Weird Problem with cat

[Alexander Dupuy]

ps72234 at naakka (Pertti Suomela) writes:
> Tcsh (I don't know if you were running it) contains a bug.

The problem is not with cat, in fact, cat is the most helpful program you
could run in the circumstance, because it tells you what the problem is.
Most commands just execute with no output whatsoever.  This is much worse
than a weird error message.  However, the cat error message is still not
helpful enough - I only figured out the bug using ofiles(1), available
from a sun-spots archive near you.

This bug exists in all versions of the csh which do filename completion on
Suns (or other NFS systems) running yellow pages.

What happens is that when getpwnam(3) is called to find the home directory
for some user (~j-user) in the tilde() function, the yellow pages opens up
a UDP socket or two to talk to the various yellow pages daemons (ypbind,
ypserv).  It closes the first of these, but leaves the second one open.
Since the csh keeps file descriptors 0-3 unused (it stashes stdin, etc. up
around 20), the file descriptor for this socket is usually 1 (aka stdout).
When the csh fork/execs a program, it moves stdin, back down to the 0-2
range.  But because the yp socket is already down there, the stdout (1)
file descriptor gets closed when all is said and done (this may be due to
close-on-exec being set for 1, or yp closing it explicitly, or something
else, I never bothered to find out).

The result of this is that your cat was invoked with no file descriptor 1.
As a result it got a write error (EBADF bad file number).  Most programs
which never checked the result of writes to stdout got errors too, but
didn't tell you about them.  You can duplicate this with 'sh -c "program
>&-"'.

The fix, in tilde() is to make a call to the undocumented, but extremely
useful yp_unbind() function after you've gotten the answer back from
getpwnam(3).  You shouls also check that the closem() function invokes
yp_unbind(), although that may not be necessary if you fix tilde() (at any
rate, it won't hurt).

@alex