sigwait - differences between Linux & FreeBSD

Kostik Belousov kostikbel at
Thu Oct 8 10:27:30 UTC 2009

On Thu, Oct 08, 2009 at 01:23:37PM +0300, Vlad Galu wrote:
> On Thu, Oct 8, 2009 at 1:02 PM, Kostik Belousov <kostikbel at> wrote:
> > On Thu, Oct 08, 2009 at 11:53:21AM +1100, Stephen Hocking wrote:
> >> Hi all,
> >>
> >> In my efforts to make the xrdp port more robust under FreeBSD, I have
> >> discovered that sigwait (kind of an analogue to select(2), but for
> >> signals rather than I/O) re-enables ignored signals in its list under
> >> Linux, but not FreeBSD. The sesman daemon uses SIGCHLD to clean up
> >> after a session has exited. Under Linux this works OK, under FreeSBD
> >> it doesn't. I have worked around it in a very hackish manner (define a
> >> dummy signal handler and enable it using signal, which means that the
> >> sigwait call can then be unblocked by it), but am wondering if anyone
> >> else has run across the same problem, and if so, if they fixed it in
> >> an elegant manner. Also, does anyone know the correct semantics of
> >> sigwait under this situation?
> >
> > ports@ is the wrong list to discuss the issue in the base system.
> >
> > Solaris 10 sigwait(2) manpage says the following:
> > If sigwait() is called on an ignored signal, then the occurrence of the
> > signal will be ignored, unless sigaction() changes the disposition.
> >
> > We have the same behaviour as Solaris, ingored signals are not queued or
> > recorded regardeless of the presence of sigwaiting thread.
> >
> This is a bit confusing. sigwait(2) says: "The signals specified by
> set should be blocked at the time of the call to sigwait()."...

Blocked and ignored are different attributes of signal.

Ignored means that signal disposition is SIG_IGN, that causes the signal
delivery event to be ignored.

Blocked means that regardeless of signal disposition, signal is not
delivered, but it is recorded somewhere and delivery happen when it
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