From Lazarus wiki

TDebuggerIntf is the base class of implementing a debugger as Lazarus-IDE plugin package. The package must have DebuggerIntf package as it's requirements. Any debugger plug-in must implement the class and register via RegisterDebugger (called at Register of registering unit).

Implementation notes:

  • the class is designed to act as asynchronous object. The interaction is implemented in form of commands sent to the class.
  • the command should be put (or not put) into an execution queue. The information about acceptance or denial of the command should be returned as soon as possible.
Command request is sent from the Main Thread. Thus any actual debugging job should be implemented in a separate thread NOT to impact the user experience.
  • once a command has been handled from the queue, the corresponding event should be called and/or debugger state changed

Debugger Workflow


IDE does following steps via TDebuggerIntf methods:

  • allocate (external debugger executable name is provided) (the state is expected dsNone)
  • allocate debug objects (Breakpoints, watches) within TDebuggerIntf
  • assign debugger properties
  • .Init() (as a result of Init method, the debugger should be in dsIdle state)
  • assigning environment
  • assigning debugged process information (WorkingDir, FileName, Arguments, etc...)


In order to set a break-point there should be a new sub-class of TGDBBreakPoint provided.

The list of break-points is stored and managed at TDBGBreakPoints class. The class constructor requires to specify the class of TGDBBreakPoint to be created (which should be the new sub-class) TDBGBreakPoints is created via protected CreateBreakPoints method. The method needs to be overridden and the code to allocate TDBGBreakPoints with the proper sub-class used.


IDE allows to create a breakpoint on any address and or source line. However, the source line where the breakpoint is created might not contain any code or debugging information. In order to indicate if the breakpoint could be stopped at TGDBBreakPoint class provides Valid property of enumeration TValidState type. The property is a public read-only property, however, it could be changed via the protected method SetValid().

  • vsUnknown - this is the initial value of Valid property. Once the debugger verifies, if the requested break point is a valid point, it should change the value to either Valid or Invalid.
  • vsValid - the breakpoint is a valid breakpoint, and the execution could stop on it.
  • vsInvalid - the breakpoint requested is at the invalid location or debugging information doesn't exist.


Whenever breakpoint is hit, TGDBBreakPoint.Hit() method should be called.

Execution Address

The execution address or execution line should be shown whenever the debugger is stopped or paused. This could occur whenever the debugger stops on a breakpoint, paused or an error occurs in the debugged process.

The debugger should call (protected) method DoCurrent(), to specify where location of the instructions be executed. The method accepts one parameter, where address and source line of code is specified. At least address field should be populated. It's understood that source line information might not be available, if so Line should be set to 0.

Threads Information

The information about threads and each thread call stack is an additional information about debugged process. TDebuggerIntf should either populate it whenever the information is available or whenever the information is explicitly requested.

In order to populate the information the debugger should update Threads property (type TThreadsSupplier). The property is a list of threads. The list could be cleaned every time before population.

The list should be populated using Add method. TThreadEntity class could be either allocated explicitly, or CreateEntity method of Threads could be used. It is important to remember that Add methods allocates a copy of TThreadEntity, thus right after the call to Add the added object could be freed (and must be freed eventually).

Once the list of threads is added to Threads, it's necessary to validate the data by calling SetValidity method, passing ddsValid as a parameter.

Finally, whenever a debugged process is stopped, it's desired to identify the "current" event thread by assinging ID of the thread to CurrentThreadId property of Threads object.

TThreadsSupplier should be sub-classed in order to get the notification when thread information is needed. (The sub-class should be created in CreateThreads of TDebuggerIntf sub-class) The sub-class must override and implement RequestMasterData method. The method is called whenever IDE needs the up-to-date information about threads. Similar to RequestCommand the method is expected to behave asynchronously and doesn't have to update threads information immediately.

Minimal Implementation

The following methods must be implemented for TDebbugerIntf

  • GetSupportedCommands (or GetCommands) - returns the set of formats that a debugger can perform
  • RequestCommand - the method actually performing commands. Only commands returned by GetSupportedCommands would be passed to this method
  • Caption - the method return user-friendly name of the debugger
  TMiniDebugger = class(TDebuggerIntf)
    function RequestCommand(const ACommand: TDBGCommand; const AParams: array of const): Boolean; override;
    function GetSupportedCommands: TDBGCommands; override;
    class function Caption: String; override;

procedure Register;


procedure Register;

class function TMiniDebugger.Caption: String;
  Result:='Initial Debugger Plug-in';

function TMiniDebugger.GetSupportedCommands: TDBGCommands;
  Result := [dcRun, dcStop];
function TMiniDebugger.RequestCommand(const ACommand: TDBGCommand;
  const AParams: array of const): Boolean;
  case ACommand of
    dcRun: begin       // the debugger was requested to run
      Result:=true;    // typically it should set the state into dsIdle (to get more data from IDE)
      SetState(dsRun); // the debugger replies as if it started immediately 
    dcStop: begin      // the debugger was requested to stop
      SetState(dsStop); // the debugger replied as if it stopped immediately



The method should return false, if the command cannot be executed.

property Environment

  property Environment: TStrings read write

Environment variables that should be passed (or updated) in a debugged project. Assigning values to Environment triggers dcEnvironment command.


The following commands are called via RequestCommand method.

Command Explanation Parameters
dcRun Starts (or Restarts) or Resumes the execution of a debugged process.

The debugged process executable and arguments to be used are available through FileName and Arguments properties of TDebuggerIntf object.

no parameters
dcPause no parameters
dcStop no parameters
dcStepOver no parameters
dcStepInto no parameters
dcStepOut no parameters
dcRunTo 0 - AnsiString - file name of the source code to run to

1 - Integer - Line number to run to

dcJumpto 0 - AnsiString - file name of the source code to jump to

1 - Integer - Line number to run to

dcAttach 0 - AnsiString - process ID to attach-to.
dcDetach no parameters
0 - AnsiString - expression to be evaluated

1 - pointer to AnsiString - the string where the result should be saved into

2 - pointer to TDBGType - the reference where the information about the result type should be saved to

3 - Integer - TDBGEvaluateFlags casted into Integer

dcModify Modifies the given expression / Assigns a value to a variable/field/property 0 - AnsiString - identified expression

1 - AnsiString - value to be assigned

Prepares/updates an environmental variable for a debugged process. 0 - AnsiString - environment variable in %name%=%value% format

1 - Boolean - the flag, if "true" then the variable needs to be set, "false" if needs to be removed


Development Hints

  • it might be helpful to recompile DebuggerIntf package, adding the following Custom Options to the compiler options of the package
these defines provide more logging output of TDebuggerIntf
  • it might be helpful to rebuild IDE adding the following defines into build Options:
VerboseDebugger logs messages debugging calls from IDE itself

See Also

  • FpGdbmiDebugger - debugger plug-in, that works over the external GDB debugger, but has a better understanding of Pascal expressions and types