What exactly does RegisterWindowMessage do?

[Ben321]

Does it add the string that's the input parameter to some entry in the Windows Registry (as hinted at by the fact it is called REGISTER window message, instead of STORE window message or SAVE window message), that can later for example be viewed in RegEdit? Or does it store it somewhere else? Wherever it stores it, when it is no longer needed, how do I reliably clean up the leftover files / registry entries that this API function leaves behind?

[Bonnie West]

I believe the "Register" in the RegisterWindowMessage function has nothing to do with the Windows Registry at all. According to Raymond Chen's blog article Some other places atoms (and the magical 0xC000) arise:

I'll start with registered window messages, created by the RegisterWindowMessage function. These are not officially atoms; they are just integers that happen to lie in the range 0xC000 to 0xFFFF, just like atoms. But yeah, internally, they're atoms. Of course, you shouldn't rely on it since it's not contractual. Think of it as a fantastic coincidence.

Based on the discussion here, registered window messages are probably stored in a system-maintained Local Atom Table.

There's no need to clean up anything because "The message remains registered until the session ends." According again to Raymond:

Registered window messages cannot be freed; once they've been created they will hang around until you log off.

[Ben321]

I believe the "Register" in the RegisterWindowMessage function has nothing to do with the Windows Registry at all. According to Raymond Chen's blog article Some other places atoms (and the magical 0xC000) arise:



Based on the discussion here, registered window messages are probably stored in a system-maintained Local Atom Table.

There's no need to clean up anything because "The message remains registered until the session ends." According again to Raymond:

So your session is as long as the program that registered the window "message" is running? Or does it refer to your Windows session (requiring you to reboot your computer to clear the registered message)?

[Bonnie West]

The session usually refers to the session of the user who launched the program that registered the window message. It will remain registered until that user logs off, not when the system shuts down.