I am able to collect output data from a process, but I don't understand the coordination of point and process-mark. In particular, the visible cursor in a non-selected window visiting the buffer does not advance, even though point is reported at correct locations when I access the variable. Selecting the window moves point to an incorrect location.

(EDIT: in an earlier version of this question, I had some basic confusion about windows, frames, selection, and focus. I consulted the documentation here and edited this question to be coherent with emacs definitions.)

I'd be grateful for some explanations and corrections to the following MVE (minimal viable example). First, I start a process:


I open another window to watch what's going on: C-x3, visit foo-buffer in that window C-xbfoo-buffer, and select my programming window window again C-xo. The window visiting foo-buffer is not selected at this time, but a shadowy cursor is visible in the window.

I set up a couple of global variables for debugging and for collecting output and a function to report on the state:

(defvar my-python-buffer-data '())
(defvar my-python-latest-output "")

(defun report ()
  (with-current-buffer "foo-buffer"
    `((:point-min     ,(point-min))
      (:point         ,(point))
      (:point-max     ,(point-max))
      (:process-mark  ,(process-mark (get-process "foo-process")))
      (:buffer-data   ,my-python-buffer-data)
      (:latest-output ,my-python-latest-output))))

Before sending data to the buffer, I check the state by calling report:


((:point-min 1)
 (:point 149)
 (:point-max 149)
 (:process-mark #<marker at 149 in foo-buffer>)
 (:buffer-data nil)
 (:latest-output ""))

Things seems consistent: the point and process marker are at the end of the buffer. I can also see that the shadowy cursor in the other window (which is not selected) is at the end of the buffer.

I now set up my custom "insertion filter," following the documentation in section 38.9.2 of the elisp manual at this link address. I am using the documented option of unconditionally setting point via goto-char at the end of the filter function. I want point to be always at the end of the new output.

I collect data about point and the process mark as I go along, so I need a function to do that:

(defun collect-python-buffer-data (sigil)
  (setq my-python-buffer-data
         `(:sigil ,sigil :marker ,(process-mark proc) :point ,(point))

and here is my filter function, copy-pasted and modded from the doc:

(defun extracting-insertion-filter (proc string)
  (setq my-python-buffer-data '())
  (setq my-python-latest-output "")
  (let ((pb (process-buffer proc)))
   (when (buffer-live-p pb)
     (with-current-buffer pb
         (collect-python-buffer-data 1)
         (goto-char (process-mark proc))
         (collect-python-buffer-data 2)
         (insert string) ; advances point
         (collect-python-buffer-data 3)
         (set-marker (process-mark proc) (point))
         (collect-python-buffer-data 4))
       (goto-char (process-mark proc))
       (collect-python-buffer-data 5)))
   ;; for reporting:
   (setq my-python-buffer-data (reverse my-python-buffer-data))
   (setq my-python-latest-output string)))

I now set the process filter for the process to my new function:

 (get-process "foo-process")

and send some data to my process:

(process-send-string "foo-process" "420 / 3\n")

Now report:


((:point-min 1)
 (:point 157)
 (:point-max 157)
 (:process-mark #<marker at 157 in foo-buffer>)
 (:buffer-data ((:sigil 1 :marker #<marker at 157 in foo-buffer> :point 149)
                (:sigil 2 :marker #<marker at 157 in foo-buffer> :point 149)
                (:sigil 3 :marker #<marker at 157 in foo-buffer> :point 157)
                (:sigil 4 :marker #<marker at 157 in foo-buffer> :point 157)
                (:sigil 5 :marker #<marker at 157 in foo-buffer> :point 157)))
 (:latest-output "140
>>> "))

Things look OK, here. I get my output, and point and process-mark are at the end, where I want them.

However, the little shadowy cursor is at the wrong place in the non-selected window that shows my buffer. Before selecting that window (and spoiling the state), let me make sure that I can send more data over:

(process-send-string "foo-process" "42 / 3\n")


((:point-min 1)
 (:point 164)
 (:point-max 164)
 (:process-mark #<marker at 164 in foo-buffer>)
 (:buffer-data ((:sigil 1 :marker #<marker at 164 in foo-buffer> :point 149)
                (:sigil 2 :marker #<marker at 164 in foo-buffer> :point 157)
                (:sigil 3 :marker #<marker at 164 in foo-buffer> :point 164)
                (:sigil 4 :marker #<marker at 164 in foo-buffer> :point 164)
                (:sigil 5 :marker #<marker at 164 in foo-buffer> :point 164)))
 (:latest-output "14
>>> "))

Everything is OK in the program, but the cursor in the non-selected window has not moved. If I select that that window and un-select it, the party is spoiled, i.e., point moved from 164 (where I want it) to 149, where it should not have been after the initialization of the process buffer. C-xoC-xo

(with-current-buffer "foo-buffer" (point))


Again, I'd be grateful for guidance on how to fix this. I tried the moving option in the documentation, but the effect is the same.

  • 2
    Each window has its own "window point". It's intentional (and very useful) that you can have multiple windows on a buffer, each showing a different part of the buffer. Forcing all windows to track a single point by default would spoil that. I suggest you look at how comint-scroll-to-bottom-on-input is used in comint.el. – phils Sep 16 '18 at 1:53
  • That's a good hint. I had some confusion on the basics and this doc tree is helping to set me straight: gnu.org/software/emacs/manual/html_node/elisp/… – Reb.Cabin Sep 16 '18 at 12:58

The following solves the problem inside my process filter. It sets the window point for every window visiting the process buffer to be the process mark, which I force to be at the end.

(mapcar (lambda (w)
          (set-window-point w
           (process-mark (get-process "foo-process"))))
        (get-buffer-window-list "foo-buffer"))

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