To make a conforming C compiler, someone has to buy the ISO C standard (208 CHF), which other folks will then spend development resources in creating C compilers that obey said standard.
The biggest open source compilers, clang and GCC, only keep being up to date with ISO C, because of the money from IBM, Red-Hat, Apple, ARM, Intel, Green-Hills,... place in assigning company resources to work on them.
I would debate that.
1. The C language spec isn't code, open source is only concerned with the code.
2. Philosophically, I would say that if you can write and compile standards conforming c code on an open source compiler, and the standard is freely available then to all intents and purposes C is open. It isn't as if GCC doesn't know what the standard is, it isn't as if they can't describe GCC as a c compiler.
It's like saying a bsd or Linux isn't unix, just because they haven't paid some fee.
(eg in practice C is more like a language family, with an attempt to standardize a common core feature set and syntax as an afterthought and call that a "standard" - it's good that it exists, but it's hardly relevant or even useful for real world projects)
In any case, it's my understanding that the C standards have largely been community driven efforts that formalized things already being done by major compilers like Gnu rather than driven by the technical needs of industry the way C++ has.
I thought having read it somewhere on one of those embedded forums like embedded.com.
Upon further investigation they are still using their own stuff.
Questions about the standard/specification are a different matter:
1. Whether the language has one? (Many languages, both open source and proprietary don't! I believe mojo doesn't have one?)
2. Whether it's available for free (gratis)?
3. Whether it's under a free (libre) license that allows redistribution with modifications?