[Chris_Newton]

There is very little that has changed that isn’t backward compatible, though, and what there is has mostly been in areas that no sensible programmer should ever have relied on in more than toy programs anyway, like inherently unsafe functions or undefined behaviour. You probably have to go back to C99 prohibiting some things that used to be implicit and therefore to code written over two decades ago to find “real” exceptions to that. Compared to many “modern” languages, I’d say that’s still an excellent track record for longevity and compatibility.

[pjmlp]

Toy programs like Linux kernel, once an heavy user of VLAs, which only GCC and clang ever implementated, while everyone else just moved from C89 to C11 without ever caring about them.

[Chris_Newton]

Sorry, I’m not sure I’m understanding your point here. Are you claiming that variable length arrays have been removed from C in recent standards after they were added in C99? Or is your objection that some compilers are very slow to support language features even after they have been standardised? Or something else?

[pjmlp]

I am not claiming, it is a fact.

It was dropped from C11, moved into optional annexes and all commercial compilers that were yet to be fully C99 compliant just moved to C11 instead, without bothering with optional annexes.

The world of C compilers isn't only GCC and clang.

[Chris_Newton]

Something being dropped is not the same as making something optional. In this case, the latter action was an explicit recognition of the reality that many compilers never fully implemented the previous standard anyway.

The point here was about backward compatibility. What standard C99 code is not also standard C11 code? What used to compile a few years ago but won’t compile today? We all understand that there are some minor exceptions, but I think it’s fair to say the answer to both question is not much.

[pjmlp]

VLAs, can't make it any clearer than that.

Any use of Annex K functions, gets(), any code that is considered semantic invalid under C11 memory model, or gets wiped away with the new UB cases from C17.