What grammar restrictions? I've converted a few C99 libraries to C89 (since i prefer that one and i often work with older compilers because they are faster) and 99% of the time the reason a library uses C99 is inline variable declaration (which IMO is a bad style, but that is subjective) and // comments (which most C89 compilers support as an extension anyway).

The versions do have other differences, of course, but they're rarely used (the next most common one is designated initializers, which for me are the most annoying to convert).

I mean, it is your code, i just found the grammar restrictions part weird since really there isn't that much of a practical difference between C99 and C89 to the point i personally never felt the need to use C99.

(note that i focus on the language bits, C99 provides some extra library functionality but even then the most useful part is stdint.h which can be implemented in almost any C89 compiler anyway)