A process filter function is a function that receives the standard output from the associated process. If a process has a filter, then all output from that process is passed to the filter. The process buffer is used directly for output from the process only when there is no filter.
The filter function can only be called when Emacs is waiting for
something, because process output arrives only at such times. Emacs
waits when reading terminal input (see the function
sleep-for (see Waiting), and in
accept-process-output (see Accepting Output).
A filter function must accept two arguments: the associated process and a string, which is output just received from it. The function is then free to do whatever it chooses with the output.
Quitting is normally inhibited within a filter function—otherwise,
the effect of typing C-g at command level or to quit a user
command would be unpredictable. If you want to permit quitting inside
a filter function, bind
nil. In most
cases, the right way to do this is with the macro
with-local-quit. See Quitting.
If an error happens during execution of a filter function, it is
caught automatically, so that it doesn't stop the execution of whatever
program was running when the filter function was started. However, if
debug-on-error is non-
nil, errors are not caught.
This makes it possible to use the Lisp debugger to debug the
filter function. See Debugger.
Many filter functions sometimes (or always) insert the output in the process's buffer, mimicking the actions of Emacs when there is no filter. Such filter functions need to make sure that they save the current buffer, select the correct buffer (if different) before inserting output, and then restore the original buffer. They should also check whether the buffer is still alive, update the process marker, and in some cases update the value of point. Here is how to do these things:
(defun ordinary-insertion-filter (proc string) (when (buffer-live-p (process-buffer proc)) (with-current-buffer (process-buffer proc) (let ((moving (= (point) (process-mark proc)))) (save-excursion ;; Insert the text, advancing the process marker. (goto-char (process-mark proc)) (insert string) (set-marker (process-mark proc) (point))) (if moving (goto-char (process-mark proc)))))))
To make the filter force the process buffer to be visible whenever new
text arrives, you could insert a line like the following just before the
(display-buffer (process-buffer proc))
To force point to the end of the new output, no matter where it was
previously, eliminate the variable
moving and call
Note that Emacs automatically saves and restores the match data while executing filter functions. See Match Data.
The output to the filter may come in chunks of any size. A program that produces the same output twice in a row may send it as one batch of 200 characters one time, and five batches of 40 characters the next. If the filter looks for certain text strings in the subprocess output, make sure to handle the case where one of these strings is split across two or more batches of output; one way to do this is to insert the received text into a temporary buffer, which can then be searched.
This function gives process the filter function filter. If filter is
nil, it gives the process no filter.
This function returns the filter function of process, or
nilif it has none.
Here is an example of the use of a filter function:
(defun keep-output (process output) (setq kept (cons output kept))) ⇒ keep-output (setq kept nil) ⇒ nil (set-process-filter (get-process "shell") 'keep-output) ⇒ keep-output (process-send-string "shell" "ls ~/other\n") ⇒ nil kept ⇒ ("lewis@slug:$ " "FINAL-W87-SHORT.MSS backup.otl kolstad.mss~ address.txt backup.psf kolstad.psf backup.bib~ david.mss resume-Dec-86.mss~ backup.err david.psf resume-Dec.psf backup.mss dland syllabus.mss " "#backups.mss# backup.mss~ kolstad.mss ")