How to stop a subprocess?

Question

I am unable to make stop-process work. I expect that it behaves as C-z in a terminal so that I can later continue it with continue-process.

Minimal not working example:

(setq ptest (start-process "test" "*yes*" "yes"))

and then

(stop-process ptest)

returns #<process test> but does nothing (i.e. the process is still writing y into the output buffer *yes*). Especially, running (process-status ptest) says run instead of stop.

What I'm missing ?

Note: GNU Emacs 28.0.50 of 2021-01-20

Answer 1

On my system, yes doesn't react to SIGTSTP, which is the signal sent by stop-process (rather than SIGSTOP as one might assume).

When you run yes in your terminal and shell, C-z is going to additionally prevent the process from having a terminal to write to -- so if the process is still running and generating output, the kernel will stop it at that point (via SIGTTOU as I understand it). My guess is that that's the difference between the two scenarios.

You can use (signal-process ptest 'STOP) to send SIGSTOP.

 -- Function: stop-process &optional process current-group
     This function stops the specified PROCESS.  If it is a real
     subprocess running a program, it sends the signal ‘SIGTSTP’ to that
     subprocess.  If PROCESS represents a network, serial, or pipe
     connection, this function inhibits handling of the incoming data
     from the connection; for a network server, this means not accepting
     new connections.  Use ‘continue-process’ to resume normal
     execution.

     Outside of Emacs, on systems with job control, the stop character
     (usually ‘C-z’) normally sends the ‘SIGTSTP’ signal to a
     subprocess.  When CURRENT-GROUP is non-‘nil’, you can think of this
     function as typing ‘C-z’ on the terminal Emacs uses to communicate
     with the subprocess.