If you're using Visual Studio to build the plugin DLL, you should be able to use "Attach to process" (in the "Debug" menu) to attach to the Basic4GL process. Something like the following: 1. Open your plugin project in Visual Studio 2. Open Basic4GL 3. Set breakpoints in Visual Studio (e.g. in the ::Load() method) 4. Click Debug->Attach to process, and select the Basic4GL process 5. Switch to Basic4GL, click "File->Plug-in Libraries..." and tick your DLL
The Visual Studio breakpoints should become active as soon as your plugin DLL file is loaded. Then you can debug your DLL like any other program.
Other C++ IDEs (like those based on GCC that use the GDB debugger) should have a similar process. The key is telling it to debug the Basic4gl.exe process, as that's where the DLLs get loaded into.