[erlang-questions] Weird behavior with the visual debugger.

Matthieu Tourne <>
Mon Nov 24 22:23:30 CET 2014

I'm using Erlang/OTP 17 on Mac OS, and I'm seeing a weird behavior with the

I'm pretty new to the Erlang world, so not sure what is expected or not ..

So here is what I'm doing :

$> ii(module_to_debug).
$> im().

This will launch the visual debugger with the module I want to debug.
I've been trying to do 2 different things :

1. Set up a break point in my code
I'm setting a break point, and setting up the option "Auto attach - On

My problem is that what I want to step through is behind a gen_server call,
and the whole thing times out before I can do any meaningful debugging.
Is there a way to prevent the timeouts *without* having to change the code
that does the call ?

2. Inspecting the call stack on exception
In this case I'm just setting the option "Auto attach - On Exit".

That is where the weirdest behaviors lies, upon reaching the exception I
get 2 blank debugger windows, one saying "State: uninterpreted" at the
bottom and the other one "Loading module..."
In my Erlang console I get the following message (which only ever happen if
I'm trying to attach the debugger this way) :

$> 13:08:38.300 [error] gen_server dbg_iserver terminated with reason: no
match of right hand value [] in dbg_iserver:handle_call/3 line 287
(I'm using lager, hence the formatting).

All subsequent calls to what leads to the exception behave weirdly :

** exception exit: {noproc,{gen_server,call,

$> 13:20:22.337 [error] CRASH REPORT Process <0.14874.0> with 2 neighbours
exited with reason: no such process or port in call to
gen_server:call(undefined, {get_meta,<0.14874.0>}, infinity) in
gen_server:init_it/6 line 330

I'm not sure what I'm doing wrong here, or if there is a real problem.
If you can eventually point me to an article that explains how to make my
two possible workflows above work, that would be great too.

Thank you,

ps: Yes I already know "dbg:tracer(),dbg:p(all,c),dbg:tpl(ess_tcp, x)."
Which doesn't entirely solve my problem.
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