Question

So I want to put a Breakpoint in a specific API or Windows message.
I don’t find any easy way to do that without writing code in any Delphi version.
Is there a way to do that similar as I can put a breakpoint in memory access?

Answer 1

To stop at any call to an API function, find it in the implementation section of Windows.pas (or wherever the function of interest is declared) and set a breakpoint. That takes care of functions you use with load-time dynamic linking. For run-time dynamic linking (LoadLibrary and GetProcAddress), you’ll need a different technique. The variable that gets the result of GetProcAddress will hold the address you want to break at, but I don’t know off-hand how to set a breakpoint at that address.

Stopping on a Window message is trickier since messages can be retrieved in many places. You’ll have to use conditional breakpoints instead.

To catch most posted messages, you can put a breakpoint in TApplication.HandleMessage on the first line after the call to PeekMessage. Set the condition to be Msg.Message = x. HandleMessage takes care of messages posted to the main thread’s message queue for the main Application.Run message loop as well as the VCL’s modal message loops. Other modal dialogs (such as Windows.MessageBox) won’t use it, though.

Observing sent messages is harder because the OS dispatches them to their target window procedures directly. You’ll have to set a breakpoint in the window procedure of every window class you’re interested in. You could get most VCL window classes by putting your conditional breakpoint in Classes.StdWndProc.

Keep in mind that conditional breakpoints can be very slow. They work by the debugger putting an unconditional breakpoint there, and when the OS triggers it, the debugger takes over, checks the condition, and then resumes execution if the conditional fails. That can involve a lot of overhead, switching between the debugger and your application; programs receive lots of messages, so if you can find a way to avoid having the debugger interrupt your program to check every one of them, do it.

If this isn’t feasible for whatever it is you’re trying to debug, then I recommend posting a new question where you describe the problem you’re really trying to solve.