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

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<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2017</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>

    <note>
      <p>In Erlang/OTP 21.0, a new API for logging was added. The
	old <c>error_logger</c> module can still be used by legacy
	code, but log events are redirected to the new Logger API. New
	code should use the Logger API directly.</p>
      <p><c>error_logger</c> is no longer started by default, but is
	automatically started when an event handler is added
	with <c>error_logger:add_report_handler/1,2</c>. The <c>error_logger</c>
	module is then also added as a handler to the new logger.</p>
      <p>See  <seealso marker="logger"><c>logger(3)</c></seealso> and
	the <seealso marker="logger_chapter">Logging</seealso> chapter
	in the User's Guide for more information.</p>
    </note>

    <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"><c>gen_event(3)</c></seealso>),
      registered as <c>error_logger</c>.</p>
    <p>Error logger is no longer started by default, but is
      automatically started when an event handler is added
      with <seealso marker="#add_report_handler/1">
	<c>add_report_handler/1,2</c></seealso>. The <c>error_logger</c>
      module is then also added as a handler to the new logger,
      causing log events to be forwarded from logger to error logger,
      and consequently to all installed error logger event
      handlers.</p>
    <p>User-defined event handlers can be added to handle application-specific
      events.</p>
    <p>Existing event handlers provided by STDLIB and SASL are still
      available, but are no longer used by OTP.</p>
    <p>Warning events were introduced in Erlang/OTP R9C and are enabled
      by default as from Erlang/OTP 18.0. To retain backwards compatibility
      with existing user-defined event handlers, the warning events can be
      tagged as <c>errors</c> or <c>info</c> using command-line flag
      <c><![CDATA[+W <e | i | w>]]></c>, thus showing up as
      <c>ERROR REPORT</c> or <c>INFO REPORT</c> in the logs.</p>
  </description>
  <datatypes>
    <datatype>
      <name name="report"/>
    </datatype>
  </datatypes>
  <funcs>
    <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"><c>gen_event(3)</c></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 events in this module, see
          section <seealso marker="#events">Events</seealso>.</p>
	<p>The first time this function is called,
	  <c>error_logger</c> is added as a Logger handler, and
	  the <c>error_logger</c> process is started.</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"><c>gen_event(3)</c></seealso>.</p>
	<p>If no more event handlers exist after the deletion,
	  <c>error_logger</c> is removed as a Logger handler, and
	  the <c>error_logger</c> process is stopped.</p>
      </desc>
    </func>
    <func>
      <name name="error_msg" arity="1"/>
      <name name="error_msg" arity="2"/>
      <name name="format" arity="2"/>
      <fsummary>Log a standard error event.</fsummary>
      <desc>
        <p>Log a standard error event. The <c><anno>Format</anno></c>
          and <c><anno>Data</anno></c> arguments are the same as the
          arguments of
	  <seealso marker="stdlib:io#format/2"><c>io:format/2</c></seealso>
	  in STDLIB.</p>
        <p>Error logger forwards the event to Logger, including
          metadata that allows backwards compatibility with legacy
          error logger event handlers.</p>
	<p>The event is handled by the default Logger handler.</p>
	<p>These functions are kept for backwards compatibility and
	  must not be used by new code. Use the <seealso marker="logger#macros">
	    <c>?LOG_ERROR</c></seealso> macro or
	  <seealso marker="logger#error-1"><c>logger:error/1,2,3</c></seealso>
	  instead.</p>
	<p><em>Example:</em></p>
        <pre>
1> <input>error_logger:error_msg("An error occurred in ~p", [a_module]).</input>
=ERROR REPORT==== 22-May-2018::11:18:43.376917 ===
An error occurred in a_module
ok</pre>
        <warning>
          <p>If the Unicode translation modifier (<c>t</c>) is used in
            the format string, all event handlers must ensure that the
            formatted output is correctly encoded for the I/O
            device.</p>
        </warning>
      </desc>
    </func>
    <func>
      <name name="error_report" arity="1"/>
      <fsummary>Log a standard error event.</fsummary>
      <desc>
        <p>Log a standard error event. Error logger forwards the event
          to Logger, including metadata that allows backwards
          compatibility with legacy error logger event handlers.</p>
	<p>The event is handled by the default Logger handler.</p>
	<p>This functions is kept for backwards compatibility and
	  must not be used by new code. Use the <seealso marker="logger#macros">
	    <c>?LOG_ERROR</c></seealso> macro or
	  <seealso marker="logger#error-1"><c>logger:error/1,2,3</c></seealso>
	  instead.</p>
	<p><em>Example:</em></p>
        <pre>
2> <input>error_logger:error_report([{tag1,data1},a_term,{tag2,data}]).</input>
=ERROR REPORT==== 22-May-2018::11:24:23.699306 ===
    tag1: data1
    a_term
    tag2: data
ok
3> <input>error_logger:error_report("Serious error in my module").</input>
=ERROR REPORT==== 22-May-2018::11:24:45.972445 ===
Serious error in my module
ok</pre>
      </desc>
    </func>
    <func>
      <name name="error_report" arity="2"/>
      <fsummary>Log a user-defined error event.</fsummary>
      <desc>
        <p>Log a user-defined error event. Error logger forwards the
          event to Logger, including metadata that allows backwards
          compatibility with legacy error logger event handlers.</p>
	<p>Error logger also adds a <c>domain</c> field with
          value <c>[<anno>Type</anno>]</c> to this event's metadata,
          causing the filters of the default Logger handler to discard
          the event. A different Logger handler, or an error logger
          event handler, must be added to handle this event.</p>
        <p>It is recommended that <c><anno>Report</anno></c> follows the same
          structure as for
	<seealso marker="#error_report/1"><c>error_report/1</c></seealso>.</p>
	<p>This functions is kept for backwards compatibility and
	  must not be used by new code. Use the <seealso marker="logger#macros">
	    <c>?LOG_ERROR</c></seealso> macro or
	  <seealso marker="logger#error-1"><c>logger:error/1,2,3</c></seealso>
	  instead.</p>
      </desc>
    </func>
    <func>
      <name name="get_format_depth" arity="0"/>
      <fsummary>Get the value of the Kernel application variable
                <c>error_logger_format_depth</c>.</fsummary>
      <desc>
	<p>Returns <c>max(10, Depth)</c>, where <c>Depth</c> is the
	value of <c>error_logger_format_depth</c>
	in the Kernel application, if Depth is an integer. Otherwise,
	<c>unlimited</c> is returned.</p>
	<note>
	  <p>The <c>error_logger_format_depth</c> variable
	    is <seealso marker="kernel_app#deprecated-configuration-parameters">
	    deprecated</seealso> since
	    the <seealso marker="logger">Logger API</seealso> was
	    introduced in Erlang/OTP 21.0. The variable, and this
	    function, are kept for backwards compatibility since they
	    still might be used by legacy report handlers.</p>
	</note>
      </desc>
    </func>
    <func>
      <name name="info_msg" arity="1"/>
      <name name="info_msg" arity="2"/>
      <fsummary>Log a standard information event.</fsummary>
      <desc>
        <p>Log a standard information event. The <c><anno>Format</anno></c>
          and <c><anno>Data</anno></c> arguments are the same as the
          arguments of
	  <seealso marker="stdlib:io#format/2"><c>io:format/2</c></seealso>
	  in STDLIB.</p>
        <p>Error logger forwards the event to Logger, including
          metadata that allows backwards compatibility with legacy
          error logger event handlers.</p>
	<p>The event is handled by the default Logger handler.</p>
	<p>These functions are kept for backwards compatibility and
	  must not be used by new code. Use the <seealso marker="logger#macros">
	    <c>?LOG_INFO</c></seealso> macro or
	  <seealso marker="logger#info-1"><c>logger:info/1,2,3</c></seealso>
	  instead.</p>
	<p><em>Example:</em></p>
        <pre>
1> <input>error_logger:info_msg("Something happened in ~p", [a_module]).</input>
=INFO REPORT==== 22-May-2018::12:03:32.612462 ===
Something happened in a_module
ok</pre>
        <warning>
          <p>If the Unicode translation modifier (<c>t</c>) is used in
            the format string, all event handlers must ensure that the
            formatted output is correctly encoded for the I/O
            device.</p>
        </warning>
      </desc>
    </func>
    <func>
      <name name="info_report" arity="1"/>
      <fsummary>Log a standard information event.</fsummary>
      <desc>
        <p>Log a standard information event. Error logger forwards the
          event to Logger, including metadata that allows backwards
          compatibility with legacy error logger event handlers.</p>
	<p>The event is handled by the default Logger handler.</p>
	<p>This functions is kept for backwards compatibility and
	  must not be used by new code. Use the <seealso marker="logger#macros">
	    <c>?LOG_INFO</c></seealso> macro or
	  <seealso marker="logger#info-1"><c>logger:info/1,2,3</c></seealso>
	  instead.</p>
	<p><em>Example:</em></p>
        <pre>
2> <input>error_logger:info_report([{tag1,data1},a_term,{tag2,data}]).</input>
=INFO REPORT==== 22-May-2018::12:06:35.994440 ===
    tag1: data1
    a_term
    tag2: data
ok
3> <input>error_logger:info_report("Something strange happened").</input>
=INFO REPORT==== 22-May-2018::12:06:49.066872 ===
Something strange happened
ok</pre>
      </desc>
    </func>
    <func>
      <name name="info_report" arity="2"/>
      <fsummary>Log a user-defined information event.</fsummary>
      <desc>
        <p>Log a user-defined information event. Error logger forwards
          the event to Logger, including metadata that allows
          backwards compatibility with legacy error logger event
          handlers.</p>
	<p>Error logger also adds a <c>domain</c> field with
          value <c>[<anno>Type</anno>]</c> to this event's metadata,
          causing the filters of the default Logger handler to discard
          the event. A different Logger handler, or an error logger
          event handler, must be added to handle this event.</p>
        <p>It is recommended that <c><anno>Report</anno></c> follows the same
          structure as for
	<seealso marker="#info_report/1"><c>info_report/1</c></seealso>.</p>
	<p>This functions is kept for backwards compatibility and
	  must not be used by new code. Use the <seealso marker="logger#macros">
	    <c>?LOG_INFO</c></seealso> macro or
	  <seealso marker="logger#info-1"><c>logger:info/1,2,3</c></seealso>
	  instead.</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 together with calling
          <c>tty(false)</c>, to have a silent system where
          all standard events are logged to a file only.
          Only one log file can be active at a time.</p>
        <p><c>Request</c> is one of the following:</p>
        <taglist>
          <tag><c>{open, <anno>Filename</anno>}</c></tag>
          <item>
            <p>Opens 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>
              cannot be opened). The file is opened with encoding UTF-8.</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>
    <func>
      <name name="tty" arity="1"/>
      <fsummary>Enable or disable printouts to the terminal.</fsummary>
      <desc>
        <p>Enables (<c><anno>Flag</anno> == true</c>) or disables
          (<c><anno>Flag</anno> == false</c>) printout of standard events
	  to the terminal.</p>
        <p>This is done by adding or deleting the standard event handler
          for output to the terminal. Thus, calling this function overrides
          the value of the Kernel <c>error_logger</c> configuration parameter.</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 command-line flag <c>+W</c>.</p>
	<p><em>Example:</em></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>Log a standard warning event.</fsummary>
      <desc>
        <p>Log a standard warning event. The <c><anno>Format</anno></c>
          and <c><anno>Data</anno></c> arguments are the same as the
          arguments of
	  <seealso marker="stdlib:io#format/2"><c>io:format/2</c></seealso>
	  in STDLIB.</p>
        <p>Error logger forwards the event to Logger, including
          metadata that allows backwards compatibility with legacy
          error logger event handlers.</p>
	<p>The event is handled by the default Logger handler. The log
	  level can be changed to error or info, see
          <seealso marker="#warning_map/0"><c>warning_map/0</c></seealso>.</p>
	<p>These functions are kept for backwards compatibility and
	  must not be used by new code. Use the <seealso marker="logger#macros">
	    <c>?LOG_WARNING</c></seealso> macro or
	  <seealso marker="logger#warning-1"><c>logger:warning/1,2,3</c></seealso>
	  instead.</p>
        <warning>
          <p>If the Unicode translation modifier (<c>t</c>) is used in
            the format string, all event handlers must ensure that the
            formatted output is correctly encoded for the I/O
            device.</p>
        </warning>
      </desc>
    </func>
    <func>
      <name name="warning_report" arity="1"/>
      <fsummary>Log a standard warning event.</fsummary>
      <desc>
        <p>Log a standard warning event. Error logger forwards the event
          to Logger, including metadata that allows backwards
          compatibility with legacy error logger event handlers.</p>
	<p>The event is handled by the default Logger handler. The log
	  level can be changed to error or info, see
          <seealso marker="#warning_map/0"><c>warning_map/0</c></seealso>.</p>
	<p>This functions is kept for backwards compatibility and
	  must not be used by new code. Use the <seealso marker="logger#macros">
	    <c>?LOG_WARNING</c></seealso> macro or
	  <seealso marker="logger#warning-1"><c>logger:warning/1,2,3</c></seealso>
	  instead.</p>
      </desc>
    </func>
    <func>
      <name name="warning_report" arity="2"/>
      <fsummary>Log a user-defined warning event.</fsummary>
      <desc>
        <p>Log a user-defined warning event. Error logger forwards the
          event to Logger, including metadata that allows backwards
          compatibility with legacy error logger event handlers.</p>
	<p>Error logger also adds a <c>domain</c> field with
          value <c>[<anno>Type</anno>]</c> to this event's metadata,
          causing the filters of the default Logger handler to discard
          the event. A different Logger handler, or an error logger
          event handler, must be added to handle this event.</p>
	<p>The log level can be changed to error or info, see
          <seealso marker="#warning_map/0"><c>warning_map/0</c></seealso>.</p>
        <p>It is recommended that <c><anno>Report</anno></c> follows the same
          structure as for
	<seealso marker="#warning_report/1"><c>warning_report/1</c></seealso>.</p>
	<p>This functions is kept for backwards compatibility and
	  must not be used by new code. Use the <seealso marker="logger#macros">
	    <c>?LOG_WARNING</c></seealso> macro or
	  <seealso marker="logger#warning-1"><c>logger:warning/1,2,3</c></seealso>
	  instead.</p>
      </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 that sent the event, and <c>Pid</c> is the process
      that 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
          if 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
          if 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
          if 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>Notice that some system-internal events can also be
      received. Therefore a catch-all clause last in the definition of
      the event handler callback function <c>Module:handle_event/2</c>
      is necessary. This also applies for
      <c>Module:handle_info/2</c>, as the event handler must also take care of
      some system-internal messages.</p>
  </section>
  <section>
    <title>See Also</title>
    <p><seealso marker="stdlib:gen_event"><c>gen_event(3)</c></seealso>,
       <seealso marker="kernel:logger"><c>logger(3)</c></seealso>,
       <seealso marker="stdlib:log_mf_h"><c>log_mf_h(3)</c></seealso>,
       <seealso marker="kernel_app"><c>kernel(6)</c></seealso>,
       <seealso marker="sasl:sasl_app"><c>sasl(6)</c></seealso></p>
  </section>
</erlref>