gen_server debug options

10 May 2023 10:22 erlang

Erlang’s gen_server (and gen_statem, etc.) allow for you to pass {debug, DebugOptions} in gen_server:start_link/4 (et al.). The documentation for it is spread over several pages, making it hard to understand at first. How should we use it?

Ordinarily when you start a gen_server, you’ll do it something like this:

% etc.

start_link(Host, Port) ->
    % Host, Port are just example names; we're not going to use them for anything.
    gen_server:start_link(?MODULE, [Host, Port], []).

That empty list [] is the “start options” for the gen_server. The start_opt() type is documented here. The one we’re interested in is this one:

{debug, Dbgs :: [sys:debug_option()]}

sys:debug_option() is defined as follows:

debug_option() =
    trace | log |
    {log, N :: integer() >= 1} |
    statistics |
    {log_to_file, FileName :: file:name()} |
     {Func :: dbg_fun(), FuncState :: term()} |
     {FuncId :: term(), Func :: dbg_fun(), FuncState :: term()}}
The FileName in {log_to_file, FileName} is relative to the current working directory. You should use a fully-qualified name.

The documentation says:

For every entry in Dbgs, the corresponding function in sys(3) is called.

I initially read this as meaning “the corresponding function […] is called with the debug event. That is: when gen_server wants to emit a debug event, it will call (e.g.) sys:trace. This is not how it works.

What it means is that this…

{ok, Pid} = gen_server:start_link(?MODULE, [], [{debug, [trace, {log_to_file, "debug.log"}]}]).

…is the same as this:

{ok, Pid} = gen_server:start_link(?MODULE, [], []),
sys:trace(Pid, true),
sys:log_to_file(Pid, "debug.log").

That’s not strictly true – it doesn’t actually call those functions – but it does the equivalent.

trace and log_to_file

Essentially, what it’s saying is that if you do this:

start_link(Host, Port) ->
    Dbgs = [trace, {log_to_file, "debug.log"}],
    gen_server:start_link(?MODULE, [Host, Port], [{debug, Dbgs}]).

…then, at appropriate points, gen_server will write debug events to stdout and to the named file. It will look something like this:

1> {ok, Pid} = example_server:start_link().
2> gen_server:call(Pid, hello).
*DBG* <0.183.0> got call hello from <0.176.0>
*DBG* <0.183.0> sent ok to <0.176.0>, new state {state}

The file debug.log contains the following:

*DBG* <0.183.0> got call hello from <0.176.0>
*DBG* <0.183.0> sent ok to <0.176.0>, new state {state}

That’s trace and log_to_file covered. What about some of the others?


If you specify {debug, [log]}, then gen_server records the events internally. You can retrieve them by using sys:log/2:

3> {ok, Events} = sys:log(Pid, get).
4> sys:log(Pid, print).
*DBG* <0.175.0> got call hello from <0.168.0>
*DBG* <0.175.0> sent ok to <0.168.0>, new state {state}

You can also see them in the output from sys:get_status/1, or in Observer’s process window, in the “State” tab.


This collects various, well, statistics about the process:

10> sys:statistics(Pid, true).
11> gen_server:call(Pid, hello).
12> sys:statistics(Pid, get).

You can see that it records the following:

  • start_time: when statistics collection was turned on.
  • current_time: the current time; you can use this with the above to work out how long statistics collection has been enabled for.
  • reductions: the number of “reductions” that have been run while statistics collection has been enabled. Reductions are a count of how many function calls have happened; they’re a rough estimate of how much work the process has done.
  • messages_in and messages_out are a count of how many messages have been sent and received through the gen_server mechanisms. They do not count OtherPid ! Message from the gen_server process.


This allows you to install your own debugging function. For example, enter the following in the console:

DebugFun = fun(DebugState, Event, ProcState) ->
    io:format("~p ~p\n", [Event, ProcState]),
    case DebugState of
        10 -> done;
        Count -> Count + 1

{ok, Pid} = example_server:start_link().
sys:install(Pid, {DebugFun, 1}).

This installs a debug function that prints out the first ten debug events and then is automatically removed.