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.
A filter function must accept two arguments: the associated process and a string, which is the output. The function is then free to do whatever it chooses with the output.
A filter function runs only while SXEmacs is waiting (e.g., for terminal
input, or for time to elapse, or for process output). This avoids the
timing errors that could result from running filters at random places in
the middle of other Lisp programs. You may explicitly cause SXEmacs to
wait, so that filter functions will run, by calling
sleep-for (see Waiting), or
(see Accepting Output). SXEmacs is also waiting when the command loop
is reading input.
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
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, the error-catching is turned
off. This makes it possible to use the Lisp debugger to debug the
filter function. See Debugger.
Many filter functions sometimes or always insert the text in the
process's buffer, mimicking the actions of SXEmacs when there is no
filter. Such filter functions need to use
set-buffer in order to
be sure to insert in that buffer. To avoid setting the current buffer
semipermanently, these filter functions must use
to make sure to restore the previous current buffer. They should also
update the process marker, and in some cases update the value of point.
Here is how to do these things:
(defun ordinary-insertion-filter (process string) (let ((old-buffer (current-buffer))) (unwind-protect (let (moving) (set-buffer (process-buffer process)) (setq moving (= (point) (process-mark process))) (save-excursion ;; Insert the text, moving the process-marker. (goto-char (process-mark process)) (insert string) (set-marker (process-mark process) (point))) (if moving (goto-char (process-mark process)))) (set-buffer old-buffer))))
The reason to use an explicit
unwind-protect rather than letting
save-excursion restore the current buffer is so as to preserve
the change in point made by
To make the filter force the process buffer to be visible whenever new
text arrives, insert the following line just before the
(display-buffer (process-buffer process))
To force point to move to the end of the new output no matter where
it was previously, eliminate the variable
moving and call
In earlier Emacs versions, every filter function that did regexp searching or matching had to explicitly save and restore the match data. SXEmacs does this automatically; filter functions never need to do it explicitly. See Match Data.
A filter function that writes the output into the buffer of the
process should check whether the buffer is still alive. If it tries to
insert into a dead buffer, it will get an error. If the buffer is dead,
(buffer-name (process-buffer process
The output to the function 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.
This function gives process the filter function filter. If filter is
nil, then the process will have no filter. If filter is
t, then no output from the process will be accepted until the filter is changed. (Output received during this time is not discarded, but is queued, and will be processed as soon as the filter is changed.)
This function returns the filter function of process, or
nilif it has none.
tmeans that output processing has been stopped.
Here is an example of 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 ")