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POProcesses exiting normally
 

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    <p>Given two linked processes <code>child</code> and <code>parent</code>, how does process <code>child</code> detect that <code>parent</code> exits (terminates) normally?</p> <p>I, as an absolute Erlang beginner, thought that a process, when it has nothing else to do, exited using <code>exit(normal)</code>. This then signals all linked processes, where</p> <ul> <li>the behaviour of processes that have <code>trap_exit</code> set to <code>false</code> is to ignore the signal, and</li> <li>the behaviour of processes that have <code>trap_exit</code> set to <code>true</code> is to generate the message <code>{'EXIT', pid, normal}</code> where <code>pid</code> is the process id of the terminating process. </li> </ul> <p>My reason for thinking this is <a href="http://learnyousomeerlang.com/errors-and-processes#its-a-trap" rel="nofollow">Learn You Some Erlang for Great Good</a> and the <a href="http://www.erlang.org/doc/reference_manual/processes.html" rel="nofollow">Erlang documentation</a> which states the following.</p> <blockquote> <p>A process is said to terminate normally, if the exit reason is the atom normal. A process with no more code to execute terminates normally.</p> </blockquote> <p>Apparently that is wrong (?), because <code>exit(normal</code>) shows <code>** exception exit: normal</code> in the command prompt and makes the code below work. Exiting because there is no more code to execute does not generate the exception and does not make my code work.</p> <p>As an example, consider the following code.</p> <pre class="lang-erlang prettyprint-override"><code>-module(test). -export([start/0,test/0]). start() -&gt; io:format("Parent (~p): started!\n",[self()]), P = spawn_link(?MODULE,test,[]), io:format( "Parent (~p): child ~p spawned. Waiting for 5 seconds\n",[self(),P]), timer:sleep(5000), io:format("Parent (~p): dies out of boredom\n",[self()]), ok. test() -&gt; io:format("Child (~p): I'm... alive!\n",[self()]), process_flag(trap_exit, true), loop(). loop() -&gt; receive Q = {'EXIT',_,_} -&gt; io:format("Child process died together with parent (~p)\n",[Q]); Q -&gt; io:format("Something else happened... (~p)\n",[Q]) after 2000 -&gt; io:format("Child (~p): still alive...\n", [self()]), loop() end. </code></pre> <p>This produces output as follows.</p> <pre><code>(erlide@127.0.0.1)&gt; test:start(). Parent (&lt;0.145.0&gt;): started! Parent (&lt;0.145.0&gt;): child &lt;0.176.0&gt; spawned. Waiting for 5 seconds Child (&lt;0.176.0&gt;): I'm... alive! Child (&lt;0.176.0&gt;): still alive... Child (&lt;0.176.0&gt;): still alive... Parent (&lt;0.145.0&gt;): dies out of boredom ok (erlide@127.0.0.1)10&gt; Child (&lt;0.176.0&gt;): still alive... Child (&lt;0.176.0&gt;): still alive... Child (&lt;0.176.0&gt;): still alive... Child (&lt;0.176.0&gt;): still alive... Child (&lt;0.176.0&gt;): still alive... Child (&lt;0.176.0&gt;): still alive... Child (&lt;0.176.0&gt;): still alive... exit(pid(0,176,0),something). Child process died together with parent ({'EXIT',&lt;0.194.0&gt;,something}) </code></pre> <p>If had to manually execute the <code>exit(pid(0,176,0),something)</code> command to keep the child from staying alive forever. Changing <code>ok.</code> in <code>start</code> to <code>exit(normal)</code> makes the execution go like this</p> <pre><code>(erlide@127.0.0.1)3&gt; test:start(). Parent (&lt;0.88.0&gt;): started! Parent (&lt;0.88.0&gt;): child &lt;0.114.0&gt; spawned. Waiting for 5 seconds Child (&lt;0.114.0&gt;): I'm... alive! Child (&lt;0.114.0&gt;): still alive... Child (&lt;0.114.0&gt;): still alive... Parent (&lt;0.88.0&gt;): dies out of boredom Child process died together with parent ({'EXIT',&lt;0.88.0&gt;,normal}) ** exception exit: normal </code></pre> <p>My concrete questions are the following.</p> <ol> <li>How can I make the above code work as expected. That is, how can I make sure the child process dies together with the parent process <em>without changing the parent process</em>?</li> <li>Why does <code>exit(normal)</code> generate a <code>** exception exit: normal</code> in the CLI? It is hard for me to think of an exception as something that is normal. What does the scentence in the Erlang documentation mean?</li> </ol> <p>I think these must be extremely basic questions, but I can't seem to figure this out.... I am using Erlang 5.9.3.1 on Windows (x64).</p>
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