Something I had never thought about, quoted from http://micro.nicholaswilson.me.uk/post/33359696556/cute-little-thought-its-obvious-why-there-arent [foreshortened, Nicholas Wilson] Methinks there should be an errno value for this...

getpeerid()

Cute little thought: It’s obvious why there aren’t APIs for finding out, for example, the current credentials of the process on the other end of a UNIX-domain socket (there might be several processes on the end!) Less obviously, there could be no process on the other end: suppose you have a socket at your end, and the remote end is dup’ed into two related processes. You pass in an fd (with an SCM_RIGHTS message), then close it. It’s not open in your process, and it can’t be associated with either receiving process (either one of them could read it first). So, who ever’s on the other end of the passed fd is in the unusal situation of having a valid, connected socket with absolutely no process on the other end! Quirky.

FreeBSD: LOCAL_PEERCRED, the manpage describes the same behavior

Solaris: not 100% clear, "A process that listens for connections retrieves the credentials of the peer at the time
the peer initiated the connection." but also "EINVAL The socket is connected but the peer credentials are unknown."? Sounds like it does the same anyway, but EINVAL leaves me with a 1% uncertainty