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SubjectRe: On SIGCHLD signal semantics
On Tue, 4 Jun 1996, Tim Wright wrote:

> > Thus there's a timing problem when the parent's SIGCHLD handler is
> > SIG_IGN. A wait call by the parent will return one of two things:

> > -ECHILD, if the child exits before the parent's wait call; or
> > the child's pid, if the child exits while the parent is waiting.

> Whoops !
> If this is the behaviour, then it's broken in that it violates the principle of
> least surprise.

> BSD always generates zombies and wait() always succeeds, regardless of the
> disposition of SIGCHLD.
> System V introduced some very dubious semantics where if the disposition of
> SIGCLD (aliased by SIGCHLD in SVR4) is explicitly set to ignore, *no* zombies
> are *ever* generated and wait() will suspend execution until all children have
> exited then return ECHILD. This behaviour was documented as being "unstable"
> or subject to change in SVR2, but is still around in SVR4 and unlikely to
> change now.

For what it's worth, I don't think System V's behaviour is so "dubious".
It seems the System V folks were trying to address BSD's apparent process
table bloat when a parent never calls wait.

BTW, pardon my ignorance, but what exactly do expiring children turn into
if not zombies (in System V)?

Marc.

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| Marc Aurele La France | work: 1-403-492-9310 |
| Computing and Network Services | fax: 1-403-492-1729 |
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| University of Alberta +-----------------------------------+
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