Re: How to debug strange value changes of a variable?

[Marcin Borkowski <mbork@wmi.amu.edu.pl>]

On 2014-12-31, at 22:09, Michael Heerdegen <michael_heerdegen@web.de> wrote:

> Marcin Borkowski <mbork@wmi.amu.edu.pl> writes:
>
>> Now somehow mu4e (I know it is to blame, since in stock message-mode,
>> with emacs -Q, and without mu4e, everything works as expected!) messes
>> around with the value of this variable /after the first increment/ (or
>> so it seems: it looks like that after some initialization - including
>> incf'ing that variable from its inital value of -1 - it somehow comes
>> back to its original (default) value).  I'd like to check what function
>> and when changes its value.  Is there any way to use Edebug (or anything
>> else) for this?
>
> Are you sure you reference the variable's value with the same buffer
> current?  That's important of course.

Yes, this I did check.

> Also note that changing a buffer's major mode removes all buffer local
> bindings in the current buffer.  That could maybe happen in your case.

Thank you, thank you, thank you, thank you so much!  I'd *never* think
of that (I didn't even know that).  That was it!  The first time my
function was called, the buffer was in fundamental mode; all subsequent
times, it was in mu4e-compose-mode.  That should explain everything.

Now the question is: what to do.  I could dive into mu4e sources (which
I'm just a bit too lazy to do now).  I could just start
mu4e-compose-mode myself in my function and see what happens (I'll try
that).  Other than that, I don't have many ideas.

>> I know about edebug-set-global-break-condition, but this doesn't help a
>> lot: it can stop when the variable has some value, but does not tell me
>> /which/ piece of code changed it.
>
> Indeed, that is not much helpful in this case, unless you instrument all
> code that could be responsible, but that may slow down things
> drastically.

What's more, some things might be done by functions written in C...

> Did you define the variable we speak about by yourself?  If you did,
> normally only your own code should change the variables binding.

Yes.

>> (A similar question is to find out what calls such-and-such function.
>> This one seems easier: I can just instrument the said function and then
>> press `d' in Edebug to see the backtrace.  Am I right?)
>
> Yes, you could.  But for such questions, the ordinary debugger is IMHO
> much simpler and more helpful (`debug-on-entry').  Load the elisp source
> files before using it to avoid seeing byte code in the debugger output.

Thanks for the tip!

> Michael.

Thank you again and a happy new year!

-- 
Marcin Borkowski
http://octd.wmi.amu.edu.pl/en/Marcin_Borkowski
Faculty of Mathematics and Computer Science
Adam Mickiewicz University