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

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  <header>
    <copyright>
      <year>1996</year><year>2011</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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      Software distributed under the License is distributed on an "AS IS"
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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.sgml</file>
  </header>
  <module>timer</module>
  <modulesummary>Timer Functions</modulesummary>
  <description>
    <p>This module provides useful functions related to time. Unless otherwise
      stated, time is always measured in <c>milliseconds</c>.  All
      timer functions return immediately, regardless of work carried
      out by another process. 
      </p>
    <p>Successful evaluations of the timer functions yield return values
      containing a timer reference, denoted <c>TRef</c> below. By using 
      <c>cancel/1</c>, the returned reference can be used to cancel any
      requested action. A <c>TRef</c> is an Erlang term, the contents
      of which must not be altered.
      </p>
    <p>The timeouts are not exact, but should be <c>at least</c> as long
      as requested.
      </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="start" arity="0"/>
      <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 should be started explicitly.  Use
          configuration parameters for <c>kernel</c> for this.</p>
      </desc>
    </func>
    <func>
      <name name="apply_after" arity="4"/>
      <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> amount of time
          has elapsed. Returns <c>{ok, <anno>TRef</anno>}</c>, or <c>{error, <anno>Reason</anno>}</c>.</p>
      </desc>
    </func>
    <func>
      <name name="send_after" arity="2"/>
      <name name="send_after" arity="3"/>
      <fsummary>Send <c>Message</c>to <c>Pid</c>after a specified <c>Time</c>.</fsummary>
      <desc>
        <p>
        <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> amount
              of time has elapsed. (<c><anno>Pid</anno></c> can also be an atom of a
              registered name.) Returns <c>{ok, <anno>TRef</anno>}</c>, or
              <c>{error, <anno>Reason</anno>}</c>.</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>
        </p>
      </desc>
    </func>
    <func>
      <name name="kill_after" arity="1"/>
      <name name="kill_after" arity="2"/>
      <name name="exit_after" arity="2"/>
      <name name="exit_after" arity="3"/>
      <fsummary>Send an exit signal with <c>Reason</c>after a specified <c>Time</c>.</fsummary>
      <desc>
        <p>
        <taglist>
          <tag><c>exit_after/3</c></tag>
          <item>
            <p>Send 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>
          </item>
          <tag><c>exit_after/2</c></tag>
          <item>
            <p>Same as <c>exit_after(<anno>Time</anno>, self(), <anno>Reason1</anno>)</c>.     </p>
          </item>
          <tag><c>kill_after/2</c></tag>
          <item>
            <p>Same as <c>exit_after(<anno>Time</anno>, <anno>Pid</anno>, kill)</c>.     </p>
          </item>
          <tag><c>kill_after/1</c></tag>
          <item>
            <p>Same as <c>exit_after(<anno>Time</anno>, self(), kill)</c>.     </p>
          </item>
        </taglist>
        </p>
      </desc>
    </func>
    <func>
      <name name="apply_interval" arity="4"/>
      <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>. Returns <c>{ok, <anno>TRef</anno>}</c>, or
          <c>{error, <anno>Reason</anno>}</c>.</p>
      </desc>
    </func>
    <func>
      <name name="send_interval" arity="2"/>
      <name name="send_interval" arity="3"/>
      <fsummary>Send <c>Message</c>repeatedly at intervals of <c>Time</c>.</fsummary>
      <desc>
        <p>
        <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>
              amount of time has elapsed. (<c><anno>Pid</anno></c> can also be an atom of
              a registered name.) 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>
        </p>
      </desc>
    </func>
    <func>
      <name name="cancel" arity="1"/>
      <fsummary>Cancel a previously requested timeout identified by <c>TRef</c>.</fsummary>
      <desc>
        <p>Cancels a previously requested timeout. <c><anno>TRef</anno></c> is a unique
          timer reference returned by the timer function in question. 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="sleep" arity="1"/>
      <fsummary>Suspend the calling process for <c>Time</c>amount of milliseconds.</fsummary>
      <desc>
        <p>Suspends the process calling this function for <c><anno>Time</anno></c> amount
          of milliseconds and then returns <c>ok</c>, or suspend 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="tc" arity="1"/>
      <name name="tc" arity="2"/>
      <name name="tc" arity="3"/>
      <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>
        <p>
        <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 <c>os:timestamp/0</c>.
              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 works
            like <c>tc/3</c>.</p>
          </item>
          <tag><c>tc/1</c></tag>
          <item>
            <p>Evaluates <c><anno>Fun</anno>()</c>. Otherwise works like <c>tc/2</c>.</p>
          </item>

        </taglist>
        </p>
      </desc>
    </func>
    <func>
      <name name="now_diff" arity="2"/>
      <fsummary>Calculate time difference between <c>now/0</c>timestamps</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> probably
          are timestamp tuples returned from <c>erlang:now/0</c>.</p>
      </desc>
    </func>
    <func>
      <name name="seconds" arity="1"/>
      <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="minutes" arity="1"/>
      <fsummary>Converts <c>Minutes</c>to <c>Milliseconds</c>.</fsummary>
      <desc>
        <p>Return the number of milliseconds in <c><anno>Minutes</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="hours" arity="1"/>
      <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="hms" arity="3"/>
      <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>
  </funcs>

  <section>
    <title>Examples</title>
    <p>This example illustrates  how to print out "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>The following coding example illustrates a process which performs a 
      certain action and if this action is not completed within a certain 
      limit, then 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>WARNING</title>
    <p>A timer can always be removed by calling <c>cancel/1</c>.
      </p>
    <p>An interval timer, i.e. a timer created by evaluating any of the 
      functions <c>apply_interval/4</c>, <c>send_interval/3</c>, and 
      <c>send_interval/2</c>, is linked to the process towards which
      the timer performs its task.
      </p>
    <p>A one-shot timer, i.e. a timer created by evaluating any of the
      functions <c>apply_after/4</c>, <c>send_after/3</c>, 
      <c>send_after/2</c>, <c>exit_after/3</c>, <c>exit_after/2</c>, 
      <c>kill_after/2</c>, and <c>kill_after/1</c> is not linked to any
      process. Hence, such a timer is removed only when it reaches its 
      timeout, or if it is explicitly removed by a call to <c>cancel/1</c>.</p>
  </section>
</erlref>