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6.21.6.5 Signals and threads

The semantics of signals looks a bit compilated in the multithread environment. Nevertheless, it is pretty comprehensible once you remember a small number of rules. Besides, Gauche sets up the default behavior easy to use, while allowing programmers to do tricky stuff.

If you don't want to be bothered by the details, just remember one thing, with one sidenote. By default, signals are handled by the primordial (main) thread. However, if the main thread is suspended on mutex or condition variable, the signal may not be handled at all, so be careful.

Now, if you are curious about the details, here are the rules:

Now, these rules have several implications.

If there are more than one thread that don't block a particular signal, you can't know which thread receives the signal. Such a situation is much less useful in Gauche than C programs because of the fact that the signal handling can be delayed indefinitely if the receiver thread is waiting on mutex or condition variable. So, it is recommended to make sure, for each signal, there is only one thread that can receive it.

In Gauche, all threads created by make-thread (See section 9.22.1 Thread procedures) blocks all the signals by default (except the reserved ones). This lets all the signals to be directed to the primordial (main) thread.

Another strategy is to create a thread dedicated for handling signals. To do so, you have to block the signals in the primordial thread, then create the signal-handling thread, and within that thread you unblock all the signals. Such a thread can just loop on sys-pause.

 
(thread-start!
  (make-thread
    (lambda ()
      (sys-sigmask SIG_SETMASK (make <sys-sigset>)) ;;empty mask
      (let loop () (sys-pause) (loop)))))

Complicated application may want to control per-thread signal handling precisely. You can do so, just make sure that at any moment only the designated thread unblocks the desired signal.


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This document was generated by Ken Dickey on November, 28 2002 using texi2html