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NAME
error_logger - Erlang error logger.
DESCRIPTION
The Erlang error logger is an event manager (see OTP Design Principles and gen_event(3erl)), registered
as error_logger. Errors, warnings, 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 the terminal.
Notice 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 has only a primitive event handler, which buffers and prints the raw event
messages. During system startup, the Kernel application replaces this with a standard event handler, by
default one that writes nicely formatted output to the terminal. Kernel can also be configured so that
events are logged to a file instead, or not logged at all, see kernel(7).
Also the SASL application, if started, adds its own event handler, which by default writes supervisor,
crash, and progress reports to the terminal. See sasl(7).
It is recommended that user-defined applications report errors through the error logger to get uniform
reports. User-defined event handlers can be added to handle application-specific events, see
add_report_handler/1,2. Also, a useful event handler is provided in STDLIB for multi-file logging of
events, see log_mf_h(3erl).
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 +W <e | i | w>, thus showing up as ERROR REPORT or INFO
REPORT in the logs.
DATA TYPES
report() =
[{Tag :: term(), Data :: term()} | term()] | string() | term()
EXPORTS
add_report_handler(Handler) -> any()
add_report_handler(Handler, Args) -> Result
Types:
Handler = module()
Args = gen_event:handler_args()
Result = gen_event:add_handler_ret()
Adds a new event handler to the error logger. The event handler must be implemented as a gen_event
callback module, see gen_event(3erl).
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.
delete_report_handler(Handler) -> Result
Types:
Handler = module()
Result = gen_event:del_handler_ret()
Deletes an event handler from the error logger by calling gen_event:delete_handler(error_logger,
Handler, []), see gen_event(3erl).
error_msg(Format) -> ok
error_msg(Format, Data) -> ok
format(Format, Data) -> ok
Types:
Format = string()
Data = list()
Sends a standard error event to the error logger. The Format and Data arguments are the same as
the arguments of io:format/2 in STDLIB. The event is handled by the standard event handler.
Example:
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
Warning:
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.
Warning:
If the Unicode translation modifier (t) is used in the format string, all error handlers must
ensure that the formatted output is correctly encoded for the I/O device.
error_report(Report) -> ok
Types:
Report = report()
Sends a standard error report event to the error logger. The event is handled by the standard
event handler.
Example:
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
error_report(Type, Report) -> ok
Types:
Type = term()
Report = report()
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.
get_format_depth() -> unlimited | integer() >= 1
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.
info_msg(Format) -> ok
info_msg(Format, Data) -> ok
Types:
Format = string()
Data = list()
Sends a standard information event to the error logger. The Format and Data arguments are the same
as the arguments of io:format/2 in STDLIB. The event is handled by the standard event handler.
Example:
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
Warning:
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.
Warning:
If the Unicode translation modifier (t) is used in the format string, all error handlers must
ensure that the formatted output is correctly encoded for the I/O device.
info_report(Report) -> ok
Types:
Report = report()
Sends a standard information report event to the error logger. The event is handled by the
standard event handler.
Example:
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
info_report(Type, Report) -> ok
Types:
Type = any()
Report = report()
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.
logfile(Request :: {open, Filename}) -> ok | {error, OpenReason}
logfile(Request :: close) -> ok | {error, CloseReason}
logfile(Request :: filename) -> Filename | {error, FilenameReason}
Types:
Filename = file:name()
OpenReason = allready_have_logfile | open_error()
CloseReason = module_not_found
FilenameReason = no_log_file
open_error() = file:posix() | badarg | system_limit
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 together with calling tty(false), 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.
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.
tty(Flag) -> ok
Types:
Flag = boolean()
Enables (Flag == true) or disables (Flag == false) printout of standard events to the terminal.
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 error_logger configuration parameter.
warning_map() -> Tag
Types:
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, 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
warning_msg(Format) -> ok
warning_msg(Format, Data) -> ok
Types:
Format = string()
Data = list()
Sends a standard warning event to the error logger. The Format and Data arguments are the same as
the arguments of io:format/2 in STDLIB. The event is handled by the standard event handler. It is
tagged as an error, warning, or info, see warning_map/0.
Warning:
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.
Warning:
If the Unicode translation modifier (t) is used in the format string, all error handlers must
ensure that the formatted output is correctly encoded for the I/O device.
warning_report(Report) -> ok
Types:
Report = report()
Sends a standard warning report event to the error logger. The event is handled by the standard
event handler. It is tagged as an error, warning, or info, see warning_map/0.
warning_report(Type, Report) -> ok
Types:
Type = any()
Report = report()
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 as an error, warning, or info, depending on the value of warning_map/0.
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(3erl), log_mf_h(3erl) kernel(7) sasl(7)
Ericsson AB kernel 5.4.1 error_logger(3erl)