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37.10 Sentinels: Detecting Process Status Changes

A process sentinel is a function that is called whenever the associated process changes status for any reason, including signals (whether sent by Emacs or caused by the process's own actions) that terminate, stop, or continue the process. The process sentinel is also called if the process exits. The sentinel receives two arguments: the process for which the event occurred, and a string describing the type of event.

The string describing the event looks like one of the following:

A sentinel runs only while Emacs 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 sentinels at random places in the middle of other Lisp programs. A program can wait, so that sentinels will run, by calling sit-for or sleep-for (see Waiting), or accept-process-output (see Accepting Output). Emacs also allows sentinels to run when the command loop is reading input. delete-process calls the sentinel when it terminates a running process.

Emacs does not keep a queue of multiple reasons to call the sentinel of one process; it records just the current status and the fact that there has been a change. Therefore two changes in status, coming in quick succession, can call the sentinel just once. However, process termination will always run the sentinel exactly once. This is because the process status can't change again after termination.

Emacs explicitly checks for output from the process before running the process sentinel. Once the sentinel runs due to process termination, no further output can arrive from the process.

A sentinel 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)) returns nil.

Quitting is normally inhibited within a sentinel—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 sentinel, bind inhibit-quit to 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 sentinel, it is caught automatically, so that it doesn't stop the execution of whatever programs was running when the sentinel 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 sentinel. See Debugger.

While a sentinel is running, the process sentinel is temporarily set to nil so that the sentinel won't run recursively. For this reason it is not possible for a sentinel to specify a new sentinel.

Note that Emacs automatically saves and restores the match data while executing sentinels. See Match Data.

— Function: set-process-sentinel process sentinel

This function associates sentinel with process. If sentinel is nil, then the process will have no sentinel. The default behavior when there is no sentinel is to insert a message in the process's buffer when the process status changes.

Changes in process sentinels take effect immediately—if the sentinel is slated to be run but has not been called yet, and you specify a new sentinel, the eventual call to the sentinel will use the new one.

          (defun msg-me (process event)
             (princ
               (format "Process: %s had the event `%s'" process event)))
          (set-process-sentinel (get-process "shell") 'msg-me)
               ⇒ msg-me
          (kill-process (get-process "shell"))
               -| Process: #<process shell> had the event `killed'
               ⇒ #<process shell>
— Function: process-sentinel process

This function returns the sentinel of process, or nil if it has none.

— Function: waiting-for-user-input-p

While a sentinel or filter function is running, this function returns non-nil if Emacs was waiting for keyboard input from the user at the time the sentinel or filter function was called, or nil if it was not.