Even if your comment both made sense and was factually accurate, it wouldn't have much relevance to the kind of C programming I'm talking about: For example, mm

Even if your comment both made sense and was factually accurate, it wouldn't have much relevance to the kind of C programming I'm talking about: For example, mmap(2) isn't defined in the C standard but in POSIX, but it's universal in the Unix and Unix-like OS world to the extent programs can safely rely on it. Standards are good and useful things, but they're only fully written down in retrospect.