What's the difference between GNU99 and C99 (Clang)?

Question

I have saw the compiler option GNU99 and C99. What\'s the difference of them? Any detail documentation? (Clang, Xcode, Mac OS X)

Answer 1

C99 is simply the version of the C standard as of 1999 as we all know it. In GCC it is not fully supported.

GNU99 is an extension to C99, just like GNU98 is an extension of C98. From the docs:

ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC, this will become the default. The name gnu9x is deprecated.

Clang supports these extensions also.