Everyone writes in vendor-specific dialects of C. Both POSIX and Win32 define behaviour that isn't defined in C (such as rules for unaligned pointers), while also undefining behaviour that is defined in C (such as what happens if you call fopen when one of the functions in your program is called "open"). Even on embedded platforms, there's no "main" function in freestanding C - your entry point is a vendor-specific extension.