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<?xml version="1.0" encoding="latin1" ?>
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<!DOCTYPE chapter SYSTEM "chapter.dtd">
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<year>2003</year><year>2009</year>
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<holder>Ericsson AB. All Rights Reserved.</holder>
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The contents of this file are subject to the Erlang Public License,
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Version 1.1, (the "License"); you may not use this file except in
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compliance with the License. You should have received a copy of the
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Erlang Public License along with this software. If not, it can be
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retrieved online at http://www.erlang.org/.
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Software distributed under the License is distributed on an "AS IS"
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basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
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the License for the specific language governing rights and limitations
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<title>Processes</title>
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<file>processes.xml</file>
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<title>Processes</title>
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<p>Erlang is designed for massive concurrency. Erlang processes are
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light-weight (grow and shrink dynamically) with small memory
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footprint, fast to create and terminate and the scheduling
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<title>Process Creation</title>
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<p>A process is created by calling <c>spawn</c>:</p>
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spawn(Module, Name, Args) -> pid()
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Module = Name = atom()
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Args = [Arg1,...,ArgN]
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<p><c>spawn</c> creates a new process and returns the pid.</p>
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<p>The new process will start executing in
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<c>Module:Name(Arg1,...,ArgN)</c> where the arguments is
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the elements of the (possible empty) <c>Args</c> argument list.</p>
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<p>There exist a number of other <c>spawn</c> BIFs, for example
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<c>spawn/4</c> for spawning a process at another node.</p>
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<title>Registered Processes</title>
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<p>Besides addressing a process by using its pid, there are also
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BIFs for registering a process under a name. The name must be an
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atom and is automatically unregistered if the process terminates:</p>
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<cell align="left" valign="middle"><c>register(Name, Pid)</c></cell>
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<cell align="left" valign="middle">Associates the name <c>Name</c>, an atom, with the process <c>Pid</c>.</cell>
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<cell align="left" valign="middle"><c>registered()</c></cell>
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<cell align="left" valign="middle">Returns a list of names which have been registered using<c>register/2</c>.</cell>
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<cell align="left" valign="middle"><c>whereis(Name)</c></cell>
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<cell align="left" valign="middle">Returns the pid registered under <c>Name</c>, or<c>undefined</c>if the name is not registered.</cell>
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<tcaption>Name Registration BIFs.</tcaption>
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<marker id="term"></marker>
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<title>Process Termination</title>
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<p>When a process terminates, it always terminates with an
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<em>exit reason</em>. The reason may be any term.</p>
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<p>A process is said to terminate <em>normally</em>, if the exit
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reason is the atom <c>normal</c>. A process with no more code to
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execute terminates normally.</p>
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<p>A process terminates with exit reason <c>{Reason,Stack}</c>
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when a run-time error occurs. See
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<seealso marker="errors#exit_reasons">Error and Error Handling</seealso>.</p>
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<p>A process can terminate itself by calling one of the BIFs
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<c>erlang:error(Reason)</c>, <c>erlang:error(Reason, Args)</c>,
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<c>erlang:fault(Reason)</c> or <c>erlang:fault(Reason, Args)</c>.
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The process then terminates with reason <c>Reason</c> for
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<c>exit/1</c> or <c>{Reason,Stack} for the others</c>.</p>
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<p>A process may also be terminated if it receives an exit signal
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with another exit reason than <c>normal</c>, see
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<seealso marker="#errors">Error Handling</seealso> below.</p>
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<title>Message Sending</title>
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<p>Processes communicate by sending and receiving messages.
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Messages are sent by using
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the <seealso marker="expressions#send">send operator !</seealso>
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and received by calling
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<seealso marker="expressions#receive">receive</seealso>.</p>
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<p>Message sending is asynchronous and safe, the message is
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guaranteed to eventually reach the recipient, provided that
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the recipient exists.</p>
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<p>Two processes can be <em>linked</em> to each other. A link
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between two processes <c>Pid1</c> and <c>Pid2</c> is created
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by <c>Pid1</c> calling the BIF <c>link(Pid2)</c> (or vice versa).
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There also exists a number a <c>spawn_link</c> BIFs, which spawns
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and links to a process in one operation.</p>
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<p>Links are bidirectional and there can only be one link between
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two processes. Repeated calls to <c>link(Pid)</c> have no effect.</p>
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<p>A link can be removed by calling the BIF <c>unlink(Pid)</c>.</p>
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<p>Links are used to monitor the behaviour of other processes, see
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<seealso marker="#errors">Error Handling</seealso> below.</p>
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<marker id="errors"></marker>
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<title>Error Handling</title>
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<p>Erlang has a built-in feature for error handling between
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processes. Terminating processes will emit exit signals to all
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linked processes, which may terminate as well or handle the exit
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in some way. This feature can be used to build hierarchical
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program structures where some processes are supervising other
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processes, for example restarting them if they terminate
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<p>Refer to OTP Design Principles for more information about
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OTP supervision trees, which uses this feature.</p>
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<title>Emitting Exit Signals</title>
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<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
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an <em>exit signal</em> to all linked processes.</p>
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<p>A process can also call the function <c>exit(Pid,Reason)</c>.
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This will result in an exit signal with exit reason
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<c>Reason</c> being emitted to <c>Pid</c>, but does not affect
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the calling process.</p>
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<title>Receiving Exit Signals</title>
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<p>The default behaviour when a process receives an exit signal
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with an exit reason other than <c>normal</c>, is to terminate
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and in turn emit exit signals with the same exit reason to its
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linked processes. An exit signal with reason <c>normal</c> is
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<p>A process can be set to trap exit signals by calling:</p>
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process_flag(trap_exit, true)</pre>
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<p>When a process is trapping exits, it will not terminate when
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an exit signal is received. Instead, the signal is transformed
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into a message <c>{'EXIT',FromPid,Reason}</c> which is put into
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the mailbox of the process just like a regular message.</p>
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<p>An exception to the above is if the exit reason is <c>kill</c>,
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that is if <c>exit(Pid,kill)</c> has been called. This will
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unconditionally terminate the process, regardless of if it is
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trapping exit signals or not.</p>
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<title>Monitors</title>
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<p>An alternative to links are <em>monitors</em>. A process
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<c>Pid1</c> can create a monitor for <c>Pid2</c> by calling
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the BIF <c>erlang:monitor(process, Pid2)</c>. The function returns
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a reference <c>Ref</c>.</p>
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<p>If <c>Pid2</c> terminates with exit reason <c>Reason</c>, a
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'DOWN' message is sent to <c>Pid1</c>:</p>
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{'DOWN', Ref, process, Pid2, Reason}</code>
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<p>If <c>Pid2</c> does not exist, the 'DOWN' message is sent
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immediately with <c>Reason</c> set to <c>noproc</c>.</p>
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<p>Monitors are unidirectional. Repeated calls to
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<c>erlang:monitor(process, Pid)</c> will create several,
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independent monitors and each one will send a 'DOWN' message when
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<c>Pid</c> terminates.</p>
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<p>A monitor can be removed by calling
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<c>erlang:demonitor(Ref)</c>.</p>
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<p>It is possible to create monitors for processes with registered
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names, also at other nodes.</p>
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<title>Process Dictionary</title>
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<p>Each process has its own process dictionary, accessed by calling
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the following BIFs:</p>