I'm working on a C++ Windows application, that runs as a process but not a windowed application (does not have WndProc). It has it's own message loop, which is used to process a quit message, and that's how we safely exit the application from within it's self.
What I want to do is somehow send a message to the process from another process, in order to tell the process to exit safely.
In Linux, we would do this by trapping signals, but I'm not sure how it's done on Windows.
Do you have the control over both processes i.e., do you have the code of both processes? If yes I suggest to expose a API to exit safely.
PostThreadMessage can post messages to threads without requiring a window proc.
In the first process, do a GetCurrentThreadId to get a system wide id for the current thread. And somehow get that id to the second app.
In the second app, OpenThread will convert to a thread handle, that you can then use with PostThreadMessage.
Do note that if your 'windowprocless' application ever pops up a message box, the message box enters its own modal message loop, which will silently destroy any thread messages. If any kind of window is ever created on the thread you would be far better off creating an invisible message window that messages can be sent to to control the app.