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    <p>I would suggest you to <em>debug/trace</em> your application to check what's going on. It's very helpful in understanding how things work in OTP.</p> <p>In your case, you might want to do the following.</p> <p>Start the tracer:</p> <pre><code>dbg:tracer(). </code></pre> <p>Trace all function calls for your supervisor and your gen_server:</p> <pre><code>dbg:p(all,c). dbg:tpl(echo_server, x). dbg:tpl(echo_sup, x). </code></pre> <p>Check which messages the processes are passing:</p> <pre><code>dbg:p(new, m). </code></pre> <p>See what's happening to your processes (crash, etc):</p> <pre><code>dbg:p(new, p). </code></pre> <p>For more information about tracing:</p> <p><a href="http://www.erlang.org/doc/man/dbg.html" rel="nofollow noreferrer">http://www.erlang.org/doc/man/dbg.html</a></p> <p><a href="http://aloiroberto.wordpress.com/2009/02/23/tracing-erlang-functions/" rel="nofollow noreferrer">http://aloiroberto.wordpress.com/2009/02/23/tracing-erlang-functions/</a></p> <p>Hope this can help for this and future situations.</p> <p><strong>HINT:</strong> The gen_server behaviour is expecting the callback terminate/2 to be defined and exported ;)</p> <p><strong>UPDATE:</strong> After the definition of the <em>terminate/2</em> the reason of the crash is evident from the trace. This is how it looks:</p> <p>We (75) call the crash/0 function. This is received by the gen_server (78).</p> <pre><code>(&lt;0.75.0&gt;) call echo_server:crash() (&lt;0.75.0&gt;) &lt;0.78.0&gt; ! {'$gen_call',{&lt;0.75.0&gt;,#Ref&lt;0.0.0.358&gt;},crash} (&lt;0.78.0&gt;) &lt;&lt; {'$gen_call',{&lt;0.75.0&gt;,#Ref&lt;0.0.0.358&gt;},crash} (&lt;0.78.0&gt;) call echo_server:handle_call(crash,{&lt;0.75.0&gt;,#Ref&lt;0.0.0.358&gt;},none) </code></pre> <p>Uh, problem on the handle call. We have a badmatch...</p> <pre><code>(&lt;0.78.0&gt;) exception_from {echo_server,handle_call,3} {error,{badmatch,2}} </code></pre> <p><strong>The terminate function is called</strong>. The server exits and it gets unregistered.</p> <pre><code>(&lt;0.78.0&gt;) call echo_server:terminate({{badmatch,2}, [{echo_server,handle_call,3}, {gen_server,handle_msg,5}, {proc_lib,init_p_do_apply,3}]},none) (&lt;0.78.0&gt;) returned from echo_server:terminate/2 -&gt; ok (&lt;0.78.0&gt;) exit {{badmatch,2}, [{echo_server,handle_call,3}, {gen_server,handle_msg,5}, {proc_lib,init_p_do_apply,3}]} (&lt;0.78.0&gt;) unregister echo_server </code></pre> <p>The Supervisor (77) receive the exit signal from the gen_server and it does its job:</p> <pre><code>(&lt;0.77.0&gt;) &lt;&lt; {'EXIT',&lt;0.78.0&gt;, {{badmatch,2}, [{echo_server,handle_call,3}, {gen_server,handle_msg,5}, {proc_lib,init_p_do_apply,3}]}} (&lt;0.77.0&gt;) getting_unlinked &lt;0.78.0&gt; (&lt;0.75.0&gt;) &lt;&lt; {'DOWN',#Ref&lt;0.0.0.358&gt;,process,&lt;0.78.0&gt;, {{badmatch,2}, [{echo_server,handle_call,3}, {gen_server,handle_msg,5}, {proc_lib,init_p_do_apply,3}]}} (&lt;0.77.0&gt;) call echo_server:start_link() </code></pre> <p>Well, it tries... Since it happens what Filippo said...</p>
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    1. COthanks for the debug tips. The "function not defined ... terminate" error confused me, too. The gen_server behaviour *shouldn't* expect terminate to be defined, since echo_server is not trapping exits. That is per the docs, anyway; I haven't read OTP's code yet.
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    2. COWell, defining and exporting the terminate/2 will remove the UNDEF, showing the real reason of the crash (badmatch on 2). The linking stuff is another story... You confused me a bit now. What do you mean exactly with "the gen_server behaviour shouldn't expect terminate to be defined, since echo_server is not trapping exits"?
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    3. COI updated the answer. Have a look.
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