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Using SWI-Prolog on CygWin
==========================
Author: Jan Wielemaker
E-mail: jan@swi.psy.uva.nl
Created: Sun Jun 17 2001
Modified:
Wat is Cygwin?
==============
Cygwin is a POSIX emulation environment for Windows (all but 3.1). It
consists of two parts: a DLL that provides a POSIX functionality on top
of Windows and ported (mostly GNU) toolchain and applications. For
further info, see http://www.cygwin.com.
Why SWI-Prolog on Cygwin?
=========================
The original and primary development platform of SWI-Prolog is Unix
(currently Linux and Solaris). 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 almost exclusively the POSIX source. The only
exception is the stack-allocation that uses the VirtualAlloc() API from
Win32 because it is better and the mmap() emulation of cygwin is not
good enough.
Cygwin is an excelent 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'. Especially signal handling is
practically non-existent. 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 you 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 lots of 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 a nightmare.
See the script ldcygwin ... In addition to pl.exe, it creates
libplimp.a the import library you need to add to your inputs
for linking foreign libraries.
So, after "make install" you'll find the following in
$(prefix)/lib/pl-<version>/runtime/i686-cygwin:
- libpl.a
Static library for creating stand-alone embedded
executables.
- libplimp.a
Import library to make foreign libraries.
* Creating a foreign library
This too is very complicated, but luckily hidden from you.
The basic sequence is
dllwrap -o mylib.dll <input files> -lplimp
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>
Mixing cygwin and native SWI-Prolog?
====================================
Sofar it appears impossible to use foreign libraries built using the
above Cygwin sequence on the native SWI-Prolog and visa versa. Most
likely this is caused by native SWI-Prolog using MSVCRT*.lib, which is
said to be mutially exclusive with the Cygwin POSIX library.
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