WIN32: Calls from DLL to parent

I am working on a plain C DLL that opens a serial comm port, monitors incoming serial data, and occasionally sends a byte back to the parent of the DLL. What is the best way to send these single bytes back to the parent app ? I have thought about passing the parent's window handle to the DLL and having the DLL post a message to that handle, but I am not sure that this is the right way to do this.
I also have looked at using a hook but it seems very complcated for this simple application.
Thanks !

Pass some pointers to functions from main app and
call them from dll.

This should work.

Delf