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Re: How to debug hang?


From: Tim X
Subject: Re: How to debug hang?
Date: Sat, 18 Nov 2006 17:37:57 +1100
User-agent: Gnus/5.11 (Gnus v5.11) Emacs/22.0.90 (gnu/linux)

Perry Smith <address@hidden> writes:

> On Nov 17, 2006, at 8:50 PM, David Abrahams wrote:
>
>     Hi,
>    
>     I'm trying to debug some kind of a hang that occurs when I try to use
>     a font-lock mode I'm developing.  I can't get control back by hitting
>     `C-g'.  I suspected it was a bad (combinatoric) regexp, but I added
>     advice before and after re-search-forward that emitted enter/exit
>     messages, and when the hang occurs I am seeing an exit message, so I'm
>     guessing that's not it.  I'm quite stumped now; hoping someone can
>     offer a clue.
>
> Check out debug-on-quit.  If that doesn't work, try debug-on-signal and then 
> kill
> the emacs from a separate window.  I'm not sure what that will do to your 
> context.
>
> There is also debug-on-entry if you have a routine that is called during the 
> hang
> but it doesn't sounds like you do.
>
> The other thing you might try is run emacs from a "terminal" (xterm or 
> similar). 
> It use to be that emacs would hook the quit signal to control-g would actually
> generate a quit unix signal so it happened immediately (via the tty code that 
> I so
> know and love :-) and not via some polling process.  Its been a long time 
> since
> I've used it and I don't know if it is still hooked up that way or not.
>
> But, while that might get you into the debugger, usually I'm too lost to 
> figure out
> why it is hanging, what is looping, why is it not finishing, etc.
>
> There is a message facility that might help too -- good ole printf debug
> technology.  (The function is called "message".)  It puts lines into the ever
> present *Messages* buffer.  So, if you ever do get control back, you can see 
> what
> happened.
>
> Good luck.
>

another alternative is to use edebug. You can instrument some
functions and then trace/step through your code. I suspect you will
find an infinite loop in there somewhere. 

Also, be wary of regular exprressions which are not anchored well. I
remember someone quite a while back who thought they were having
problems with the system hanging, but it turned out that their RE was
not well anchored and so it was taking a really long time for the RE
to do its work - made things look like the system was hung, but in
fact it just hadn't finished trying all the possible RE combinations
and subsequent backtracking. 

HTH

Tim

-- 
tcross (at) rapttech dot com dot au


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