System shutdown messages

[Jon Turney]

On 09/05/2016 14:35, Ken Brown wrote:
> A Cygwin emacs user has asked
> (http://debbugs.gnu.org/cgi/bugreport.cgi?bug=23483) whether emacs can
> auto-save files when he shuts down Windows from the Start Menu.  Is
> there a way for a Cygwin process to listen for Windows system shutdown
> messages?

This seems entirely possible using the Win32 API.

For emacs-w32, it's probably straightforward to handle WM_ENDSESSION in 
the message loop similarly to WM_CLOSE

(Note that the process will be terminated if you return from processing 
a WM_ENDSESSION message)

For emacs-nox and emacs-x11 it's more complex, as you'd have to create 
your own hidden window to receive the WM_ENDSESSION, and either select() 
on a /dev/windows fd or block in GetMessage() on a separate thread.

(see [1] for an example of the latter approach as used in the X server)

But I'm not sure that's the best approach, or if wouldn't be more 
appropriate to arrange for SIGTERM to be sent to emacs (or whatever) by 
mintty or the X server when an WM_ENDSESSION is received...

[1] https://cgit.freedesktop.org/xorg/xserver/tree/hw/xwin/winmsgwindow.c

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