Comment by pjmlp

Ok, but that is the split certification due to the way Sun started to introduce on UNIX world the SDK concept, where customers had to additionally pay for the development tools, the UNIX version of UNIX Home and UNIX Pro editions.

Naturally UNIX Home users are also using a UNIX, and UNIX Pro, users have anyway a C compiler.

Also to note, exactly because nothing else is required, there used to be UNIX vendors, like Sun, that only included C and C++ on their base SDK. Fortran and Ada compilers were additional products to acquire on top of the SDK. Naturally most folks didn't even bother.