Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!seismo!husc6!hao!oddjob!gargoyle!ihnp4!cbosgd!osu-cis!tut!karl
From: karl@tut.cis.ohio-state.edu (Karl Kleinpaste)
Newsgroups: comp.sources.bugs
Subject: Re: another tcsh bug
Message-ID: <24@tut.cis.ohio-state.edu>
Date: Fri, 4-Sep-87 11:07:37 EDT
Article-I.D.: tut.24
Posted: Fri Sep  4 11:07:37 1987
Date-Received: Sat, 5-Sep-87 21:59:19 EDT
References: <16196@teknowledge-vaxc.ARPA>
Organization: Ohio State University Computer and Information Science Dept
Lines: 50

mkhaw@teknowledge-vaxc.ARPA writes:
>I accidentally pressed ^C twice (SIGINT) at the tcsh prompt just a few
>moments ago, and it spat out "Faulty alias 'periodic' removed."  Normally,
>I have "periodic" aliased to "checknews" and tperiod unset.  After the
>message, sure enough, "periodic" was no longer one of my aliases.
>
>This is repeatable on my system (Ultrix 1.2).  If I set tperiod to a *nonzero*
>value, this doesn't happen.

It is quite repeatable with a nonzero $tperiod; you just have to catch
it when the time period has passed the point where periodic has to be
re-executed.  If you happen to catch it at that point, the same thing
will happen again.  It should be reproducible in a manner like this:
	% alias periodic checknews
	[checknews executes once right away due to no $tperiod.]
	% set tperiod=1
	[now checknews doesn't execute; 1 minute hasn't expired.]
	[so sit there for 61 seconds, and then:]
	% <CR>
	[checknews now re-executes since the $tperiod has expired.]
	^C
	Faulty alias 'periodic' removed.
	%

I put periodic/$tperiod in my version of csh several years ago; a lot
of it has been absorbed by Paul into tcsh.  The reason for the
catch-and-remove-alias is to prevent the halt-and-catch-fire bug which
it fixes; consider the following:
	% alias periodic 'bad double quote"'
Given that execution of the periodic alias happens just before the
printing of your prompt, right after executing this alias, the
newly-created periodic alias will attempt to execute before printing
another "%" prompt.  But it will catch a quoting error in the process,
resulting in a call to bferr() [builtin function error], which will
toss the csh back to a point just before the printing of the prompt,
where it will execute the periodic alias...ad infinitum.  The catch
code is there to annihilate a (supposedly) failing periodic alias.  A
side effect of this is that an interruption of a valid periodic alias
will leave the flag set that the csh uses to determine a faulty
periodic alias.  Poof - your periodic alias is blown away.

I haven't any good ideas for how to get around this problem.  The code
is sh.c; if you're feeling motivated, look it over and see if you can
improve on it.

[By the way, checknews is an *awfully* expensive periodic alias to be
running before every single prompt (which is what happens when there
is no $tperiod).]
-- 
Karl