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#14640
SA_RESTART prevents execution of signal handlers
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Message #10 received at 14640-done <at> debbugs.gnu.org (full text, mbox):
On Mon 17 Jun 2013 15:54, ludovic.courtes <at> inria.fr (Ludovic Courtès) writes:
> When using SA_RESTART, signal handlers are never executed, as in this
> example (checked on 2.0.9+):
>
> (sigaction SIGALRM
> (lambda (signum)
> (pk 'sig signum))
> SA_RESTART)
> (alarm 3)
> (pk 'char (read-char))
>
> Presumably this is because the read(2) syscall is automatically
> restarted, leaving no chance for the handler async to run.
Thinking about this a bit -- since we always handle signals
asynchronously and have no intention of handling them synchronously,
then we just have to document this behavior. Done in e877e1b:
Guile handles signals asynchronously. When it receives a signal, the
synchronous signal handler just records the fact that a signal was
received and sets a flag to tell the relevant Guile thread that it has a
pending signal. When the Guile thread checks the pending-interrupt
flag, it will arrange to run the asynchronous part of the signal
handler, which is the handler attached by @code{sigaction}.
This strategy has some perhaps-unexpected interactions with the
@code{SA_RESTART} flag, though: because the synchronous handler doesn't
do very much, and notably it doesn't run the Guile handler, it's
impossible to interrupt a thread stuck in a long-running system call via
a signal handler that is installed with @code{SA_RESTART}: the
synchronous handler just records the pending interrupt, but then the
system call resumes and Guile doesn't have a chance to actually check
the flag and run the asynchronous handler. That's just how it is.
Andy
This bug report was last modified 8 years and 363 days ago.
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