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From: Paul Eggert <eggert@cs.ucla.edu>
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Subject: Re: Concurrency, again
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Cc: Eli Zaretskii <eliz@gnu.org>, Emacs-Devel devel <emacs-devel@gnu.org>
To: Ken Raeburn <raeburn@raeburn.org>, "Herring, Davis" <herring@lanl.gov>
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On 10/20/2016 12:55 AM, Ken Raeburn wrote:
> I think the last time I had to deal with the wait* interfaces in portable code was back before POSIX support could be assumed

Emacs doesn't assume full POSIX support, and in practice waitid is 
problematic. The Gnulib manual says:

    This function is missing on some platforms: Mac OS X 10.4, FreeBSD 
6.0, NetBSD 5.0, OpenBSD 3.8, Minix 3.1.8, Cygwin, mingw, MSVC 9, 
Interix 3.5, BeOS. As of 2005, no system is known on which waitid with 
flag WNOWAIT works correctly.

Although WNOWAIT is probably in better shape now, I wouldn't assume it 
works on all current Emacs targets.