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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
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.
    
      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.
    
    </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>

