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			Using SWI-Prolog on Cygwin
			==========================

Author:   Jan Wielemaker
E-mail:   wielemak@science.uva.nl
Created:  Jun 17 2001
Modified: Nov 21 2005
	  Nov 23, 2005 by Corinna Vinschen

Wat is Cygwin?
==============

Cygwin is a POSIX emulation  environment for Windows (95 and newer, NT 4
and newer).  It consists of a DLL which  provides POSIX functionality on
top of Windows  and ported (mostly GNU) toolchain  and applications. For
further info, see http://cygwin.com.


Why SWI-Prolog on Cygwin?
=========================

The original and primary development  platform   of  SWI-Prolog  is Unix
(currently SuSE Linux). SWI-Prolog has been   ported to MS-Windows using
various emulators of the required basic   POSIX functonality. First this
was the WATCOM environment. Right now  it   is  MSVC  using the NT Posix
layer with an  additional  library   (src/win32/uxnt)  to  provide  some
additional functionality and work around some  bugs and dubious features
of the NT POSIX layer.

Cygwin provides a fairly complete emulation of the POSIX standard, while
allowing access to the native Win32  API   at  the same time. The cygwin
ports therefore uses exclusively  the  POSIX source.  The only exception
is the  stack-allocation that uses  the VirtualAlloc() API from Win32 on
older versions of Cygwin, because the mmap() emulation of cygwin was not
good enough up to Cygwin release 1.5.19.

Cygwin is an excellent platform for  porting native Unix applications to
Windows without much work. Though the native Win32 version of SWI-Prolog
can run under Cygwin, it is limited  in this environment. It cannot load
shared objects created in  Cygwin  and   it  doesn't  use  Cygwins POSIX
extensions to the Windows  filesystem   (mount-points,  symbolic links).
Therefore only a Cygwin version  of   SWI-Prolog  can use advanced POSIX
behaviour, which is  especially  useful   to  realise  portable advanced
process and server management.


Drawbacks?
==========

Cygwin  is just `fairly  complete', it doesn't support every POSIX call,
mostly due to lack of manpower.  Cygwin is big, so if SWI-Prolog is your
only  cygwin  application  it isn't  very interesting.   Cygwin is slow,
notably handling calls for which you really want it such as fork().

So, it is useful if  your  boss   says  your  application  *must* run on
Windows and the application uses lots   of  advanced Unix primitives. In
all other cases I would recomment using   a  real Unix machine. They are
cheaper and as easy to manage as windows machines these days.


Problems?
=========

Most today Unix systems are based on   the  ELF executable format, while
Windows uses XCOFF (eXtended COFF).   This   has  some consequences  for
making foreign libraries for SWI-Prolog.  With   the  cygwin  port we've
started to put all these system dependencies   in  plld, but it is still
good to know the ins and outs.

	* Linking pl.exe
	Linking the executable, such that it exports systems and you
	can import them in your foreign libraries is now fairly easy,
	since gcc and binutils have full support to create DLLs on
	Cygwin for some time now.

	If you just make a default build, you will get just a static
	library libpl.a in $(prefix)/lib/pl-<version>/lib/i686-cygwin,
	and the binaries are all linked statically against this library.

	If you choose to configure with --enable-shared and after calling
	"make install", you'll get a DLL, called cygpl.dll in 
	$(prefix)/lib/pl-<version>/bin/i686-cygwin, as well as a link
	library libpl.dll.a in $(prefix)/lib/pl-<version>/lib/i686-cygwin.
	This file is used for linking stand-alone embedded executables
	and foreign libraries against the cygpl DLL.

	* Creating a foreign library
	This is a bit complicated, but luckily hidden from you.  The
	basic sequence is

		gcc -shared -o mylib.dll <input files> -lpl ...

	plus a couple of system libraries.

	To make life a bit easier in your Makefiles, we've put this
	in plld, so you can do on any platform for which plld is
	aware of the proper sequence:

		plld -shared -o mylib <input files>

	If you use the -v option as in
	
		plld -v -shared ...

	you can see what system libraries are necessary when linking.
	Or simply ask pl:

		pl -dump-runtime-variables | grep PLLIBS


Mixing cygwin and native SWI-Prolog?
====================================

It's not possible to use  foreign libraries built using the above Cygwin
sequence on the  native  SWI-Prolog  and  visa versa.  This is caused by
native   SWI-Prolog using MSVCRT*.lib,  which is mutually exclusive with
the Cygwin POSIX library.