D.3 Software support to keep track of license conditions

Given the above, it is possible that SWI-Prolog packages and extensions will rely on the GPL. (71) The predicates below allow for registering license requirements for Prolog files and foreign modules. The predicate eval_license/0 reports which components from the currenly configured system are distributed under copy-left and open source enforcing licenses (the GPL) and therefore must be replaced before distributing linked applications under non-free license conditions.

eval_license

Evaluate the license conditions of all loaded components. If the system contains one or more components that are licenced under GPL-like restrictions the system indicates this program may only be distributed under the GPL license as well as which components prohibit the use of other license conditions.

license(+LicenseId, +Component)

Register the fact that Component is distributed under a license identified by LicenseId. The most important LicenseId's are:

swipl

Indicates this module is distributed under the GNU General Public License (GPL) with the SWI-Prolog exception: (72)

As a special exception, if you link this library with other files, compiled with SWI-Prolog, 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.

This should be the default for software contributed to the SWI-Prolog project as it allows the community to prosper both in the free and non-free world. Still, people using SWI-Prolog to create non-free applications must contribute sources to improvements they make to the community.

lgpl

This is the default license for foreign-libraries linked with SWI-Prolog. Use PL_license() to register the condition from foreign code.

gpl

Indicates this module is strictly Free Software, which implies it cannot be used together with any module that is incompatible to the GPL. Please only use these conditions when forced by other code used in the component.

license(+LicenseId)

Intented as a directive in Prolog source files. It takes the current filename and calls license/2.

void PL_license(const char *LicenseId, const char *Component)

Intended for the install() procedure of foreign libraries. This call can be made before PL_initialise().