

Hey!

When working with ancillary data, it is difficult to not use cmsgData from the syscall package. The package implements FD passing but everything else will need access to the data. A previous proposition was made to make the function public but was rejected due to returning unsafe.Pointer.

I would propose to use the following signature for the public function:

func CmsgData(h *Cmsghdr, len int) []byte

It could still panic if len is incorrect. Is that admissible?

[ianlancetaylor]

Why can't you use syscall.ParseSocketControlMessage?

[vincentbernat]

Oooh, terribly sorry. You are right. I just need the "reverse" function (SocketControlMessage?) taking []SocketControlMessage and returning []byte.

[ianlancetaylor]

Closing since I don't think there is anything to do in the syscall package, which is more or less frozen anyhow.