os.getlogin() raises OSError (errno 25)

[Donn Cave]

Quoth davidccarson at hotmail.com (David Carson):
| Could someone please explain why the following runs fine if I type the
| input and end stdin with ctrl-D, but bombs when I pipe stdin to it.
...
|     loginName = os.getlogin()
| OSError: [Errno 25] Inappropriate ioctl for device
...
| mandrake Linux 8.2

On NetBSD, it doesn't fail on a pipe, but man 2 getlogin mentions
that it used to, in some earlier version of NetBSD, specifically if
the process was not associated with a "login terminal."  Details
vary from one platform to another (you don't want to know about the
setpenv business on AIX), but characteristically it's a function of
the terminal login session.  In theory, you'd get at the controlling
terminal (if that's the same thing) by opening /dev/tty, and if you're
really desperate you could try that out to see if you can work around
some Linux brain damage:  open /dev/tty read/write, dup unit 0/1/2 to
keep them around, dup2 the tty unit to 0/1/2, call getlogin, dup2 the
saved units back and close the dups.  Of course it will be for naught
if /dev/tty isn't a terminal - I'm just speculating that the brain
damage is that GNU getlogin assumes /dev/tty is on 0 (or 1 or 2.)

Incidentally, on NetBSD I get "root" although I am not root.  That's
probably because my sshd failed to call setlogin() - I don't know, but
I've seen the same thing on other platforms for similar reasons.  One
would not want to rely on this function for sensitive matters.  It's
only good for a hint - if getuid() returns 0, you can try getlogin.

	Donn Cave, donn at drizzle.com