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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html><head><title>SWI-Prolog License Conditions</title></head>
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<blockquote>
<blockquote>
<h1 align="center">
<hr>
SWI-Prolog License Conditions
<hr>
</h1>
</blockquote>
</blockquote>
<h2>Preamble</h2>
<p>
SWI-Prolog licensing aims at a large audience, combining ideas from
the <a href="http://www.fsf.org/">Free Software Foundation</a> and the less principal
<a href="http://www.opensource.org/">Open Source Initiative</a>. The license aims at:
</p><ul>
<li>Make SWI-Prolog itself and its libraries are `As free as possible'.
</li><li>Allow for easy integration of <a href="#contrib">contributions</a>.
</li><li>Free software can build on SWI-Prolog without limitations.
</li><li>Non-free (open or proprietary) software can be produced using
SWI-Prolog, although contributed pure <a href="http://www.gnu.org/copyleft/gpl.html">GPL</a> components
cannot be used.
</li></ul>
<p>
To achieve this, different parts of the system have different licenses.
SWI-Prolog programs consists of a mixture of `native' code (source compiled
to machine instructions) and `virtual machine' code (Prolog source compiled to
SWI-Prolog virtual machine instructions, covering both compiled SWI-Prolog
libraries and your compiled application).
</p><h2>The SWI-Prolog kernel and foreign libraries</h2>
<p>
The SWI-Prolog kernel and our foreign libraries are distributed
under the <a href="http://www.gnu.org/copyleft/lesser.html">Lesser GNU Public License</a>, also called the
<b>LGPL</b>. A Prolog executable consists of the combination of these
`native' code components and Prolog virtual machine code. The SWI-Prolog
<b>swipl-rc</b> utility allows for disassembling and re-assembling these
parts, a process satisfying article <b>6b</b> of the LGPL.
</p><p>
Under the LGPL SWI-Prolog can be linked to code distributed under
arbitrary licenses, provided a number of requirements are fullfilled.
The most important requirement is that, if an application relies on a
<em>modified</em> version of SWI-Prolog, the modified sources must be
made available.
</p><h3>Contributed foreign libraries</h3>
<p>
Foreign libraries <em>contributed</em> to the SWI-Prolog project must have
license conditions that are compatible to the <a href="http://www.gnu.org/copyleft/lesser.html">LGPL</a> or the
<a href="http://www.gnu.org/copyleft/gpl.html">GPL</a>. Applications using SWI-Prolog must satisfy the license
restrictions of all modules.
</p><h2>The SWI-Prolog Prolog libraries</h2>
<p>
Lacking a satisfactory technical solution to handle article <b>6</b> of the
LGPL, this license cannot be used for the Prolog source code that is part
of the SWI-Prolog system (both libraries and kernel code). This situation
is comparable to <code>libgcc</code>, the runtime library used with the
GNU C-compiler. Therefore, we use the same proven license terms as this
library. The libgcc license is the <a href="http://www.gnu.org/copyleft/gpl.html">GPL</a>, accompanied with a
special exception. Below we rephrased this exception adjusted to our needs:
</p><blockquote><em>
As a special exception, if you link this library with other files,
compiled with a Free Software compiler, to produce an executable, this
library does not by itself cause the resulting executable to be covered
by the GNU General Public License. This exception does not however
invalidate any other reasons why the executable file might be covered by
the GNU General Public License.
</em></blockquote>
<h2>Keeping it free, open and practical</h2>
<p>
The above sounds a bit complicated. To facilitate this heterogeneous
license system we have added some machinery to SWI-Prolog:
</p><dl>
<dt><b>:- license(License)</b></dt><dd>
This directive can be used in any SWI-Prolog source file. <var>License</var>
denotes one of the established free software licenses or <code>swipl</code>,
referring to the GPL plus the SWI-Prolog Prolog code exception.
<p>
Please check the SWI-Prolog manual for details on accepted license identifiers.
</p></dd><dt><b>PL_license(const char *module-id, enum PL_license_t license)</b></dt><dd>
Allows for registering license information from foreign code through the
foreign-language interface. For example, the GNU readline library is linked
to the system using
<p></p><table align="center" bgcolor="#e0e0e0" border="6" width="80%"><tbody><tr><td nowrap="nowrap"><pre>...
PL_license("libreadline", LICENSE_GPL);
...
</pre></td></tr></tbody></table>
</dd><dt><b>license/0</b></dt><dd>
This toplevel predicate summarises the license requirements of all loaded
modules.
</dd></dl>
<h2><a name="contrib">Contributing to the SWI-Prolog project</a></h2>
<p>
To achieve maximal coherence using SWI-Prolog for Free and Non-Free software
we advice the use of the LGPL for contributed foreign code and the use of
the GPL with SWI-Prolog exception for Prolog code for contributed modules.
</p><p>
As a rule of thumb it is advised to use the above licenses whenever possible
and only use a strict GPL compliant license only if the module contains other
code under strict GPL compliant licenses.
</p><p>
</p><hr>
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