path: root/lib/kernel/doc/src/error_logger.xml

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<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2016</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>error_logger</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module>error_logger</module>
  <modulesummary>Erlang Error Logger</modulesummary>
  <description>
    <p>The Erlang <em>error logger</em> is an event manager (see
      <seealso marker="doc/design_principles:des_princ">OTP Design Principles</seealso> and
      <seealso marker="stdlib:gen_event">gen_event(3)</seealso>),
      registered as <c>error_logger</c>. Error, warning and info events
      are sent to the error logger from the Erlang runtime system and
      the different Erlang/OTP applications. The events are, by default,
      logged to tty. Note that an event from a process <c>P</c> is
      logged at the node of the group leader of <c>P</c>. This means
      that log output is directed to the node from which a process was
      created, which not necessarily is the same node as where it is
      executing.</p>
    <p>Initially, <c>error_logger</c> only has a primitive event
      handler, which buffers and prints the raw event messages. During
      system startup, the application Kernel replaces this with a
      <em>standard event handler</em>, by default one which writes
      nicely formatted output to tty. Kernel can also be configured so
      that events are logged to file instead, or not logged at all, see
      <seealso marker="kernel_app">kernel(6)</seealso>.</p>
    <p>Also the SASL application, if started, adds its own event
      handler, which by default writes supervisor, crash and progress
      reports to tty. See
      <seealso marker="sasl:sasl_app">sasl(6)</seealso>.</p>
    <p>It is recommended that user defined applications should report
      errors through the error logger, in order to get uniform reports.
      User defined event handlers can be added to handle application
      specific events. (<c>add_report_handler/1,2</c>). Also, there is
      a useful event handler in STDLIB for multi-file logging of events,
      see <c>log_mf_h(3)</c>.</p>
    <p>Warning events were introduced in Erlang/OTP R9C and are enabled
      by default as of 18.0. To retain backwards compatibility with existing
      user defined event handlers, these may be tagged as errors or info
      using the command line flag <c><![CDATA[+W <e | i | w>]]></c>, thus
      showing up as error or info reports in the logs.</p>
  </description>
  <datatypes>
    <datatype>
      <name name="report"/>
    </datatype>
  </datatypes>
  <funcs>
    <func>
      <name name="error_msg" arity="1"/>
      <name name="error_msg" arity="2"/>
      <name name="format" arity="2"/>
      <fsummary>Send an standard error event to the error logger</fsummary>
      <desc>
        <p>Sends a standard error event to the error logger.
          The <c><anno>Format</anno></c> and <c><anno>Data</anno></c> arguments are the same as
          the arguments of <c>io:format/2</c>. The event is handled by
          the standard event handler.</p>
        <pre>
1> <input>error_logger:error_msg("An error occurred in ~p~n", [a_module]).</input>

=ERROR REPORT==== 11-Aug-2005::14:03:19 ===
An error occurred in a_module
ok</pre>
        <warning>
          <p>If called with bad arguments, this function can crash
            the standard event handler, meaning no further events are
            logged. When in doubt, use <c>error_report/1</c> instead.</p>
        </warning>
      </desc>
    </func>
    <func>
      <name name="error_report" arity="1"/>
      <fsummary>Send a standard error report event to the error logger</fsummary>
      <desc>
        <p>Sends a standard error report event to the error logger.
          The event is handled by the standard event handler.</p>
        <pre>
2> <input>error_logger:error_report([{tag1,data1},a_term,{tag2,data}]).</input>

=ERROR REPORT==== 11-Aug-2005::13:45:41 ===
    tag1: data1
    a_term
    tag2: data
ok
3> <input>error_logger:error_report("Serious error in my module").</input>

=ERROR REPORT==== 11-Aug-2005::13:45:49 ===
Serious error in my module
ok</pre>
      </desc>
    </func>
    <func>
      <name name="error_report" arity="2"/>
      <fsummary>Send a user defined error report event to the error logger</fsummary>
      <desc>
        <p>Sends a user defined error report event to the error logger.
          An event handler to handle the event is supposed to have been
          added. The event is ignored by the standard event handler.</p>
        <p>It is recommended that <c><anno>Report</anno></c> follows the same
          structure as for <c>error_report/1</c>.</p>
      </desc>
    </func>
    <func>
      <name name="warning_map" arity="0"/>
      <fsummary>Return the current mapping for warning events</fsummary>
      <desc>
        <p>Returns the current mapping for warning events. Events sent
          using <c>warning_msg/1,2</c> or <c>warning_report/1,2</c>
          are tagged as errors, warnings (default) or info, depending
          on the value of the command line flag <c>+W</c>.</p>
        <pre>
os$ <input>erl</input>
Erlang (BEAM) emulator version 5.4.8 [hipe] [threads:0] [kernel-poll]

Eshell V5.4.8  (abort with ^G)
1> <input>error_logger:warning_map().</input>
warning
2> <input>error_logger:warning_msg("Warnings tagged as: ~p~n", [warning]).</input>

=WARNING REPORT==== 11-Aug-2005::15:31:55 ===
Warnings tagged as: warning
ok
3> 
User switch command
 --> q
os$ <input>erl +W e</input>
Erlang (BEAM) emulator version 5.4.8 [hipe] [threads:0] [kernel-poll]

Eshell V5.4.8  (abort with ^G)
1> <input>error_logger:warning_map().</input>
error
2> <input>error_logger:warning_msg("Warnings tagged as: ~p~n", [error]).</input>

=ERROR REPORT==== 11-Aug-2005::15:31:23 ===
Warnings tagged as: error
ok</pre>
      </desc>
    </func>
    <func>
      <name name="warning_msg" arity="1"/>
      <name name="warning_msg" arity="2"/>
      <fsummary>Send a standard warning event to the error logger</fsummary>
      <desc>
        <p>Sends a standard warning event to the error logger.
          The <c><anno>Format</anno></c> and <c><anno>Data</anno></c> arguments are the same as
          the arguments of <c>io:format/2</c>. The event is handled by
          the standard event handler. It is tagged either as an error,
          warning or info, see
          <seealso marker="#warning_map/0">warning_map/0</seealso>.</p>
        <warning>
          <p>If called with bad arguments, this function can crash
            the standard event handler, meaning no further events are
            logged. When in doubt, use <c>warning_report/1</c> instead.</p>
        </warning>
      </desc>
    </func>
    <func>
      <name name="warning_report" arity="1"/>
      <fsummary>Send a standard warning report event to the error logger</fsummary>
      <desc>
        <p>Sends a standard warning report event to the error logger.
          The event is handled by the standard event handler. It is
          tagged either as an error, warning or info, see
          <seealso marker="#warning_map/0">warning_map/0</seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="warning_report" arity="2"/>
      <fsummary>Send a user defined warning report event to the error logger</fsummary>
      <desc>
        <p>Sends a user defined warning report event to the error
          logger. An event handler to handle the event is supposed to
          have been added. The event is ignored by the standard event
          handler. It is tagged either as an error, warning or info,
          depending on the value of
          <seealso marker="#warning_map/0">warning_map/0</seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="info_msg" arity="1"/>
      <name name="info_msg" arity="2"/>
      <fsummary>Send a standard information event to the error logger</fsummary>
      <desc>
        <p>Sends a standard information event to the error logger.
          The <c><anno>Format</anno></c> and <c><anno>Data</anno></c> arguments are the same as
          the arguments of <c>io:format/2</c>. The event is handled by
          the standard event handler.</p>
        <pre>
1> <input>error_logger:info_msg("Something happened in ~p~n", [a_module]).</input>

=INFO REPORT==== 11-Aug-2005::14:06:15 ===
Something happened in a_module
ok</pre>
        <warning>
          <p>If called with bad arguments, this function can crash
            the standard event handler, meaning no further events are
            logged. When in doubt, use <c>info_report/1</c> instead.</p>
        </warning>
      </desc>
    </func>
    <func>
      <name name="info_report" arity="1"/>
      <fsummary>Send a standard information report event to the error logger</fsummary>
      <desc>
        <p>Sends a standard information report event to the error
          logger. The event is handled by the standard event handler.</p>
        <pre>
2> <input>error_logger:info_report([{tag1,data1},a_term,{tag2,data}]).</input>

=INFO REPORT==== 11-Aug-2005::13:55:09 ===
    tag1: data1
    a_term
    tag2: data
ok
3> <input>error_logger:info_report("Something strange happened").</input>

=INFO REPORT==== 11-Aug-2005::13:55:36 ===
Something strange happened
ok</pre>
      </desc>
    </func>
    <func>
      <name name="info_report" arity="2"/>
      <fsummary>Send a user defined information report event to the error logger</fsummary>
      <desc>
        <p>Sends a user defined information report event to the error
          logger. An event handler to handle the event is supposed to
          have been added. The event is ignored by the standard event
          handler.</p>
        <p>It is recommended that <c><anno>Report</anno></c> follows the same
          structure as for <c>info_report/1</c>.</p>
      </desc>
    </func>
    <func>
      <name name="add_report_handler" arity="1"/>
      <name name="add_report_handler" arity="2"/>
      <fsummary>Add an event handler to the error logger</fsummary>
      <desc>
        <p>Adds a new event handler to the error logger. The event
          handler must be implemented as a <c>gen_event</c> callback
          module, see
          <seealso marker="stdlib:gen_event">gen_event(3)</seealso>.</p>
        <p><c><anno>Handler</anno></c> is typically the name of the callback module
          and <c><anno>Args</anno></c> is an optional term (defaults to []) passed
          to the initialization callback function <c><anno>Handler</anno>:init/1</c>.
          The function returns <c>ok</c> if successful.</p>
        <p>The event handler must be able to handle the
          <seealso marker="#events">events</seealso> described below.</p>
      </desc>
    </func>
    <func>
      <name name="delete_report_handler" arity="1"/>
      <fsummary>Delete an event handler from the error logger</fsummary>
      <desc>
        <p>Deletes an event handler from the error logger by calling
          <c>gen_event:delete_handler(error_logger, <anno>Handler</anno>, [])</c>,
          see <seealso marker="stdlib:gen_event">gen_event(3)</seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="tty" arity="1"/>
      <fsummary>Enable or disable printouts to the tty</fsummary>
      <desc>
        <p>Enables (<c><anno>Flag</anno> == true</c>) or disables
          (<c><anno>Flag</anno> == false</c>) printout of standard events to the tty.</p>
        <p>This is done by adding or deleting the standard event handler
          for output to tty, thus calling this function overrides
          the value of the Kernel <c>error_logger</c> configuration
          parameter.</p>
      </desc>
    </func>
    <func>
      <name name="logfile" arity="1" clause_i="1"/>
      <name name="logfile" arity="1" clause_i="2"/>
      <name name="logfile" arity="1" clause_i="3"/>
      <fsummary>Enable or disable error printouts to a file</fsummary>
      <type variable="Filename"/>
      <type variable="OpenReason" name_i="1"/>
      <type variable="CloseReason" name_i="2"/>
      <type variable="FilenameReason" name_i="3"/>
      <type name="open_error"/>
      <desc>
        <p>Enables or disables printout of standard events to a file.</p>
        <p>This is done by adding or deleting the standard event handler
          for output to file, thus calling this function overrides
          the value of the Kernel <c>error_logger</c> configuration
          parameter.</p>
        <p>Enabling file logging can be used in combination with calling
          <c>tty(false)</c>, in order to have a silent system, where
          all standard events are logged to a file only.
          There can only be one active log file at a time.</p>
        <p><c>Request</c> is one of:</p>
        <taglist>
          <tag><c>{open, <anno>Filename</anno>}</c></tag>
          <item>
            <p>Opens the log file <c><anno>Filename</anno></c>. Returns <c>ok</c> if
              successful, or <c>{error, allready_have_logfile}</c> if
              logging to file is already enabled, or an error tuple if
              another error occurred. For example, if <c><anno>Filename</anno></c>
              could not be opened.</p>
          </item>
          <tag><c>close</c></tag>
          <item>
            <p>Closes the current log file. Returns <c>ok</c>, or
              <c>{error, module_not_found}</c>.</p>
          </item>
          <tag><c>filename</c></tag>
          <item>
            <p>Returns the name of the log file <c><anno>Filename</anno></c>, or
              <c>{error, no_log_file}</c> if logging to file is not
              enabled.</p>
          </item>
        </taglist>
      </desc>
    </func>
  </funcs>

  <section>
    <marker id="events"></marker>
    <title>Events</title>
    <p>All event handlers added to the error logger must handle
      the following events. <c>Gleader</c> is the group leader pid of
      the process which sent the event, and <c>Pid</c> is the process
      which sent the event.</p>
    <taglist>
      <tag><c>{error, Gleader, {Pid, Format, Data}}</c></tag>
      <item>
        <p>Generated when <c>error_msg/1,2</c> or <c>format</c> is
          called.</p>
      </item>
      <tag><c>{error_report, Gleader, {Pid, std_error, Report}}</c></tag>
      <item>
        <p>Generated when <c>error_report/1</c> is called.</p>
      </item>
      <tag><c>{error_report, Gleader, {Pid, Type, Report}}</c></tag>
      <item>
        <p>Generated when <c>error_report/2</c> is called.</p>
      </item>
      <tag><c>{warning_msg, Gleader, {Pid, Format, Data}}</c></tag>
      <item>
        <p>Generated when <c>warning_msg/1,2</c> is called, provided
          that warnings are set to be tagged as warnings.</p>
      </item>
      <tag><c>{warning_report, Gleader, {Pid, std_warning, Report}}</c></tag>
      <item>
        <p>Generated when <c>warning_report/1</c> is called, provided
          that warnings are set to be tagged as warnings.</p>
      </item>
      <tag><c>{warning_report, Gleader, {Pid, Type, Report}}</c></tag>
      <item>
        <p>Generated when <c>warning_report/2</c> is called, provided
          that warnings are set to be tagged as warnings.</p>
      </item>
      <tag><c>{info_msg, Gleader, {Pid, Format, Data}}</c></tag>
      <item>
        <p>Generated when <c>info_msg/1,2</c> is called.</p>
      </item>
      <tag><c>{info_report, Gleader, {Pid, std_info, Report}}</c></tag>
      <item>
        <p>Generated when <c>info_report/1</c> is called.</p>
      </item>
      <tag><c>{info_report, Gleader, {Pid, Type, Report}}</c></tag>
      <item>
        <p>Generated when <c>info_report/2</c> is called.</p>
      </item>
    </taglist>
    <p>Note that also a number of system internal events may be
      received, a catch-all clause last in the definition of
      the event handler callback function <c>Module:handle_event/2</c>
      is necessary. This also holds true for
      <c>Module:handle_info/2</c>, as there are a number of system
      internal messages the event handler must take care of as well.</p>
  </section>

  <section>
    <title>SEE ALSO</title>
    <p>gen_event(3), log_mf_h(3), kernel(6), sasl(6)</p>
  </section>
</erlref>