[erlang-questions] Cannot use io:format() in a handle_call() callback in an application?

Calle Dybedahl calle@REDACTED
Thu Jul 3 09:04:24 CEST 2014

Hi there.

I've just run into a bit of behavior that I do not understand: if I have an
io:format() in a handle_call, I get a timeout instead of a return from
gen_server:call(), but only if the gen_server module was started as part of
an application. I ran into it while trying to do something real, but it's
easy enough to reproduce in a clean environment.

So. I start with an application created with "rebar create-app appid=wtf"
and "rebar create template=simplesrv srvid=wtf", and the wtf module added
as a child of the supervisor. If I then do "application:start(wtf)." in a
shell, "gen_server:call(wtf,something)." returns "ok" as expected. If I now
add the single line "io:format("In handle_call.~n")," at the start of
wtf:handle_call(), and try to run the same two commands, instead of "ok" I
get a pause followed by:

** exception exit: {timeout,{gen_server,call,[wtf,something]}}
     in function  gen_server:call/2 (gen_server.erl, line 182)

I totally don't understand why this happens, and only when it was started
with "application:start(wtf)". If I start either the wtf_sup or wtf modules
manually, the io:format() runs and prints what it should without problems.

I'm using Erlang 17.1 (installed with Homebrew) under OSX 10.9.
Calle Dybedahl -*- calle@REDACTED
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