How to debug an erlang application that won't boot

Question

If i write some erlang code to build a supervision tree and then launch the application at the boot whith the following command, it may be very hard to find out why it doesn't work :

erl -s myapp -pa ebin ... ...

(myapp example module)

-module(myapp).
-export([start/0]).
start() -> application:start(myapp).

Let's say that my app launches a supervisor, myapp_sup. myapp_sup launches at its turn several supervisors (let's say server_sup, database_sup, another_sup).

Theese supervisors will launch some gen_servers.

At some point, if there is a mistake in my code, i cannot find it out !

I wrote a call to somemodule:functionthatdoesntexists() in the init callback of some gen_server.

All the vm says is "init terminating in do boot" and then display error location of a badmatch, precising file and line of my top module (myapp).

(Badmatch because ok = application:start(...) will not match).

I look in th erl_crash.dump file, and there is no information about this undefined function (yet i find it in the atoms list).

So, i wrote some log to see approximatively where is the error, but then i'll have to launch my gen_servers by the hand to get back the correct error information.

What am i missing, how could i figure it out faster ?

Thanks

Answer 1

If your application makes a call to an unknown module, your erl_crash.dump file will contain a line like this one:

41DABB8:t4:A8:nonexistent_module,A7:unknown,N,N

where the "unknown" in the line indicates that the module nonexistent_module can't be found. Searching your erl_crash.dump file for the string "unknown" can help in these cases.

If you suspect a certain module makes a call to a function that doesn't exist, you can find it using the xref tool in an interactive erl shell. Make sure you compile the module with debug information (typically via erlc +debug_info), then:

1> xref:m(my_module).
[{deprecated,[]},
 {undefined,[{{my_module,init,1},{another_module,unknown,0}}]},
 {unused,[]}]

Here, xref shows us that the my_module:init/1 function makes a call to the another_module:unknown/0 function, but the unknown/0 function isn't defined in another_module.

You can also use xref to check entire applications; see the documentation for details

Answer 2

Add -init_debug to your erl command :)