Question

Is it possible to check from within an elisp function if the current emacs frame is an X window or a terminal?

I have a function that maximizes the window, and it’s set to run whenever a new frame is created. However, when I open a terminal-only session I get error messages whenever a new frame is created.

I want the function to check whether it is an X window, and do nothing otherwise. Is that possible?

For the record, here is the current function:

(defun fullscreen (&optional f)     
  (interactive)                      ;if called interactively, use current frame
  (if f (select-frame f))            ;if called as hook, use new frame
  (x-send-client-message nil 0 nil "_NET_WM_STATE" 32
                    '(2 "_NET_WM_STATE_MAXIMIZED_VERT" 0))
  (x-send-client-message nil 0 nil "_NET_WM_STATE" 32
                    '(2 "_NET_WM_STATE_MAXIMIZED_HORZ" 0))
  )

Answer 1

You may look at the window-system function. It accepts a frame optional argument (defaults to current frame). Alternatively, display-graphic-p is more recent (as per the documentation) and allows checking a whole display containing several frames. In your example, you could just write:

(if (display-graphic-p) ...)