path: root/lib/stdlib/doc/src/sys.xml

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<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2019</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.
    
    </legalnotice>

    <title>sys</title>
    <prepared>Martin Bj&ouml;rklund</prepared>
    <responsible>Bjarne D&auml;cker</responsible>
    <docno></docno>
    <approved>Bjarne D&auml;cker</approved>
    <checked></checked>
    <date>1996-06-06</date>
    <rev></rev>
    <file>sys.xml</file>
  </header>
  <module since="">sys</module>
  <modulesummary>A functional interface to system messages.</modulesummary>
  <description>
    <p>This module contains functions for sending system messages used by
      programs, and messages used for debugging purposes.</p>
    <p>Functions used for implementation of processes are also expected to
      understand system messages, such as debug messages and code change. These
      functions must be used to implement the use of system messages for a
      process; either directly, or through standard behaviors, such as
      <seealso marker="gen_server"><c>gen_server</c></seealso>.</p>
    <p>The default time-out is 5000 ms, unless otherwise specified.
      <c>timeout</c> defines the time to wait for the process to
      respond to a request. If the process does not respond, the
      function evaluates <c>exit({timeout, {M, F, A}})</c>.
      </p>
    <marker id="dbg_opt"/>
    <p>The functions make references to a debug structure.
      The debug structure is a list of <c>dbg_opt()</c>, which is an internal
      data type used by function <seealso marker="#handle_system_msg/6">
      <c>handle_system_msg/6</c></seealso>. No debugging is performed if it is 
      an empty list.</p>
  </description>

  <section>
    <title>System Messages</title>
    <p>Processes that are not implemented as one of the standard
      behaviors must still understand system messages. The following 
      three messages must be understood:</p>
    <list type="bulleted">
      <item>
        <p>Plain system messages. These are received as
          <c>{system, From, Msg}</c>. The content and meaning of
          this message are not interpreted by the
          receiving process module. When a system message is received, function 
	  <seealso marker="#handle_system_msg/6">
          <c>handle_system_msg/6</c></seealso>
          is called to handle the request.</p>
      </item>
      <item>
        <p>Shutdown messages. If the process traps exits, it must
          be able to handle a shutdown request from its parent, the
          supervisor. The message <c>{'EXIT', Parent, Reason}</c>
          from the parent is an order to terminate. The process must 
	  terminate when this message is received, normally with the
          same <c>Reason</c> as <c>Parent</c>.
          </p>
      </item>
      <item>
        <p>If the modules used to implement the process change dynamically
          during runtime, the process must understand one more message. An
          example is the <seealso marker="gen_event"><c>gen_event</c></seealso>
          processes. The message is <c>{_Label, {From, Ref}, get_modules}</c>.
          The reply to this message is <c>From ! {Ref, Modules}</c>, where
          <c>Modules</c> is a list of the currently active modules in the
          process.</p>
        <p>This message is used by the release handler to find which
          processes that execute a certain module. The process can later be
          suspended and ordered to perform a code change for one of its
          modules.</p>
      </item>
    </list>
  </section>

  <section>
    <title>System Events</title>
    <p>When debugging a process with the functions of this
      module, the process generates <em>system_events</em>, which are
      then treated in the debug function. For example, <c>trace</c>
      formats the system events to the terminal.
      </p>
    <p>Four predefined system events are used when a
      process receives or sends a message. The process can also define its
      own system events. It is always up to the process itself
      to format these events.</p>
  </section>

  <datatypes>
    <datatype>
      <name name="name"/>
    </datatype>
    <datatype>
      <name name="system_event"/>
      <desc>
	<taglist>
	  <tag><c>{in,<anno>Msg</anno>}</c></tag>
	  <item>
	    <p>
	      Is produced by <c>gen_server</c> and <c>gen_event</c>
	      when the message <c>Msg</c> arrives.
	    </p>
	  </item>
	  <tag><c>{in,<anno>Msg</anno>,<anno>State</anno>}</c></tag>
	  <item>
	    <p>
	      Is produced by <c>gen_statem</c>
	      when the message <c>Msg</c> arrives in state <c>State</c>.
	    </p>
	    <p>
	      For <c>gen_statem</c> the <c><anno>Msg</anno></c> term is
	      an <c>{EventType,EventContent}</c> tuple.
	    </p>
	  </item>
	  <tag><c>{out,<anno>Msg</anno>,<anno>To</anno>}</c></tag>
	  <item>
	    <p>
	      Is produced by <c>gen_statem</c> when the reply <c>Msg</c>
	      is sent back to <c>To</c> by returning
	      a <c>{reply,To,Msg}</c> action from the callback module.
	    </p>
	    <p>
	      <c><anno>To</anno></c> is of the same type
	      as the first argument to <c>gen_statem:reply/2</c>.
	    </p>
	  </item>
	  <tag>
	    <c>{out,<anno>Msg</anno>,<anno>To</anno>,<anno>State</anno>}</c>
	  </tag>
	  <item>
	    <p>
	      Is produced by <c>gen_server</c>
	      when the reply <c><anno>Msg</anno></c>
	      is sent back to <c><anno>To</anno></c>
	      by returning a <c>{reply,...}</c> tuple
	      from the callback module.
	    </p>
	    <p>
	      <c><anno>To</anno></c> is of the same type
	      as the first argument to <c>gen_server:reply/2</c>.
	    </p>
	    <p>
	      <c><anno>State</anno></c> is the new server state.
	    </p>
	  </item>
	  <tag>
	    <c>{noreply,<anno>State</anno>}</c>
	  </tag>
	  <item>
	    <p>
	      Is produced by <c>gen_server</c>
	      when a <c>{noreply,...}</c> tuple is returned
	      from the callback module.
	    </p>
	    <p>
	      <c><anno>State</anno></c> is the new server state.
	    </p>
	  </item>
	  <tag>
	    <c>{continue,<anno>Continuation</anno>}</c>
	  </tag>
	  <item>
	    <p>
	      Is produced by <c>gen_server</c>
	      when a <c>{continue,<anno>Continuation</anno>}</c>
	      tuple is returned from the callback module.
	    </p>
	  </item>
	  <tag>
	    <c>{code_change,<anno>Event</anno>,<anno>State</anno>}</c>
	  </tag>
	  <item>
	    <p>
	      Is produced by <c>gen_statem</c>
	      when the message <c><anno>Event</anno></c>
	      arrives in state <c><anno>State</anno></c>
	      as the first event after a code change.
	    </p>
	    <p>
	      <c><anno>Event</anno></c> is
	      an <c>{EventType,EventContent}</c> tuple.
	    </p>
	  </item>
	  <tag>
	    <c>
	      {postpone,<anno>Event</anno>,<anno>State</anno>,<anno>NextState</anno>}
	    </c>
	  </tag>
	  <item>
	    <p>
	      Is produced by <c>gen_statem</c>
	      when the message <c><anno>Event</anno></c>
	      is postponed in state <c><anno>State</anno></c>.
	      <c><anno>NextState</anno></c> is the new state.
	    </p>
	    <p>
	      <c><anno>Event</anno></c> is
	      an <c>{EventType,EventContent}</c> tuple.
	    </p>
	  </item>
	  <tag>
	    <c>
	      {consume,<anno>Event</anno>,<anno>State</anno>,<anno>NextState</anno>}
	    </c>
	  </tag>
	  <item>
	    <p>
	      Is produced by <c>gen_statem</c>
	      when the message <c><anno>Event</anno></c>
	      is consumed in state <c><anno>State</anno></c>.
	      <c><anno>NextState</anno></c> is the new state.
	    </p>
	    <p>
	      <c><anno>Event</anno></c> is
	      an <c>{EventType,EventContent}</c> tuple.
	    </p>
	  </item>
	  <tag>
	    <c>
	      {start_timer,<anno>Action</anno>,<anno>State</anno>}
	    </c>
	  </tag>
	  <item>
	    <p>
	      Is produced by <c>gen_statem</c>
	      when the action <c><anno>Action</anno></c>
	      starts a timer in state <c><anno>State</anno></c>.
	    </p>
	  </item>
	  <tag>
	    <c>
	      {insert_timeout,<anno>Event</anno>,<anno>State</anno>}
	    </c>
	  </tag>
	  <item>
	    <p>
	      Is produced by <c>gen_statem</c> when a timeout zero action
	      inserts event <c><anno>Event</anno></c>
	      in state <c><anno>State</anno></c>.
	    </p>
	    <p>
	      <c><anno>Event</anno></c> is
	      an <c>{EventType,EventContent}</c> tuple.
	    </p>
	  </item>
	  <tag>
	    <c>
	      {enter,<anno>State</anno>}
	    </c>
	  </tag>
	  <item>
	    <p>
	      Is produced by <c>gen_statem</c>
	      when the first state <c><anno>State</anno></c> is entered.
	    </p>
	  </item>
	  <tag>
	    <c>
	      {terminate,<anno>Reason</anno>,<anno>State</anno>}
	    </c>
	  </tag>
	  <item>
	    <p>
	      Is produced by <c>gen_statem</c>
	      when it terminates with reason <c><anno>Reason</anno></c>
	      in state <c><anno>State</anno></c>.
	    </p>
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="dbg_opt"/>
      <desc><p>See the introduction of this manual page.</p></desc>
    </datatype>
    <datatype>
      <name name="dbg_fun"/>
    </datatype>
    <datatype>
      <name name="debug_option"/>
    </datatype>
    <datatype>
      <name name="format_fun"/>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="change_code" arity="4" since=""/>
      <name name="change_code" arity="5" since=""/>
      <fsummary>Send the code change system message to the process.</fsummary>
      <desc>
        <p>Tells the process to change code. The process must be
          suspended to handle this message. Argument <c><anno>Extra</anno></c>
          is reserved for each process to use as its own. Function
          <c><anno>Module</anno>:system_code_change/4</c> is called.
	  <c><anno>OldVsn</anno></c> is the old version of the
          <c><anno>Module</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="get_state" arity="1" since="OTP R16B01"/>
      <name name="get_state" arity="2" since="OTP R16B01"/>
      <fsummary>Get the state of the process.</fsummary>
      <desc>
        <p>Gets the state of the process.</p>
        <note>
          <p>These functions are intended only to help with debugging. They are
            provided for convenience, allowing developers to avoid having to
            create their own state extraction functions and also avoid having
            to interactively extract the state from the return values of
            <seealso marker="#get_status-1"><c>get_status/1</c></seealso> or
            <seealso marker="#get_status-2"><c>get_status/2</c></seealso>
            while debugging.</p>
        </note>
        <p>The value of <c><anno>State</anno></c> varies for different types of
          processes, as follows:</p>
        <list type="bulleted">
          <item>
            <p>For a
              <seealso marker="gen_server"><c>gen_server</c></seealso>
              process, the returned <c><anno>State</anno></c>
              is the state of the callback module.</p>
          </item>
          <item>
            <p>For a
              <seealso marker="gen_statem"><c>gen_statem</c></seealso>
              process, <c><anno>State</anno></c> is the tuple
              <c>{CurrentState,CurrentData}</c>.</p>
          </item>
          <item>
            <p>For a
              <seealso marker="gen_event"><c>gen_event</c></seealso>
              process, <c><anno>State</anno></c> is a list of tuples,
              where each tuple corresponds to an event handler registered
              in the process and contains <c>{Module, Id, HandlerState}</c>,
              as follows:</p>
            <taglist>
              <tag><c>Module</c></tag>
              <item>
                <p>The module name of the event handler.</p>
              </item>
              <tag><c>Id</c></tag>
              <item>
                <p>The ID of the handler (which is <c>false</c> if it was 
                  registered without an ID).</p>
              </item>
              <tag><c>HandlerState</c></tag>
              <item>
                <p>The state of the handler.</p>
              </item>
            </taglist>
          </item>
        </list>
        <p>If the callback module exports a function <c>system_get_state/1</c>,
          it is called in the target process to get its state. Its argument is
          the same as the <c>Misc</c> value returned by
          <seealso marker="#get_status-1"><c>get_status/1,2</c></seealso>, and
          function <seealso marker="#Module:system_get_state/1">
          <c>Module:system_get_state/1</c></seealso> is expected to extract the
          state of the callback module from it. Function
          <c>system_get_state/1</c> must return <c>{ok, State}</c>, where
          <c>State</c> is the state of the callback module.</p>
        <p>If the callback module does not export a <c>system_get_state/1</c>
          function, <c>get_state/1,2</c> assumes that the <c>Misc</c> value is
          the state of the callback module and returns it directly instead.</p>
        <p>If the callback module's <c>system_get_state/1</c> function crashes
          or throws an exception, the caller exits with error 
          <c>{callback_failed, {Module, system_get_state}, {Class, Reason}}</c>,
          where <c>Module</c> is the name of the callback module and
          <c>Class</c> and <c>Reason</c> indicate details of the exception.</p>
        <p>Function <c>system_get_state/1</c> is primarily useful for
          user-defined behaviors and modules that implement OTP
          <seealso marker="#special_process">special processes</seealso>. 
          The <c>gen_server</c>,
	  <c>gen_statem</c>, and <c>gen_event</c> OTP
          behavior modules export this function, so callback modules for those
          behaviors need not to supply their own.</p>
        <p>For more information about a process, including its state, see
          <seealso marker="#get_status-1"><c>get_status/1</c></seealso> and
          <seealso marker="#get_status-2"><c>get_status/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="get_status" arity="1" since=""/>
      <name name="get_status" arity="2" since=""/>
      <fsummary>Get the status of the process.</fsummary>
      <desc>
        <p>Gets the status of the process.</p>
        <p>The value of <c><anno>Misc</anno></c> varies for different types of
          processes, for example:</p>
        <list type="bulleted">
          <item>
            <p>A <seealso marker="gen_server"><c>gen_server</c></seealso>
              process returns the state of the callback module.</p>
          </item>
          <item>
            <p>A <seealso marker="gen_statem"><c>gen_statem</c></seealso>
              process returns information, such as its current
              state name and state data.</p>
          </item>
          <item>
            <p>A <seealso marker="gen_event"><c>gen_event</c></seealso>
              process returns information about each of its
              registered handlers.</p>
          </item>
        </list>
        <p>Callback modules for <c>gen_server</c>,
	  <c>gen_statem</c>, and <c>gen_event</c>
	  can also change the value of <c><anno>Misc</anno></c>
          by exporting a function <c>format_status/2</c>, which contributes
          module-specific information. For details, see
          <seealso marker="gen_server#Module:format_status/2">
          <c>gen_server:format_status/2</c></seealso>,
          <seealso marker="gen_statem#Module:format_status/2">
          <c>gen_statem:format_status/2</c></seealso>, and
          <seealso marker="gen_event#Module:format_status/2">
          <c>gen_event:format_status/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="install" arity="2" since=""/>
      <name name="install" arity="3" since=""/>
      <fsummary>Install a debug function in the process.</fsummary>
      <desc>
        <p>Enables installation of alternative debug functions. An example of
          such a function is a trigger, a function that waits for some
          special event and performs some action when the event is
          generated. For example, turning on low-level tracing.</p>
        <p><c><anno>Func</anno></c> is called whenever a system event is
          generated. This function is to return <c>done</c>, or a new
          <c>Func</c> state. In the first case, the function is removed. It is 
          also removed if the function fails. If one debug function should be 
          installed more times, a unique <c><anno>FuncId</anno></c> must be 
          specified for each installation.</p>
      </desc>
    </func>

    <func>
      <name name="log" arity="2" since=""/>
      <name name="log" arity="3" since=""/>
      <fsummary>Log system events in memory.</fsummary>
      <desc>
        <p>Turns the logging of system events on or off. If on, a
          maximum of <c><anno>N</anno></c> events are kept in the
          debug structure (default is 10).</p>
        <p>If <c><anno>Flag</anno></c> is <c>get</c>, a list of all logged
          events is returned.</p>
        <p>If <c><anno>Flag</anno></c> is <c>print</c>, the logged events
          are printed to <c>standard_io</c>.</p>
        <p>The events are formatted with a function that is defined by the
          process that generated the event (with a call to
          <seealso marker="#handle_debug/4">
          <c>handle_debug/4</c>)</seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="log_to_file" arity="2" since=""/>
      <name name="log_to_file" arity="3" since=""/>
      <fsummary>Log system events to the specified file.</fsummary>
      <desc>
        <p>Enables or disables the logging of all system events in text
          format to the file. The events are formatted with a function that is 
          defined by the process that generated the event (with a call to
          <seealso marker="#handle_debug/4"><c>handle_debug/4</c></seealso>).
	  The file is opened with encoding UTF-8.</p>
      </desc>
    </func>

    <func>
      <name name="no_debug" arity="1" since=""/>
      <name name="no_debug" arity="2" since=""/>
      <fsummary>Turn off debugging.</fsummary>
      <desc>
        <p>Turns off all debugging for the process. This includes
          functions that are installed explicitly with function
          <seealso marker="#install/2"><c>install/2,3</c></seealso>,
          for example, triggers.</p>
      </desc>
    </func>

    <func>
      <name name="remove" arity="2" since=""/>
      <name name="remove" arity="3" since=""/>
      <fsummary>Remove a debug function from the process.</fsummary>
      <desc>
        <p>Removes an installed debug function from the
          process. <c><anno>Func</anno></c> or <c><anno>FuncId</anno></c> must be 
          the same as previously installed.</p>
      </desc>
    </func>

    <func>
      <name name="replace_state" arity="2" since="OTP R16B01"/>
      <name name="replace_state" arity="3" since="OTP R16B01"/>
      <fsummary>Replace the state of the process.</fsummary>
      <desc>
        <p>Replaces the state of the process, and returns the new state.</p>
        <note>
          <p>These functions are intended only to help with debugging, and are
            not to be called from normal code. They are provided for
            convenience, allowing developers to avoid having to create their own
            custom state replacement functions.</p>
        </note>
        <p>Function <c><anno>StateFun</anno></c> provides a new state for the
          process. Argument <c><anno>State</anno></c> and the
          <c><anno>NewState</anno></c> return value of
          <c><anno>StateFun</anno></c> vary for different types of 
          processes as follows:</p>
        <list type="bulleted">
          <item>
            <p>For a <seealso marker="gen_server"><c>gen_server</c></seealso>
              process, <c><anno>State</anno></c> is the state of the callback
              module and <c><anno>NewState</anno></c> 
              is a new instance of that state.</p>
          </item>
          <item>
            <p>For a <seealso marker="gen_statem"><c>gen_statem</c></seealso>
	      process, <c><anno>State</anno></c> is the
	      tuple <c>{CurrentState,CurrentData}</c>,
	      and <c><anno>NewState</anno></c> is a
              similar tuple, which can contain
              a new current state, new state data, or both.</p>
          </item>
          <item>
            <p>For a <seealso marker="gen_event"><c>gen_event</c></seealso>
              process, <c><anno>State</anno></c> is the
              tuple <c>{Module, Id, HandlerState}</c> as follows:</p>
            <taglist>
              <tag><c>Module</c></tag>
              <item>
                <p>The module name of the event handler.</p>
              </item>
              <tag><c>Id</c></tag>
              <item>
                <p>The ID of the handler (which is <c>false</c> if it was
                  registered without an ID).</p>
              </item>
              <tag><c>HandlerState</c></tag>
              <item>
                <p>The state of the handler.</p>
              </item>
            </taglist>
            <p><c><anno>NewState</anno></c> is a similar tuple where
              <c>Module</c> and <c>Id</c> are to have the same values as in
              <c><anno>State</anno></c>, but the value of <c>HandlerState</c>
              can be different. Returning a <c><anno>NewState</anno></c>, whose
              <c>Module</c> or <c>Id</c> values differ from those of
              <c><anno>State</anno></c>, leaves the state of the event handler
              unchanged. For a <c>gen_event</c> process,
              <c><anno>StateFun</anno></c> is called once for each event handler
              registered in the <c>gen_event</c> process.</p>
          </item>
        </list>
        <p>If a <c><anno>StateFun</anno></c> function decides not to effect any
          change in process state, then regardless of process type, it can
          return its <c><anno>State</anno></c> argument.</p>
        <p>If a <c><anno>StateFun</anno></c> function crashes or throws an
          exception, the original state of the process is unchanged for
          <c>gen_server</c>, and <c>gen_statem</c> processes.
	  For <c>gen_event</c> processes, a crashing or
	  failing <c><anno>StateFun</anno></c> function
          means that only the state of the particular event handler it was
          working on when it failed or crashed is unchanged; it can still
          succeed in changing the states of other event
          handlers registered in the same <c>gen_event</c> process.</p>
        <p>If the callback module exports a 
          <seealso marker="#Module:system_replace_state/2">
          <c>system_replace_state/2</c></seealso> function, it is called in the
          target process to replace its state using <c>StateFun</c>. Its two
          arguments are <c>StateFun</c> and <c>Misc</c>, where 
          <c>Misc</c> is the same as the <c>Misc</c> value returned by
          <seealso marker="#get_status-1"><c>get_status/1,2</c></seealso>.
          A <c>system_replace_state/2</c> function is expected to return 
          <c>{ok, NewState, NewMisc}</c>, where <c>NewState</c> is the new state
          of the callback module, obtained by calling <c>StateFun</c>, and
          <c>NewMisc</c> is
          a possibly new value used to replace the original <c>Misc</c>
          (required as <c>Misc</c> often contains the state of the callback
          module within it).</p>
        <p>If the callback module does not export a
          <c>system_replace_state/2</c> function,
          <seealso marker="#replace_state/2"><c>replace_state/2,3</c></seealso>
          assumes that <c>Misc</c> is the state of the callback module,
	  passes it to <c>StateFun</c> and uses the return value as
	  both the new state and as the new value of <c>Misc</c>.</p>
        <p>If the callback module's function <c>system_replace_state/2</c>
          crashes or throws an exception, the caller exits with error
          <c>{callback_failed, {Module, system_replace_state}, {Class,
          Reason}}</c>, where <c>Module</c> is the name of the callback module
          and <c>Class</c> and <c>Reason</c> indicate details of the exception.
          If the callback module does not provide a
          <c>system_replace_state/2</c> function and <c>StateFun</c> crashes or
          throws an exception, the caller exits with error
          <c>{callback_failed, StateFun, {Class, Reason}}</c>.</p>
        <p>Function <c>system_replace_state/2</c> is primarily useful for
          user-defined behaviors and modules that implement OTP
          <seealso marker="#special_process">special processes</seealso>. The
          OTP behavior modules <c>gen_server</c>,
	  <c>gen_statem</c>, and <c>gen_event</c>
	  export this function, so callback modules for those
          behaviors need not to supply their own.</p>
      </desc>
    </func>

    <func>
      <name name="resume" arity="1" since=""/>
      <name name="resume" arity="2" since=""/>
      <fsummary>Resume a suspended process.</fsummary>
      <desc>
        <p>Resumes a suspended process.</p>
      </desc>
    </func>

    <func>
      <name name="statistics" arity="2" since=""/>
      <name name="statistics" arity="3" since=""/>
      <fsummary>Enable or disable the collections of statistics.</fsummary>
      <desc>
        <p>Enables or disables the collection of statistics. If 
          <c><anno>Flag</anno></c> is <c>get</c>, 
          the statistical collection is returned.</p>
      </desc>
    </func>

    <func>
      <name name="suspend" arity="1" since=""/>
      <name name="suspend" arity="2" since=""/>
      <fsummary>Suspend the process.</fsummary>
      <desc>
        <p>Suspends the process. When the process is suspended, it
          only responds to other system messages, but not other
          messages.</p>
      </desc>
    </func>

    <func>
      <name name="terminate" arity="2" since="OTP 18.0"/>
      <name name="terminate" arity="3" since="OTP 18.0"/>
      <fsummary>Terminate the process.</fsummary>
      <desc>
        <p>Orders the process to terminate with the
          specified <c><anno>Reason</anno></c>. The termination is done
          asynchronously, so it is not guaranteed that the process is
          terminated when the function returns.</p>
      </desc>
    </func>

    <func>
      <name name="trace" arity="2" since=""/>
      <name name="trace" arity="3" since=""/>
      <fsummary>Print all system events on <c>standard_io</c>.</fsummary>
      <desc>
        <p>Prints all system events on <c>standard_io</c>. The events are
          formatted with a function that is defined by the process that
          generated the event (with a call to
          <seealso marker="#handle_debug/4"><c>handle_debug/4</c></seealso>).
        </p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Process Implementation Functions</title>
    <marker id="special_process"/>
    <p>The following functions are used when implementing a
      special process. This is an ordinary process, which does not use a
      standard behavior, but a process that understands the standard system
     messages.</p>
  </section>

  <funcs>
    <func>
      <name name="debug_options" arity="1" since=""/>
      <fsummary>Convert a list of options to a debug structure.</fsummary>
      <desc>
        <p>Can be used by a process that initiates a debug
          structure from a list of options. The values of argument
          <c><anno>Opt</anno></c> are the same as for the corresponding
          functions.</p>
      </desc>
    </func>

    <func>
      <name name="get_debug" arity="3" since=""/>
      <fsummary>Get the data associated with a debug option.</fsummary>
      <desc>
	<warning>
	  <p>
	    <c>get_debug/3</c> is deprecated since it returns
	    data of an internal type only useful for debugging.
	  </p>
	</warning>
        <p>Gets the data associated with a debug option.
          <c><anno>Default</anno></c>
          is returned if <c><anno>Item</anno></c> is not found. Can be
          used by the process to retrieve debug data for printing before it
          terminates.</p>
      </desc>
    </func>

    <func>
      <name name="handle_debug" arity="4" since=""/>
      <fsummary>Generate a system event.</fsummary>
      <desc>
        <p>This function is called by a process when it generates a
          system event. <c><anno>FormFunc</anno></c> is a formatting
          function, called as <c><anno>FormFunc</anno>(Device,
          <anno>Event</anno>, <anno>Extra</anno>)</c> to print the events, 
          which is necessary if tracing is activated.
          <c><anno>Extra</anno></c> is any extra information that the
          process needs in the format function, for example, the process
          name.</p>
      </desc>
    </func>

    <func>
      <name name="handle_system_msg" arity="6" since=""/>
      <fsummary>Take care of system messages.</fsummary>
      <desc>
        <p>This function is used by a process module to take care of system
          messages. The process receives a 
          <c>{system, <anno>From</anno>, <anno>Msg</anno>}</c> message and
          passes <c><anno>Msg</anno></c> and <c><anno>From</anno></c> to this
          function.</p>
        <p>This function <em>never</em> returns. It calls either of the
          following functions:</p>
        <list type="bulleted">
          <item>
            <p><c><anno>Module</anno>:system_continue(<anno>Parent</anno>,
              NDebug, <anno>Misc</anno>)</c>,
	      where the process continues the execution.</p>
          </item>
          <item>
            <p><c><anno>Module</anno>:system_terminate(Reason,
              <anno>Parent</anno>, <anno>Debug</anno>, <anno>Misc</anno>)</c>,
              if the process is to terminate.</p>
          </item>
        </list>
        <p><c><anno>Module</anno></c> must export the following:</p>
        <list type="bulleted">
          <item><c>system_continue/3</c></item>
          <item><c>system_terminate/4</c></item>
          <item><c>system_code_change/4</c></item>
          <item><c>system_get_state/1</c></item>
          <item><c>system_replace_state/2</c></item>
        </list>
        <p>Argument <c><anno>Misc</anno></c> can be used to save internal data
          in a process, for example, its state. It is sent to
          <c><anno>Module</anno>:system_continue/3</c> or
          <c><anno>Module</anno>:system_terminate/4</c>.</p>
      </desc>
    </func>

    <func>
      <name name="print_log" arity="1" since=""/>
      <fsummary>Print the logged events in the debug structure.</fsummary>
      <desc>
        <p>Prints the logged system events in the debug structure,
          using <c>FormFunc</c> as defined when the event was
          generated by a call to
          <seealso marker="#handle_debug/4"><c>handle_debug/4</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="get_log" arity="1" since="OTP-22.0"/>
      <fsummary>Return the logged events in the debug structure.</fsummary>
      <desc>
        <p>
	  Returns the logged system events in the debug structure,
	  that is the last argument to
          <seealso marker="#handle_debug/4"><c>handle_debug/4</c></seealso>.
	</p>
      </desc>
    </func>

    <func>
      <name since="">Module:system_code_change(Misc, Module, OldVsn, Extra) ->
        {ok, NMisc}</name>
      <fsummary>Called when the process is to perform a code change.</fsummary>
      <type>
        <v>Misc = term()</v>
        <v>OldVsn = undefined | term()</v>
        <v>Module = atom()</v>
        <v>Extra = term()</v>
        <v>NMisc = term()</v>
      </type>
      <desc>
        <p>Called from <seealso marker="#handle_system_msg/6">
          <c>handle_system_msg/6</c></seealso> when the process is to perform a
          code change. The code change is used when the
          internal data structure has changed. This function
          converts argument <c>Misc</c> to the new data
          structure. <c>OldVsn</c> is attribute <em>vsn</em> of the
          old version of the <c>Module</c>. If no such attribute is
          defined, the atom <c>undefined</c> is sent.</p>
      </desc>
    </func>

    <func>
      <name since="">Module:system_continue(Parent, Debug, Misc) -> none()</name>
      <fsummary>Called when the process is to continue its execution.</fsummary>
      <type>
        <v>Parent = pid()</v>
        <v>Debug = [<seealso marker="#type-dbg_opt">dbg_opt()</seealso>]</v>
        <v>Misc = term()</v>
      </type>
      <desc>
        <p>Called from <seealso marker="#handle_system_msg/6">
          <c>handle_system_msg/6</c></seealso> when the process is to continue
          its execution (for example, after it has been
          suspended). This function never returns.</p>
      </desc>
    </func>

    <func>
      <name since="OTP 17.0">Module:system_get_state(Misc) -> {ok, State}</name>
      <fsummary>Called when the process is to return its current state.
      </fsummary>
      <type>
        <v>Misc = term()</v>
        <v>State = term()</v>
      </type>
      <desc>
        <p>Called from <seealso marker="#handle_system_msg/6">
          <c>handle_system_msg/6</c></seealso>
          when the process is to return a term that reflects its current state. 
          <c>State</c> is the value returned by 
          <seealso marker="#get_state/2"><c>get_state/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name since="OTP 17.0">Module:system_replace_state(StateFun, Misc) ->
        {ok, NState, NMisc}</name>
      <fsummary>Called when the process is to replace its current state.
      </fsummary>
      <type>
        <v>StateFun = fun((State :: term()) -> NState)</v>
        <v>Misc = term()</v>
        <v>NState = term()</v>
        <v>NMisc = term()</v>
      </type>
      <desc>
        <p>Called from <seealso marker="#handle_system_msg/6">
          <c>handle_system_msg/6</c></seealso> when the process is to replace
          its current state. <c>NState</c> is the value returned by
          <seealso marker="#replace_state/3"><c>replace_state/3</c></seealso>.
          </p>
      </desc>
    </func>

    <func>
      <name since="">Module:system_terminate(Reason, Parent, Debug, Misc) -> none()</name>
      <fsummary>Called when the process is to terminate.</fsummary>
      <type>
        <v>Reason = term()</v>
        <v>Parent = pid()</v>
        <v>Debug = [<seealso marker="#type-dbg_opt">dbg_opt()</seealso>]</v>
        <v>Misc = term()</v>
      </type>
      <desc>
        <p>Called from <seealso marker="#handle_system_msg/6">
          <c>handle_system_msg/6</c></seealso> when the process is to terminate.
          For example, this function is called when
          the process is suspended and its parent orders shutdown.
          It gives the process a chance to do a cleanup. This function never
          returns.</p>
      </desc>
    </func>
  </funcs>
</erlref>