How to pass a Window Handle to a sub-process?

Question

I am working on a special project with an unusual design.

In my application/process, I will create a sub process to do some work. In my process, I need to get the feedback from sub process. I want to pass the Windows handle of my application/process to this sub process, so I am post message from this sub process.

How can I pass Window Handle to sub process? My sub process is an command line application without Window UI and with the main function like this:

int APIENTRY _tWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
                   LPTSTR lpCmdLine, int nCmdShow)

Should I pass Window Handle to the main function? How?

Thanks

Answer 1

Window handles (HWNDs) are global to the system so you can just print the handle as a decimal number into a string buffer, pass it as a string on the command line when you spawn your sub-process with CreateProcess, and call _wtoi() or similar to convert the string back to a handle again.

Answer 2

1. Create a named mutex (global), so both processes could access it, and take it.
2. Spawn the second process. It should wait for the mutex to be freed.
3. You can then pass the window handle using any Inter-Process Communication method; the easiest one may be Shared Memory. Just write the duplicated handle into shared memory.
4. Release the mutex, so that second process could grab it.
5. Read the handle from the shared memory. It is now safe to use it.

The whole mutex manipulation is only to make sure second process will not read from shared memory before anything is written there.

(thanks to @JonathanPotter for comments)