11. Licensing Requirements

All artifacts shall adhere to RTEMS Project licensing requirements. Currently, the preferred licenses are:

  • “Two Clause BSD” (BSD-2-Clause) for source code, and
  • CC-BY-SA-4.0 license for documentation

Historically, RTEMS has been licensed under the GPL v2 with linking exception (https://www.rtems.org/license). It is preferred that new submissions be under one of the two preferred licenses. If you have previously submitted code to RTEMS under a historical license, please grant the project permission to relicense. See https://devel.rtems.org/ticket/3053 for details.

For example templates for what to include in source code and documentation, see Copyright and License Block.

11.1. Rationale

RTEMS is intended for use in real-time embedded systems in which the application is statically linked with the operating system and all support libraries. Given this use case, the RTEMS development team evaluated a variety of licenses with with the goal of promoting use while protecting both users and the developers.

Using the GNU General Public License Version 2 (GPLv2) unmodified was considered but discarded because the GPL can only be linked statically with other GPL code. Put simply, linking your application code statically with GPL code would cause your code to become GPL code. This would force both licensing and redistribution requirements onto RTEMS users. This was completely unacceptable.

The GNU Lesser General Public License Version 2 (LGPLv2) was also considered and deemed to not be a suitable license for RTEMS. This is because it either requires use of a shared library that can be re-linked, or release of the linked (application) code. This would require an RTEMS-based embedded system to provide a “relinking kit.” Again, this license would force an unacceptable requirement on RTEMS users and deemed unacceptable.

Newer versions of the GPL (i.e. version 3) are completely unsuitable for embedded systems due to the additions which add further restrictions on end user applications.

The historical RTEMS License is a modified version of the GPL version 2 that includes an exception to permit including headers and linking against RTEMS object files statically. This was based on the license used by GCC language runtime libraries at that time. This license allows the static linking of RTEMS with applications without forcing obligations and restrictions on users.

A problem for RTEMS is there are no copyleft licenses that are compatible with the deployment model of RTEMS. Thus, RTEMS Project has to reject any code that uses the GPL or LGPL, even though RTEMS has historically appeared to use the GPL itself – but with the exception for static linking, and also because an upstream GPL version 2 project could at any time switch to GPL version 3 and become totally unusable. In practice, RTEMS can only accept original code contributed under the RTEMS License and code that has a permissive license.

As stated above, the RTEMS Project has defined its preferred licenses. These allow generation of documentation and software from specification as well as allow end users to statically link with RTEMS and not incur obligations.

In some cases, RTEMS includes software from third-party projects. In those cases, the license is carefully evaluated to meet the project licensing goals. The RTEMS Project can only include software under licenses which follow these guidelines:

  • 2- and 3-clause BSD, MIT, and other OSI-approved non-copyleft licenses that permit statically linking with the code of different licenses are acceptable.
  • The historical RTEMS License is acceptable for software already in the tree. This software is being relicensed to BSD-2-Clause, rewritten, or removed.
  • GPL licensed code is NOT acceptable, neither is LGPL.
  • Software which is dual-licensed in a manner which prevents free use in commercial applications is not acceptable.
  • Advertising obligations are not acceptable.
  • Some license restrictions may be permissible. These will be considered on a case-by-case basis.

In practice, these guidelines are not hard to follow. Critically, they protect the freedom of the RTEMS source code and that of end users to select the license and distribution terms they prefer for their RTEMS-based application.