From:	rdparker AT butlermfg DOT org (Parker, Ron)
Subject:	RE: Crazy fork idea
3 Apr 1998 15:30:40 -0800 :
Message-ID:	<c=US%a=_%p=Butler_Manufactu%l=BUILDINGS/BLDGSHUB/00090E5C.cygnus.cygwin32.developers@nttwr2.tower.bldgs.butlermfg.org>
Mime-Version:	1.0
To:	Larry Hall <lhall AT rfk DOT com>, "cgf AT bbc DOT com" <cgf AT bbc DOT com>,
	"cygwin32-developers AT cygnus DOT com" <cygwin32-developers AT cygnus DOT com>

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Actually the fork code is implemented in the brain-dead POSIX 
subsystem of Windows NT.  It is not built into the kernel. The POSIX 
subsystem is inaccessible from cygwin32, which runs in the Win32 
subsystem.

The APIs for implementing or replacing a subsystem are not documented. 
 Also there are some virtual memory functions at the kernel level 
which are also undocumented.

There is a replacement for the MS POSIX subsystem.  I think it is 
called OpenNT.  If you have run it, you may have noticed that its 
fork() is comparable to a modern UNIX fork.  They were able to do this 
because they have licensed, at *great* expense the source code for 
Windows NT.  As you may have guessed from all of the "hints" about 
copy-on-write (COW) on NTInternals, in various MS publications and on 
the MSDN CDs, it was used to implement their fork.

I was trying figuring out how to allocate a brand new user-level 
process.  One whose virtual memory table I could then setup for 
myself.  From the time I spent looking into COW, I believe that OpenNT 
may have used some of the undocumented kernel level process functions 
in their fork.  FYI, OpenNT did not work on NT 5.0 when I tried it.

Check out PAGE_WRITECOPY on MSDN or in the MS KnowledgeBase. 
 PAGE_WRITECOPY is available on both platforms for the VirtualXXX 
functions and the file mapping functions.

If we have to resort to undocumented NT features or NT only features, 
we still need the best fallback possible for the Win9X world.


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