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:
"exited abnormally with codeexitcode
A sentinel runs only while Emacs is waiting (⁖, for terminal
input, or for time to elapse, or for process output). This avoids the
timing errors that could result from running them at random places in
the middle of other Lisp programs. A program can wait, so that
sentinels will run, by calling
(see Waiting), or
accept-process-output (see Accepting Output). Emacs also allows sentinels to run when the command loop is
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
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
nil. In most cases, the
right way to do this is with the macro
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, the error-catching is turned
off. 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
nil so that the sentinel won't run recursively.
For this reason it is not possible for a sentinel to specify
a new sentinel.
In earlier Emacs versions, every sentinel that did regular expression searching or matching had to explicitly save and restore the match data. Now Emacs does this automatically for sentinels; they never need to do it explicitly. See Match Data.
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 sentinel 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>
This function returns the sentinel of process, or
nilif it has none.
While a sentinel or filter function is running, this function returns non-
nilif Emacs was waiting for keyboard input from the user at the time the sentinel or filter function was called,
nilif it was not.