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  <header>
    <copyright>
      <year>1996</year><year>2018</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.
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    </legalnotice>

    <title>timer</title>
    <prepared>Sebastian Strollo</prepared>
    <responsible>Bjarne D&auml;cker</responsible>
    <docno>1</docno>
    <approved>Bjarne D&auml;cker</approved>
    <checked></checked>
    <date>1998-09-09</date>
    <rev>D</rev>
    <file>timer.xml</file>
  </header>
  <module since="">timer</module>
  <modulesummary>Timer functions.</modulesummary>
  <description>
    <p>This module provides useful functions related to time. Unless otherwise
      stated, time is always measured in <em>milliseconds</em>. All
      timer functions return immediately, regardless of work done by another
      process.</p>
    <p>Successful evaluations of the timer functions give return values
      containing a timer reference, denoted <c>TRef</c>. By using
      <seemfa marker="#cancel/1"><c>cancel/1</c></seemfa>,
      the returned reference can be used to cancel any
      requested action. A <c>TRef</c> is an Erlang term, which contents
      must not be changed.</p>
    <p>The time-outs are not exact, but are <em>at least</em> as long
      as requested.</p>
    <p>Creating timers using
      <seemfa marker="erts:erlang#send_after/3">erlang:send_after/3</seemfa> and
      <seemfa marker="erts:erlang#start_timer/3">erlang:start_timer/3</seemfa>
      is much more efficient than using the timers provided by this module. See
      <seeguide marker="system/efficiency_guide:commoncaveats#timer-module">the
      Timer Module section in the Efficiency Guide</seeguide>.</p>
  </description>

  <datatypes>
    <datatype>
      <name name="time"/>
      <desc><p>Time in milliseconds.</p></desc>
    </datatype>
    <datatype>
      <name name="tref"/>
      <desc><p>A timer reference.</p></desc>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="apply_after" arity="4" since=""/>
      <fsummary>Apply <c>Module:Function(Arguments)</c> after a specified
        <c>Time</c>.</fsummary>
      <desc>
        <p>Evaluates <c>apply(<anno>Module</anno>, <anno>Function</anno>,
          <anno>Arguments</anno>)</c> after <c><anno>Time</anno></c>
          milliseconds.</p>
        <p>Returns <c>{ok, <anno>TRef</anno>}</c> or
          <c>{error, <anno>Reason</anno>}</c>.</p>
      </desc>
    </func>

    <func>
      <name name="apply_interval" arity="4" since=""/>
      <fsummary>Evaluate <c>Module:Function(Arguments)</c> repeatedly at
        intervals of <c>Time</c>.</fsummary>
      <desc>
        <p>Evaluates <c>apply(<anno>Module</anno>, <anno>Function</anno>,
          <anno>Arguments</anno>)</c> repeatedly at intervals of
          <c><anno>Time</anno></c>.</p>
        <p>Returns <c>{ok, <anno>TRef</anno>}</c> or
          <c>{error, <anno>Reason</anno>}</c>.</p>
      </desc>
    </func>

    <func>
      <name name="cancel" arity="1" since=""/>
      <fsummary>Cancel a previously requested time-out identified by
        <c>TRef</c>.</fsummary>
      <desc>
        <p>Cancels a previously requested time-out. <c><anno>TRef</anno></c> is
          a unique
          timer reference returned by the related timer function.</p>
        <p>Returns <c>{ok, cancel}</c>, or <c>{error, <anno>Reason</anno>}</c>
          when <c><anno>TRef</anno></c> is not a timer reference.</p>
      </desc>
    </func>

    <func>
      <name name="exit_after" arity="2" since=""/>
      <name name="exit_after" arity="3" since=""/>
      <fsummary>Send an exit signal with <c>Reason</c> after a specified
        <c>Time</c>.</fsummary>
      <desc>
        <p><c>exit_after/2</c> is the same as
          <c>exit_after(<anno>Time</anno>, self(),
          <anno>Reason1</anno>)</c>.</p>
        <p><c>exit_after/3</c> sends an exit signal with reason
          <c><anno>Reason1</anno></c> to
          pid <c><anno>Pid</anno></c>. Returns <c>{ok, <anno>TRef</anno>}</c>
          or <c>{error, <anno>Reason2</anno>}</c>.</p>
      </desc>
    </func>

    <func>
      <name name="hms" arity="3" since=""/>
      <fsummary>Convert <c>Hours</c>+<c>Minutes</c>+<c>Seconds</c> to
        <c>Milliseconds</c>.</fsummary>
      <desc>
        <p>Returns the number of milliseconds in <c><anno>Hours</anno> +
          <anno>Minutes</anno> + <anno>Seconds</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="hours" arity="1" since=""/>
      <fsummary>Convert <c>Hours</c> to <c>Milliseconds</c>.</fsummary>
      <desc>
        <p>Returns the number of milliseconds in <c><anno>Hours</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="kill_after" arity="1" since=""/>
      <name name="kill_after" arity="2" since=""/>
      <fsummary>Send an exit signal with <c>Reason</c> after a specified
        <c>Time</c>.</fsummary>
      <desc>
        <p><c>kill_after/1</c> is the same as
          <c>exit_after(<anno>Time</anno>, self(), kill)</c>.</p>
        <p><c>kill_after/2</c> is the same as
          <c>exit_after(<anno>Time</anno>, <anno>Pid</anno>, kill)</c>.</p>
      </desc>
    </func>

    <func>
      <name name="minutes" arity="1" since=""/>
      <fsummary>Converts <c>Minutes</c> to <c>Milliseconds</c>.</fsummary>
      <desc>
        <p>Returns the number of milliseconds in
          <c><anno>Minutes</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="now_diff" arity="2" since=""/>
      <fsummary>Calculate time difference between time stamps.</fsummary>
      <type_desc variable="Tdiff">In microseconds</type_desc>
      <desc>
        <p>Calculates the time difference <c><anno>Tdiff</anno> =
          <anno>T2</anno> - <anno>T1</anno></c> in <em>microseconds</em>,
          where <c><anno>T1</anno></c> and <c><anno>T2</anno></c>
          are time-stamp tuples on the same format as returned from
          <seemfa marker="erts:erlang#timestamp/0">
          <c>erlang:timestamp/0</c></seemfa> or
          <seemfa marker="kernel:os#timestamp/0">
          <c>os:timestamp/0</c></seemfa>.</p>
      </desc>
    </func>

    <func>
      <name name="seconds" arity="1" since=""/>
      <fsummary>Convert <c>Seconds</c> to <c>Milliseconds</c>.</fsummary>
      <desc>
        <p>Returns the number of milliseconds in
          <c><anno>Seconds</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="send_after" arity="2" since=""/>
      <name name="send_after" arity="3" since=""/>
      <fsummary>Send <c>Message</c> to <c>Pid</c> after a specified
        <c>Time</c>.</fsummary>
      <desc>
        <taglist>
          <tag><c>send_after/3</c></tag>
          <item>
            <p>Evaluates <c><anno>Pid</anno> ! <anno>Message</anno></c> after
              <c><anno>Time</anno></c> milliseconds. (<c><anno>Pid</anno></c>
              can also be an atom of a registered name.)</p>
            <p>Returns <c>{ok, <anno>TRef</anno>}</c> or
              <c>{error, <anno>Reason</anno>}</c>.</p>
            <p>See also
              <seeguide marker="system/efficiency_guide:commoncaveats#timer-module">
              the Timer Module section in the Efficiency Guide</seeguide>.</p>
          </item>
          <tag><c>send_after/2</c></tag>
          <item>
            <p>Same as <c>send_after(<anno>Time</anno>, self(),
              <anno>Message</anno>)</c>.</p>
          </item>
        </taglist>
      </desc>
    </func>

    <func>
      <name name="send_interval" arity="2" since=""/>
      <name name="send_interval" arity="3" since=""/>
      <fsummary>Send <c>Message</c> repeatedly at intervals of <c>Time</c>.
      </fsummary>
      <desc>
        <taglist>
          <tag><c>send_interval/3</c></tag>
          <item>
            <p>Evaluates <c><anno>Pid</anno> ! <anno>Message</anno></c>
              repeatedly after <c><anno>Time</anno></c> milliseconds.
              (<c><anno>Pid</anno></c> can also be
              an atom of a registered name.)</p>
            <p>Returns <c>{ok, <anno>TRef</anno>}</c> or
              <c>{error, <anno>Reason</anno>}</c>.</p>
          </item>
          <tag><c>send_interval/2</c></tag>
          <item>
            <p>Same as <c>send_interval(<anno>Time</anno>, self(),
              <anno>Message</anno>)</c>.</p>
          </item>
        </taglist>
      </desc>
    </func>

    <func>
      <name name="sleep" arity="1" since=""/>
      <fsummary>Suspend the calling process for <c>Time</c> milliseconds.
      </fsummary>
      <desc>
        <p>Suspends the process calling this function for
          <c><anno>Time</anno></c> milliseconds and then returns <c>ok</c>,
          or suspends the process forever if <c><anno>Time</anno></c> is the
          atom <c>infinity</c>. Naturally, this
          function does <em>not</em> return immediately.</p>
      </desc>
    </func>

    <func>
      <name name="start" arity="0" since=""/>
      <fsummary>Start a global timer server (named <c>timer_server</c>).
      </fsummary>
      <desc>
        <p>Starts the timer server. Normally, the server does not need
          to be started explicitly. It is started dynamically if it
          is needed. This is useful during development, but in a
          target system the server is to be started explicitly. Use
          configuration parameters for
          <seeapp marker="kernel:index">Kernel</seeapp> for this.</p>
      </desc>
    </func>

    <func>
      <name name="tc" arity="1" since="OTP R14B03"/>
      <name name="tc" arity="2" since="OTP R14B"/>
      <name name="tc" arity="3" since=""/>
      <fsummary>Measure the real time it takes to evaluate <c>apply(Module,
        Function, Arguments)</c> or <c>apply(Fun, Arguments)</c>.</fsummary>
      <type_desc variable="Time">In microseconds</type_desc>
      <desc>
        <taglist>
          <tag><c>tc/3</c></tag>
          <item>
            <p>Evaluates <c>apply(<anno>Module</anno>, <anno>Function</anno>,
              <anno>Arguments</anno>)</c> and measures the elapsed real time as 
              reported by <seemfa marker="erts:erlang#monotonic_time/0">
              <c>erlang:monotonic_time/0</c></seemfa>.</p>
            <p>Returns <c>{<anno>Time</anno>, <anno>Value</anno>}</c>, where
              <c><anno>Time</anno></c> is the elapsed real time in
              <em>microseconds</em>, and <c><anno>Value</anno></c> is what is
              returned from the apply.</p>
          </item>
          <tag><c>tc/2</c></tag>
          <item>
            <p>Evaluates <c>apply(<anno>Fun</anno>, <anno>Arguments</anno>)</c>.
              Otherwise the same as <c>tc/3</c>.</p>
          </item>
          <tag><c>tc/1</c></tag>
          <item>
            <p>Evaluates <c><anno>Fun</anno>()</c>. Otherwise the same as
              <c>tc/2</c>.</p>
          </item>
        </taglist>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Examples</title>
    <p><em>Example 1</em></p>
    <p>The following example shows how to print "Hello World!" in 5 seconds:</p>
    <pre>
1> <input>timer:apply_after(5000, io, format, ["~nHello World!~n", []]).</input>
{ok,TRef}
Hello World!</pre>

    <p><em>Example 2</em></p>
    <p>The following example shows a process performing a 
      certain action, and if this action is not completed within a certain 
      limit, the process is killed:</p>
    <code type="none">
Pid = spawn(mod, fun, [foo, bar]),
%% If pid is not finished in 10 seconds, kill him
{ok, R} = timer:kill_after(timer:seconds(10), Pid),
...
%% We change our mind...
timer:cancel(R),
...</code>
  </section>

  <section>
    <title>Notes</title>
    <p>A timer can always be removed by calling
      <seemfa marker="#cancel/1"><c>cancel/1</c></seemfa>.</p>

    <p>An interval timer, that is, a timer created by evaluating any of the
      functions
      <seemfa marker="#apply_interval/4"><c>apply_interval/4</c></seemfa>,
      <seemfa marker="#send_interval/3"><c>send_interval/3</c></seemfa>, and
      <seemfa marker="#send_interval/2"><c>send_interval/2</c></seemfa>
      is linked to the process to which the timer performs its task.</p>

    <p>A one-shot timer, that is, a timer created by evaluating any of the
      functions
      <seemfa marker="#apply_after/4"><c>apply_after/4</c></seemfa>,
      <seemfa marker="#send_after/3"><c>send_after/3</c></seemfa>,
      <seemfa marker="#send_after/2"><c>send_after/2</c></seemfa>,
      <seemfa marker="#exit_after/3"><c>exit_after/3</c></seemfa>,
      <seemfa marker="#exit_after/2"><c>exit_after/2</c></seemfa>,
      <seemfa marker="#kill_after/2"><c>kill_after/2</c></seemfa>, and
      <seemfa marker="#kill_after/1"><c>kill_after/1</c></seemfa>
      is not linked to any process. Hence, such a timer is removed only
      when it reaches its time-out, or if it is explicitly removed by a call to
      <seemfa marker="#cancel/1"><c>cancel/1</c></seemfa>.</p>
  </section>
</erlref>

