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Re: Question about handling SIGINT properly in Guile

From: Olivier Dion
Subject: Re: Question about handling SIGINT properly in Guile
Date: Sat, 13 Apr 2024 12:46:04 -0400

On Sat, 13 Apr 2024, Vijay Marupudi <> wrote:
> Hello folk of guile-user,
> I am trying to create a simple Guile program that continues to perform a
> task until an interrupt signal is sent. However, I am unable to
> deterministically end the program when SIGINT is sent, as the
> interrupt handler does not run soon after the call to (sleep ...) ends.
> Try running the simple reproduction included below. Try typing ^C
> (Ctrl+C) soon after the program begins. Sometimes "WORK" is run
> once, and sometimes "WORK" is run twice. What do I need to do to make
> "WORK" run only once in this instance?

Depending where the signal is sent, you will have two WORKs.  For

--8<---------------cut here---------------start------------->8---
(define quit? #f)

(sigaction SIGINT (lambda (arg)
                    (pk "SIG" arg)
                    (set! quit? #t)))

(let loop ()
  (pk "QUIT-CHECK")
  (if quit?
      (begin (format #t "Quitting!~%") (exit 0))
      (begin (pk "WORK" (do ((i 0 (+ i 1))
                             (sum 0 (+ sum i)))
                            ((= i 1000) sum)))
             (pk "SLEEP" (sleep 10))
             (kill (getpid) SIGINT)    ;; Ctrl-C emulation.
--8<---------------cut here---------------end--------------->8---

> I understand that signal handlers are asynchronous (Asyncs) and aren't
> guaranteed to run immediately. However, I expect them to be run before
> the next primitive call. Am I doing something wrong?

So there is two things with signals.  First, when a process get a signal
queued, the OS only deliver the signal -- at least on linux -- when
going back to user-space.  Typically, before the process get
re-scheduled or after a system call.  So sending a signal is not
immediate.  Furthermore, Guile also queues the signals and deliver them
to threads with async marks I think.

Olivier Dion

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