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error_logger Erlang error logger.

In Erlang/OTP 21.0, a new API for logging was added. The old error_logger 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.

error_logger is no longer started by default, but is automatically started when an event handler is added with error_logger:add_report_handler/1,2. The error_logger module is then also added as a handler to the new logger.

See logger(3) and the Logging chapter in the User's Guide for more information.

The Erlang error logger is an event manager (see OTP Design Principles and gen_event(3)), registered as error_logger.

Error logger is no longer started by default, but is automatically started when an event handler is added with add_report_handler/1,2. The error_logger 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.

User-defined event handlers can be added to handle application-specific events.

Existing event handlers provided by STDLIB and SASL are still available, but are no longer used by OTP.

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 errors or info using command-line flag ]]>, thus showing up as ERROR REPORT or INFO REPORT in the logs.

 Add an event handler to the error logger.

Adds a new event handler to the error logger. The event handler must be implemented as a gen_event callback module, see gen_event(3).

Handler is typically the name of the callback module and Args is an optional term (defaults to []) passed to the initialization callback function Handler:init/1. The function returns ok if successful.

The event handler must be able to handle the events in this module, see section Events.

The first time this function is called, error_logger is added as a Logger handler, and the error_logger process is started.

 Delete an event handler from the error logger.

Deletes an event handler from the error logger by calling gen_event:delete_handler(error_logger, Handler, []), see gen_event(3).

If no more event handlers exist after the deletion, error_logger is removed as a Logger handler, and the error_logger process is stopped.

 Log a standard error event.

Log a standard error event. The Format and Data arguments are the same as the arguments of io:format/2 in STDLIB.

Error logger forwards the event to Logger, including metadata that allows backwards compatibility with legacy error logger event handlers.

The event is handled by the default Logger handler.

These functions are kept for backwards compatibility and must not be used by new code. Use the ?LOG_ERROR macro or logger:error/1,2,3 instead.

Example:

1> error_logger:error_msg("An error occurred in ~p", [a_module]).
=ERROR REPORT==== 22-May-2018::11:18:43.376917 ===
An error occurred in a_module
ok

If the Unicode translation modifier (t) is used in the format string, all event handlers must ensure that the formatted output is correctly encoded for the I/O device.

 Log a standard error event.

Log a standard error event. Error logger forwards the event to Logger, including metadata that allows backwards compatibility with legacy error logger event handlers.

The event is handled by the default Logger handler.

This functions is kept for backwards compatibility and must not be used by new code. Use the ?LOG_ERROR macro or logger:error/1,2,3 instead.

Example:

2> error_logger:error_report([{tag1,data1},a_term,{tag2,data}]).
=ERROR REPORT==== 22-May-2018::11:24:23.699306 ===
    tag1: data1
    a_term
    tag2: data
ok
3> error_logger:error_report("Serious error in my module").
=ERROR REPORT==== 22-May-2018::11:24:45.972445 ===
Serious error in my module
ok
 Log a user-defined error event.

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.

Error logger also adds a domain field with value [Type] 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.

It is recommended that Report follows the same structure as for error_report/1.

This functions is kept for backwards compatibility and must not be used by new code. Use the ?LOG_ERROR macro or logger:error/1,2,3 instead.

 Get the value of the Kernel application variable error_logger_format_depth.

Returns max(10, Depth), where Depth is the value of error_logger_format_depth in the Kernel application, if Depth is an integer. Otherwise, unlimited is returned.

The error_logger_format_depth variable is deprecated since the Logger API 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.

 Log a standard information event.

Log a standard information event. The Format and Data arguments are the same as the arguments of io:format/2 in STDLIB.

Error logger forwards the event to Logger, including metadata that allows backwards compatibility with legacy error logger event handlers.

The event is handled by the default Logger handler.

These functions are kept for backwards compatibility and must not be used by new code. Use the ?LOG_INFO macro or logger:info/1,2,3 instead.

Example:

1> error_logger:info_msg("Something happened in ~p", [a_module]).
=INFO REPORT==== 22-May-2018::12:03:32.612462 ===
Something happened in a_module
ok

If the Unicode translation modifier (t) is used in the format string, all event handlers must ensure that the formatted output is correctly encoded for the I/O device.

 Log a standard information event.

Log a standard information event. Error logger forwards the event to Logger, including metadata that allows backwards compatibility with legacy error logger event handlers.

The event is handled by the default Logger handler.

This functions is kept for backwards compatibility and must not be used by new code. Use the ?LOG_INFO macro or logger:info/1,2,3 instead.

Example:

2> error_logger:info_report([{tag1,data1},a_term,{tag2,data}]).
=INFO REPORT==== 22-May-2018::12:06:35.994440 ===
    tag1: data1
    a_term
    tag2: data
ok
3> error_logger:info_report("Something strange happened").
=INFO REPORT==== 22-May-2018::12:06:49.066872 ===
Something strange happened
ok
 Log a user-defined information event.

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.

Error logger also adds a domain field with value [Type] 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.

It is recommended that Report follows the same structure as for info_report/1.

This functions is kept for backwards compatibility and must not be used by new code. Use the ?LOG_INFO macro or logger:info/1,2,3 instead.

 Enable or disable error printouts to a file.

Enables or disables printout of standard events to a file.

This is done by adding or deleting the error_logger_file_h event handler, and thus indirectly adding error_logger as a Logger handler.

Notice that this function does not manipulate the Logger configuration directly, meaning that if the default Logger handler is already logging to a file, this function can potentially cause logging to a second file.

This function is useful as a shortcut during development and testing, but must not be used in a production system. See section Logging in the Kernel User's Guide, and the logger(3) manual page for information about how to configure Logger for live systems.

Request is one of the following:

{open, Filename}

Opens log file Filename. Returns ok if successful, or {error, allready_have_logfile} if logging to file is already enabled, or an error tuple if another error occurred (for example, if Filename cannot be opened). The file is opened with encoding UTF-8.

 close

Closes the current log file. Returns ok, or {error, module_not_found}.

 filename

Returns the name of the log file Filename, or {error, no_log_file} if logging to file is not enabled.

 Enable or disable printouts to the terminal.

Enables (Flag == true) or disables (Flag == false) printout of standard events to the terminal.

This is done by manipulating the Logger configuration. The function is useful as a shortcut during development and testing, but must not be used in a production system. See section Logging in the Kernel User's Guide, and the logger(3) manual page for information about how to configure Logger for live systems.

 Return the current mapping for warning events.

Returns the current mapping for warning events. Events sent using warning_msg/1,2 or warning_report/1,2 are tagged as errors, warnings (default), or info, depending on the value of command-line flag +W.

Example:

os$ erl
Erlang (BEAM) emulator version 5.4.8 [hipe] [threads:0] [kernel-poll]

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

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

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

=ERROR REPORT==== 11-Aug-2005::15:31:23 ===
Warnings tagged as: error
ok
 Log a standard warning event.

Log a standard warning event. The Format and Data arguments are the same as the arguments of io:format/2 in STDLIB.

Error logger forwards the event to Logger, including metadata that allows backwards compatibility with legacy error logger event handlers.

The event is handled by the default Logger handler. The log level can be changed to error or info, see warning_map/0.

These functions are kept for backwards compatibility and must not be used by new code. Use the ?LOG_WARNING macro or logger:warning/1,2,3 instead.

If the Unicode translation modifier (t) is used in the format string, all event handlers must ensure that the formatted output is correctly encoded for the I/O device.

 Log a standard warning event.

Log a standard warning event. Error logger forwards the event to Logger, including metadata that allows backwards compatibility with legacy error logger event handlers.

The event is handled by the default Logger handler. The log level can be changed to error or info, see warning_map/0.

This functions is kept for backwards compatibility and must not be used by new code. Use the ?LOG_WARNING macro or logger:warning/1,2,3 instead.

 Log a user-defined warning event.

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.

Error logger also adds a domain field with value [Type] 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.

The log level can be changed to error or info, see warning_map/0.

It is recommended that Report follows the same structure as for warning_report/1.

This functions is kept for backwards compatibility and must not be used by new code. Use the ?LOG_WARNING macro or logger:warning/1,2,3 instead.
Events

All event handlers added to the error logger must handle the following events. Gleader is the group leader pid of the process that sent the event, and Pid is the process that sent the event.

{error, Gleader, {Pid, Format, Data}}

Generated when error_msg/1,2 or format is called.

 {error_report, Gleader, {Pid, std_error, Report}}

Generated when error_report/1 is called.

 {error_report, Gleader, {Pid, Type, Report}}

Generated when error_report/2 is called.

 {warning_msg, Gleader, {Pid, Format, Data}}

Generated when warning_msg/1,2 is called if warnings are set to be tagged as warnings.

 {warning_report, Gleader, {Pid, std_warning, Report}}

Generated when warning_report/1 is called if warnings are set to be tagged as warnings.

 {warning_report, Gleader, {Pid, Type, Report}}

Generated when warning_report/2 is called if warnings are set to be tagged as warnings.

 {info_msg, Gleader, {Pid, Format, Data}}

Generated when info_msg/1,2 is called.

 {info_report, Gleader, {Pid, std_info, Report}}

Generated when info_report/1 is called.

 {info_report, Gleader, {Pid, Type, Report}}

Generated when info_report/2 is called.

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 Module:handle_event/2 is necessary. This also applies for Module:handle_info/2, as the event handler must also take care of some system-internal messages.

See Also

gen_event(3), logger(3), log_mf_h(3), kernel(6), sasl(6)