elisp: Window Systems
37.24 Window Systems
====================
Emacs works with several window systems, most notably the X Window
System. Both Emacs and X use the term “window”, but use it differently.
An Emacs frame is a single window as far as X is concerned; the
individual Emacs windows are not known to X at all.
-- Variable: window-system
This terminal-local variable tells Lisp programs what window system
Emacs is using for displaying the frame. The possible values are
‘x’
Emacs is displaying the frame using X.
‘w32’
Emacs is displaying the frame using native MS-Windows GUI.
‘ns’
Emacs is displaying the frame using the Nextstep interface
(used on GNUstep and macOS).
‘pc’
Emacs is displaying the frame using MS-DOS direct screen
writes.
‘nil’
Emacs is displaying the frame on a character-based terminal.
-- Variable: initial-window-system
This variable holds the value of ‘window-system’ used for the first
frame created by Emacs during startup. (When Emacs is invoked with
the ‘--daemon’ option, it does not create any initial frames, so
‘initial-window-system’ is ‘nil’, except on MS-Windows, where it is
still ‘w32’. 
daemon (emacs)Initial Options.)
-- Function: window-system &optional frame
This function returns a symbol whose name tells what window system
is used for displaying FRAME (which defaults to the currently
selected frame). The list of possible symbols it returns is the
same one documented for the variable ‘window-system’ above.
Do _not_ use ‘window-system’ and ‘initial-window-system’ as
predicates or boolean flag variables, if you want to write code that
works differently on text terminals and graphic displays. That is
because ‘window-system’ is not a good indicator of Emacs capabilities on
a given display type. Instead, use ‘display-graphic-p’ or any of the
other ‘display-*-p’ predicates described in Display Feature
Testing.
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