The FSF GNAT version, in contrast to the AdaCore GNAT version, comes with the RLE (see https://www.gnu.org/licenses/gcc-exception-3.1.de.html). This means that the code generated from closed-source Ada applications may be linked with the runtime library included with FSF GNAT without GPL violation. With the runtime library included with the AdaCore GNAT this is not true. So in practice you have to use FSF GNAT for closed-source applications unless you're willing to pay for the AdaCore GNAT. This is mainly an issue if you need to compile for a processor architecture for which there is no FSF runtime library.
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Thank you! Is the FSF GNAT version ancient or pretty close to the Adacore version in terms of updates?
I'm not up to date. Last time I checked (about five years ago) the FSF GNAT was about a year behind AdaCore GPL GNAT. Maybe someone else has more recent information.
At least as of a year or two ago, AdaCore seemed to be putting in more effort to shore up the differences between FSF and GPL GNAT.
Do you have evidence/references of this?
After all, FSF GNAT was already compatible with Ada 2012 at that time. And a company that makes large investments in software using Ada undoubtedly needs support and will not shy away from the comparatively low additional costs for licenses and maintenance contracts in its own interest. But it is important that the technology is absolutely free for small companies and people who are simply interested in the technology.
Nothing concrete I can point you to offhand. An AdaCore employee popped up in the community (freenode #ada, probably comp.lang.ada too) and started talking about it. I know several people pointed him to differences that were later addressed, or he had already tackled.
What abour Spark?
SPARK is a model checker for a subset of Ada. In contrast to the GNAT runtime library there is no need to deploy anything related to SPARK when you use it. As long as you don't deploy GPL software there is no issue with closed-source applications. Just use the GPL version of GNAT and SPARK to build and check your application.