Interesting comment they made about using C89 for portability. What's the state of C99 today? I know gcc doesn't support all C99 features, but what about other compilers? Is it worthwhile to use C99 in a new project?
You mean, adopting a memory model that properly supports multi-threaded code?
> memcopy is for instance impossible to implement in C17.
What? I assume you mean memcpy? How is it impossible to implement given that it's implemented in the standard library?
> Due to these issues the Linux kernel is no longer using standrard C.
As others have mentioned, the Linux kernel never used standard C. Just search for, say, the substring "__builtin" where it uses compiler builtins directly. It's also quite recent that Linux can be compiled by clang; if it were using standard C, such compatibility issue would not arise in the first place.
The Linux kernel never used standard C, since the very first release (0.01) it already depended on GCC extensions.
void set_value_to_zero(int * p) { if(p == NULL) throw_an_error_and_exit(); *p = 0; }
In c89 writing to NULL is gives you undefined behavior. In c99 the compiler can assume that no code path will produce code that writes to NULL. That's a huge difference! It means that looking at the above code the compiler can reason: since there is a path to writing to the pointer p, the compiler can assume that p will never be NULL. Therfore p == NULL is known to be FALSE at compile time and the null test can be optimized out. I (and Linus) think this is insane.