Re: SIGCHLD and sigaction()

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

> When I install a SIGCHLD handler via sigaction() using SA_SIGINFO, is
> it guaranteed that my handler is called (at least) once per
> death-of-a-child?

"Maybe."  It depends on how portable you want to be.

Historically, "no": in some older systems, a second SIGCHLD delivered
when there's already one pending delivery gets, lost same as any other
signal.

Then someone - POSIX? SVR4? I don't know - decided to invent a flavour
of signal that's more like writes to a pipe: multiple of them can be
pending at once.  Some systems decided this was sane and implemented
it.

Personally, I don't like it; I think signals should be much like
hardware interrupts in that a second instance happening before the
first is serviced gets silently merged.  If you want some sort of
queued notification of child death, it seems to me a much righter, much
more UNIXy, tack to take is to add something like AF_CHILD sockets or
some such and get child death notifications that way.  (Actually, I'm
not sure sockets would work without severe hackery; too much of the
socket machinery assumes the data in a socket is independent of who
reads from it.  Perhaps it would need a new flavour of file descriptor,
akin to kevent or timerfd descriptors.  I have no doubt it could be
done one way or another.  But arguably it's best to just use SIGCHLD
and a loop that uses WNOHANG.)

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