The 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. Normally, the GPL can only be linked statically with other GPL code, or rather, linking statically with GPL code would cause your code to become GPL code. The LGPL is not a suitable alternative, because it either requires use of a shared library that can be re-linked, or release of the linked (application) code. And newer versions (GPL version 3) are completely unsuitable for embedded systems due to the relinking restriction which is technically challenging.
A problem for RTEMS is there are no copyleft licenses that are compatible with the RTEMS license. Thus, RTEMS Project has to reject any code that uses the GPL or LGPL, even though RTEMS seems to use the GPL itself---this is because of 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.
I could not find any license that provides the copyleft protection of a software project while still allowing static linking of proprietary software. Maybe there is some subtle legal or technical issue that I do not understand, but it seems like such a license ought to exist somewhere that protects the free software while permitting applications to use it; a sort-of "Embedded GPL".
Some things that RTEMS could do better include:
- Collect all of the copyright and license disclaimers for users
- Collect all of the advertising restrictions, or move those encumbered files to a secondary repository
- Switch from the GPL + linking exception, but to what I do not know
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