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error_logger Erlang Error Logger

The Erlang error logger is an event manager (see OTP Design Principles and gen_event(3)), registered as error_logger. 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 P is logged at the node of the group leader of P. 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.

Initially, error_logger only has a primitive event handler, which buffers and prints the raw event messages. During system startup, the application Kernel replaces this with a standard event handler, 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 kernel(6).

Also the SASL application, if started, adds its own event handler, which by default writes supervisor, crash and progress reports to tty. See sasl(6).

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. (add_report_handler/1,2). Also, there is a useful event handler in STDLIB for multi-file logging of events, see log_mf_h(3).

Warning events was introduced in Erlang/OTP R9C. To retain backwards compatibility, these are by default tagged as errors, thus showing up as error reports in the logs. By using the command line flag ]]>, they can instead be tagged as warnings or info. Tagging them as warnings may require rewriting existing user defined event handlers.

 Send an standard error event to the error logger

Sends a standard error event to the error logger. The Format and Data arguments are the same as the arguments of io:format/2. The event is handled by the standard event handler.

1> error_logger:error_msg("An error occurred in ~p~n", [a_module]).

=ERROR REPORT==== 11-Aug-2005::14:03:19 ===
An error occurred in a_module
ok

If called with bad arguments, this function can crash the standard event handler, meaning no further events are logged. When in doubt, use error_report/1 instead.

 Send a standard error report event to the error logger

Sends a standard error report event to the error logger. The event is handled by the standard event handler.

2> error_logger:error_report([{tag1,data1},a_term,{tag2,data}]).

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

=ERROR REPORT==== 11-Aug-2005::13:45:49 ===
Serious error in my module
ok
 Send a user defined error report event to the error logger

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.

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

 warning_map() -> Tag Return the current mapping for warning events Tag = error | warning | info

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

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().
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
3> 
User switch command
 --> q
os$ erl +W w
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
 Send a standard warning event to the error logger

Sends a standard warning event to the error logger. The Format and Data arguments are the same as the arguments of io:format/2. The event is handled by the standard event handler. It is tagged either as an error, warning or info, see warning_map/0.

If called with bad arguments, this function can crash the standard event handler, meaning no further events are logged. When in doubt, use warning_report/1 instead.

 Send a standard warning report event to the error logger

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 warning_map/0.

 Send a user defined warning report event to the error logger

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 warning_map/0.

 Send a standard information event to the error logger

Sends a standard information event to the error logger. The Format and Data arguments are the same as the arguments of io:format/2. The event is handled by the standard event handler.

1> error_logger:info_msg("Something happened in ~p~n", [a_module]).

=INFO REPORT==== 11-Aug-2005::14:06:15 ===
Something happened in a_module
ok

If called with bad arguments, this function can crash the standard event handler, meaning no further events are logged. When in doubt, use info_report/1 instead.

 Send a standard information report event to the error logger

Sends a standard information report event to the error logger. The event is handled by the standard event handler.

2> error_logger:info_report([{tag1,data1},a_term,{tag2,data}]).

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

=INFO REPORT==== 11-Aug-2005::13:55:36 ===
Something strange happened
ok
 Send a user defined information report event to the error logger

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.

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

 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 described below.

 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).

 Enable or disable printouts to the tty

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

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 error_logger configuration parameter.

 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 standard event handler for output to file, thus calling this function overrides the value of the Kernel error_logger configuration parameter.

Enabling file logging can be used in combination with calling tty(false), 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.

Request is one of:

{open, Filename}

Opens the 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 could not be opened.

 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.
Events

All event handlers added to the error logger must handle the following events. Gleader is the group leader pid of the process which sent the event, and Pid is the process which 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, provided that warnings are set to be tagged as warnings.

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

Generated when warning_report/1 is called, provided that warnings are set to be tagged as warnings.

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

Generated when warning_report/2 is called, provided that 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.

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 Module:handle_event/2 is necessary. This also holds true for Module:handle_info/2, as there are a number of system internal messages the event handler must take care of as well.

SEE ALSO

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