Some unmentioned incompatibilities I've encountered that makes a C header not directly usable in C++:

- C `_Atomic(T)` and C++ `std::atomic<T>`. C++23 has C compatible header `stdatomic.h` that defines `_Atomic(T)`, but it's still problematic

- C `_Noreturn/noreturn` and C++ `[[noreturn]]`. C23 `[[noreturn]]` makes them compatible

- C inline and C++ inline are different. Good news is their `static inline` are the same

- C has anonymous struct. C++ doesn't. Both have anonymous union though