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Subject: Re: WinAPI spawn() not used by Cygwin posix_spawn()? Re: [PATCH]
 Cygwin: Add spawn family of functions to docs
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From: Lionel Cons via Cygwin <cygwin AT cygwin DOT com>
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On Mon, 17 Feb 2025 at 11:08, Corinna Vinschen via Cygwin
<cygwin AT cygwin DOT com> wrote:
>
> On Feb 16 23:33, Lionel Cons via Cygwin wrote:
> > On Sun, 16 Feb 2025 at 22:47, Mark Geisert <mark AT maxrnd DOT com> wrote:
> > >
> > > In the doc tree, change the title of section "Other UNIX system
> > > interfaces..." to "Other system interfaces...".  Add the spawn family of
> > > functions noting their origin as Windows.
> >
> > re spawn() family: Cygwin posix_spawn() seems to rely on the rather
> > inefficient vfork(), while Opengroup intended it to be an API to
> > Windows spawn().
> >
> > Is there a technical limitation why Cygwin posix_spawn() cannot use
> > WinAPI spawn() directly?
>
> The requirements of posix_spawn and their helper functions are so
> that we can't easily fulfill them without doing the fork/exec
> twist.

How did UWIN do that? I did ask around - Glenn Fowler of AT&T Research
demonstrated a prototype of UWIN posix_spawn() before posix_spawn()
was finalised by the Austin Group as Opengroup POSIX standard. So this
IS possible, and because non fork() or page cloning has to be done it
should be significantly faster than the
fork()-and-throw-copied-data-away-at-exec() approach.

> See https://man7.org/linux/man-pages/man3/posix_spawn.3.html.> Windows CreateProcess() is not quite the same as Linux clone().

Here might be the misunderstanding:
posix_spawn() is intended NOT to copy anything except the file
descriptors and requested attributes, no memory pages and no
whatsoever. Everything should be done on the caller's process side,
nothing in the child process.

Lionel

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