How to use the dbg tracer in Elixir

Questions / Help
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1

I’m trying to use the dbg tracer from Erlang in Elixir. At the moment I have the following line:

:dbg.tracer(process, fn (msg, _) -> IO.puts("DEBUG: #{msg}") end)

This raises the error:

** (CompileError) iex:1: undefined function process/0
    (stdlib 3.16.1) lists.erl:1358: :lists.mapfoldl/3

Any help with this would be appreciated. I’m myself pretty new to Elixir and Erlang, so it’s very possible it’s something very simple that I’m doing wrong.

Kind regards!

2

The first argument of :dbg.tracer/2 is type which is one of the atoms :port | :process | :module.
You probably got that example mixed from some source in Erlang. process in Erlang is an atom. So the equivalent in elixir would be :process

:dbg.tracer(:process, fn (msg, _) -> IO.puts("DEBUG: #{msg}") end)
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3

I usually suggest GitHub - redink/extrace: Elixir wrapper for Recon Trace., which translates quite well from and to erlang.

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4

I’ve read the page you linked but this seems to only be able to trace function calls? In my case I am trying to trace message sends and receives.

5

This works, I was indeed confused by the documentation (which is for Erlang). It raises another error though:

iex(1)> :dbg.tracer(:process, fn (msg, _) -> IO.puts("DEBUG:  #{msg}") end) 
** (FunctionClauseError) no function clause matching in :dbg.tracer/2    

    The following arguments were given to :dbg.tracer/2:

        # 1
        :process

        # 2
        #Function<43.65746770/2 in :erl_eval.expr/5>

    (runtime_tools 1.17) dbg.erl:308: :dbg.tracer/2

It seems to not find the function. Note, this function does exist in Erlang (as can be seen in the documentation I linked) and :dbg.tracer() works as well.

6

From looking at the docs it appears as though that function does not take a two-arity function as its second argument. It should either be a zero-arity function, or a tuple with a two-arity function and the initial data for that function. This is the Erlang type syntax which specifies this:

Data = PortGenerator | HandlerSpec | ModuleSpec
PortGenerator = fun() (no arguments)
Error = term()
HandlerSpec = {HandlerFun, InitialData}
HandlerFun = fun() (two arguments)
ModuleSpec = fun() (no arguments) |{TracerModule, TracerState}
TracerModule = atom()
InitialData = TracerState = term()
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7

This seems to have fixed it! I simply added a dummy initial value (since I don’t need one yet). Thanks a lot!