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The unit LazLogger is supplied with Lazarus and provides logging to file or console.


Adding logging support to your source code

By just including the unit lazlogger you can use the following:

  • DebugLn: which works about the same as WriteLn.
  • DebugLnEnter/DebugLnExit: same as debugln, but DebugLnEnter increases (and DebugLnExit decreases) the indentation of the written line.
  • DbgOut: which works about the same as Write.

All of the above can be called with either:

  • A single string, or list of (up to 15) strings: DebugLn('Foo'); DbgOut('a','b');
  • An array of const: DebugLn(['Foo=', 1, ' Bar=', anInteger]);
  • Same arguments as Format(). String + array of const: DbgOut('a %d',[1]);

Also there are many dbgs functions. Each converts a common type to a string for display. For example dbgs(aRect) converts a variable of type TRect to a string. You can write your own dbgs() functions for type conversions Lazarus does not provide, such as your custom records, which you can then use in DebugLn etc.

Activate/Deactivate LazLogger by using the correct unit

LazLogger provides 3 units for your "uses" clause

Use this unit in your main program only. Using this unit activates the functionality of LazLogger.
Use this unit whenever you want to use DebugLn. What LazLoggerBase does depends on whether the unit LazLogger is used in any other unit of your application. If the LazLogger unit is not used, LazLoggerBase installs a "black hole" logger which discards all log messages. If LazLogger is used anywhere, then all units using LazLoggerBase will write the log messages as described under "Logging Output"
This unit provides you with dummy DebugLn calls (and all other LazLogger functions). They are just calls to empty methods. This allows you to disable all logging in a unit, without the need to remove any debugln statement.

Logging output


By default the output created by DebugLn is written to the stdout device. The output is silently discarded if stdout is not open. On Windows this is the default behavior, if project type "Application" is used to create a Windows GUI application. To get a console alongside your Windows GUI application, you need to uncheck the option "Win32 gui application (-WG)" under Project options / Compiler options / Config and Target / Target specific options. Alternatively you can add {$APPTYPE CONSOLE} at the top of your project .lpr file, or configure multiple "Build Modes" to control the -WG switch.


Debug output can also be written to file. The initialization code of the logging unit

  • checks your program's command line parameters for --debug-log=<file> (In Lazarus: Run / Run Parameters / Command line parameters). On finding this parameter, any subsequent debug output is sent to <file>.
The param name can be changed with DebugLogger.ParamForLogFileName:='--debug-log'; This property exists only if the unit LazLogger is used. (LazLoggerBase does not define this property)
  • If no '--debug-log' command line parameter has been given, it next checks if an operating system environment variable xxx_debuglog exists, where xxx is the program file name without extension. E.g. for Lazarus this would be lazarus_debuglog. If such an environment variable exists, it uses the file specified in that environment variable as file to receive debug output.

Example: if you do:

set lazarus_debuglog=c:\lazarus\debug.txt

and run Lazarus, debug output will be written to c:\lazarus\debug.txt.

Debug server

Not available in LazLogger; see DebugServer.

Log Groups

You can add log groups.

var MY_LOG_GROUP: PLazLoggerLogGroup; 

  MY_LOG_GROUP := DebugLogger.FindOrRegisterLogGroup('fancy_name_for_mylog' {$IFDEF MY_LOG_GROUP_ON_BY_DEFAULT} , True {$ENDIF} );

In your code use

debugln(MY_LOG_GROUP, 'log this text');
debugln(MY_LOG_GROUP, ['a=',a,' b=',b]); // a,b must be basic types: integer, byte, ansistring. See types available for "array of const"

and to enable it call your app with


--debug-enable= takes a comma separated list

An already enabled group ( DebugLogger.FindOrRegisterLogGroup('grpname', True) ) can be disabled by specifying the group name with a minus.


The parameter name can be set via DebugLogger.ParamForEnabledLogGroups='--debug-enable';


For more features, see the unit itself and the LCL documentation.

Using LazLogger/LazLoggerBase your code can access the DebugLogger instance. This offers some properties to fine tune logging.

You can also write your own logger class and use it with LazLogger/Base.


  • DebugLn is thread safe, since Lazarus 2.0.
That is once the logger is created, see below
Earlier versions of this text stated "since 1.0", but a further race condition was found and fixed for 2.0.
  • Calls to DbgOut, DebugLnEnter, DebugLnExit can be made from threads, but may not always format/indent the output correctly.
  • The creation/setup/destruction/configuration of the logger is not thread-safe, and must be done before threads are started.
Creation is usually done with the first "debugln" call, maybe just debugln('') passing an empty string before any multi threaded code. Any other LazLogger access will also trigger creation and, once done, its done for the whole application.
But any properties (eg ParamForLogFileName) should be set before accessing the logger from threads.
  • Callback to events like OnDebugLn are made in each thread. The code in the callback must be thread-safe by itself.
Changing the property is NOT thread-safe

See also

  • TEventLog documentation Built in support for logging in FPC/Lazarus
  • DebugServer Allows sending log entries to a viewer via IPC
  • MultiLog
  • log4delphi
  • The package IdeLazLogger, which adds a dialog to the IDE allowing to enable/disable log groups used in the IDE itself.