As of version 7.4.0180Actually pre-release 7.3.33, the SWI-Prolog source code is distributed under the Simplified BSD license:
Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
This, unfortunately, does not mean you can any version of SWI-Prolog under the above license. The SWI-Prolog core may be linked to libraries that are more restrictive and in addition your code may have loaded extension packages that have more restrictive conditions. In particular, the core is by default linked to libgmp, distributed under the Lesser GNU Public license.
The above implies you need to configure and recompile the system without these components. For this we provide options to the configure script:
./configure --without-gpl ./configure --without-lgpl
The GNU MP Bignum Library provides unbounded integers, rational numbers and some cryptographical functionality. As libgmp is provided under the Lesser GNU Public license it may legally be combined with proprietary software as long as libgmp is dynamically linked (default) and the end user can replace the libgmp shared object and use your application with their (possibly modified) version of libgmp. In practice this leads to problems if the application is not accessible (e.g., embedded in closed hardware) or you want to avoid customers to peek around in the process memory as they can easily do so by adding a backdoor to the modified LGPL component. Note that such a protection is in general not possible anyway if the customer has unrestricted access to the machine on which the application runs.