[Openmcl-devel] Unix signal handling

Gary Byers gb at clozure.com
Tue Jul 6 18:52:09 UTC 2010

On Tue, 6 Jul 2010, Daniel Weinreb wrote:

> You added  ccl:*quit-interrupt-hook*, which is called
> on SIGTERM.  Does it have these problems?


There's a handler for SIGTERM (and SIGINT and IIRC SIGQUIT)
in the lisp kernel; it just sets a flag in memory indicating
which of those signals have been received since the flag was
last cleared.

Some lisp code that runs a few times a second checks/clears that flag
and interrupts some thread.  There are a few hooks that give a little
bit of control over what thread gets interrupted and (in the case of
SIGQUIT/SIGTERM) what function gets called.  That's (at most) just a
matter of interrupting a thread via PROCESS-INTERRUPT, and
PROCESS-INTERRUPT tries to be very careful about when/how the target
thread is interrupted.

A more general mechanism would allow you to do something like:

(define-signal-handler SIGQUIT
   ;; This runs on an arbitrary thread at a more-or-less arbitrary
   ;; time.  The implementation has ensured that it's GC-safe to
   ;; run lisp code now; beyond that, you have enough rope to hang
   ;; yourself in lots of ways, and you may not be able to safely
   ;; do too much more than set a flag here.
   (arbitrary-code ...))

but that mechanism doesn't exist and would require some implementation
support to even offer the GC-safety that the comment describes.

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