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<chapter>
  <header>
    <copyright>
      <year>2003</year><year>2009</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
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    <title>Processes</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>processes.xml</file>
  </header>

  <section>
    <title>Processes</title>
    <p>Erlang is designed for massive concurrency. Erlang processes are
      light-weight (grow and shrink dynamically) with small memory
      footprint, fast to create and terminate and the scheduling
      overhead is low.</p>
  </section>

  <section>
    <title>Process Creation</title>
    <p>A process is created by calling <c>spawn</c>:</p>
    <pre>
spawn(Module, Name, Args) -> pid()
  Module = Name = atom()
  Args = [Arg1,...,ArgN]
    ArgI = term()</pre>
    <p><c>spawn</c> creates a new process and returns the pid.</p>
    <p>The new process will start executing in
      <c>Module:Name(Arg1,...,ArgN)</c> where the arguments is
      the elements of the (possible empty) <c>Args</c> argument list.</p>
    <p>There exist a number of other <c>spawn</c> BIFs, for example
      <c>spawn/4</c> for spawning a process at another node.</p>
  </section>

  <section>
    <title>Registered Processes</title>
    <p>Besides addressing a process by using its pid, there are also
      BIFs for registering a process under a name. The name must be an
      atom and is automatically unregistered if the process terminates:</p>
    <table>
      <row>
        <cell align="left" valign="middle"><c>register(Name, Pid)</c></cell>
        <cell align="left" valign="middle">Associates the name <c>Name</c>, an atom, with the process <c>Pid</c>.</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>registered()</c></cell>
        <cell align="left" valign="middle">Returns a list of names which have been registered using<c>register/2</c>.</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>whereis(Name)</c></cell>
        <cell align="left" valign="middle">Returns the pid registered under <c>Name</c>, or<c>undefined</c>if the name is not registered.</cell>
      </row>
      <tcaption>Name Registration BIFs.</tcaption>
    </table>
  </section>

  <section>
    <marker id="term"></marker>
    <title>Process Termination</title>
    <p>When a process terminates, it always terminates with an
      <em>exit reason</em>. The reason may be any term.</p>
    <p>A process is said to terminate <em>normally</em>, if the exit
      reason is the atom <c>normal</c>. A process with no more code to
      execute terminates normally.</p>
    <p>A process terminates with exit reason <c>{Reason,Stack}</c>
      when a run-time error occurs. See
      <seealso marker="errors#exit_reasons">Error and Error Handling</seealso>.</p>
    <p>A process can terminate itself by calling one of the BIFs
      <c>exit(Reason)</c>, 
      <c>erlang:error(Reason)</c>, <c>erlang:error(Reason, Args)</c>,
      <c>erlang:fault(Reason)</c> or <c>erlang:fault(Reason, Args)</c>. 
      The process then terminates with reason <c>Reason</c> for
      <c>exit/1</c> or <c>{Reason,Stack} for the others</c>.</p>
    <p>A process may also be terminated if it receives an exit signal
      with another exit reason than <c>normal</c>, see
      <seealso marker="#errors">Error Handling</seealso> below.</p>
  </section>

  <section>
    <title>Message Sending</title>
    <p>Processes communicate by sending and receiving messages.
      Messages are sent by using
      the <seealso marker="expressions#send">send operator !</seealso>
      and received by calling
      <seealso marker="expressions#receive">receive</seealso>.</p>
    <p>Message sending is asynchronous and safe, the message is
      guaranteed to eventually reach the recipient, provided that
      the recipient exists.</p>
  </section>

  <section>
    <title>Links</title>
    <p>Two processes can be <em>linked</em> to each other. A link
      between two processes <c>Pid1</c> and <c>Pid2</c> is created
      by <c>Pid1</c> calling the BIF <c>link(Pid2)</c> (or vice versa).
      There also exists a number a <c>spawn_link</c> BIFs, which spawns
      and links to a process in one operation.</p>
    <p>Links are bidirectional and there can only be one link between
      two processes. Repeated calls to <c>link(Pid)</c> have no effect.</p>
    <p>A link can be removed by calling the BIF <c>unlink(Pid)</c>.</p>
    <p>Links are used to monitor the behaviour of other processes, see
      <seealso marker="#errors">Error Handling</seealso> below.</p>
  </section>

  <section>
    <marker id="errors"></marker>
    <title>Error Handling</title>
    <p>Erlang has a built-in feature for error handling between
      processes. Terminating processes will emit exit signals to all
      linked processes, which may terminate as well or handle the exit
      in some way. This feature can be used to build hierarchical
      program structures where some processes are supervising other
      processes, for example restarting them if they terminate
      abnormally.</p>
    <p>Refer to OTP Design Principles for more information about
      OTP supervision trees, which uses this feature.</p>

    <section>
      <title>Emitting Exit Signals</title>
      <p>When a process terminates, it will terminate with an <em>exit reason</em> as explained in <seealso marker="#term">Process Termination</seealso> above. This exit reason is emitted in
        an <em>exit signal</em> to all linked processes.</p>
      <p>A process can also call the function <c>exit(Pid,Reason)</c>.
        This will result in an exit signal with exit reason
        <c>Reason</c> being emitted to <c>Pid</c>, but does not affect
        the calling process.</p>
    </section>

    <section>
      <title>Receiving Exit Signals</title>
      <p>The default behaviour when a process receives an exit signal
        with an exit reason other than <c>normal</c>, is to terminate
        and in turn emit exit signals with the same exit reason to its
        linked processes. An exit signal with reason <c>normal</c> is
        ignored.</p>
      <p>A process can be set to trap exit signals by calling:</p>
      <pre>
process_flag(trap_exit, true)</pre>
      <p>When a process is trapping exits, it will not terminate when
        an exit signal is received. Instead, the signal is transformed
        into a message <c>{'EXIT',FromPid,Reason}</c> which is put into
        the mailbox of the process just like a regular message.</p>
      <p>An exception to the above is if the exit reason is <c>kill</c>,
        that is if <c>exit(Pid,kill)</c> has been called. This will
        unconditionally terminate the process, regardless of if it is
        trapping exit signals or not.</p>
    </section>
  </section>

  <section>
    <title>Monitors</title>
    <p>An alternative to links are <em>monitors</em>. A process
      <c>Pid1</c> can create a monitor for <c>Pid2</c> by calling
      the BIF <c>erlang:monitor(process, Pid2)</c>. The function returns
      a reference <c>Ref</c>.</p>
    <p>If <c>Pid2</c> terminates with exit reason <c>Reason</c>, a
      'DOWN' message is sent to <c>Pid1</c>:</p>
    <code type="none">
{'DOWN', Ref, process, Pid2, Reason}</code>
    <p>If <c>Pid2</c> does not exist, the 'DOWN' message is sent
      immediately with <c>Reason</c> set to <c>noproc</c>.</p>
    <p>Monitors are unidirectional. Repeated calls to
      <c>erlang:monitor(process, Pid)</c> will create several,
      independent monitors and each one will send a 'DOWN' message when
      <c>Pid</c> terminates.</p>
    <p>A monitor can be removed by calling
      <c>erlang:demonitor(Ref)</c>.</p>
    <p>It is possible to create monitors for processes with registered
      names, also at other nodes.</p>
  </section>

  <section>
    <title>Process Dictionary</title>
    <p>Each process has its own process dictionary, accessed by calling
      the following BIFs:</p>
    <pre>
put(Key, Value)
get(Key)
get()
get_keys(Value)
erase(Key)
erase()</pre>
  </section>
</chapter>

