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copied!<p>The basic steps of tracing for function calls are on a non-live node:</p> <pre><code>> dbg:start(). % start dbg > dbg:tracer(). % start a simple tracer process > dbg:tp(Module, Function, Arity, []). % specify MFA you are interested in > dbg:p(all, c). % trace calls (c) of that MFA for all processes. ... trace here > dbg:stop_clear(). % stop tracer and clear effect of tp and p calls. </code></pre> <p>You can trace for multiple functions at the same time. Add functions by calling <code>tp</code> for each function. If you want to trace for non-exported functions, you need to call <code>tpl</code>. To remove functions, call <code>ctp</code> or <code>ctpl</code> in a similar manner. Some general tp calls are:</p> <pre><code>> dbg:tpl(Module, '_', []). % all calls in Module > dbg:tpl(Module, Function, '_', []). % all calls to Module:Function with any arity. > dbg:tpl(Module, Function, Arity, []). % all calls to Module:Function/Arity. > dbg:tpl(M, F, A, [{'_', [], [{return_trace}]}]). % same as before, but also show return value. </code></pre> <p>The last argument is a match specification. You can play around with that by using <code>dbg:fun2ms</code>.</p> <p>You can select the processes to trace on with the call to p(). The items are described under erlang:trace. Some calls are:</p> <pre><code>> dbg:p(all, c). % trace calls to selected functions by all functions > dbg:p(new, c). % trace calls by processes spawned from now on > dbg:p(Pid, c). % trace calls by given process > dbg:p(Pid, [c, m]). % trace calls and messages of a given process </code></pre> <p>I guess you will never need to directly call <code>erlang:trace</code>, as <code>dbg</code> does pretty much everything for you.</p> <p>A golden rule for a live node is to generate only an amount of trace output to the shell, which lets you to type in <code>dbg:stop_clear().</code>. :)</p> <p>I often use a tracer that will auto-stop itself after a number of events. For example:</p> <pre><code>dbg:tracer(process, {fun (_,100) -> dbg:stop_clear(); (Msg, N) -> io:format("~p~n", [Msg]), N+1 end, 0 }). </code></pre> <p>If you are looking for debugging on remote nodes (or multiple nodes), search for <code>pan</code>, <code>eper</code>, <code>inviso</code> or <code>onviso</code>.</p>

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