It’s entirely opt-in, Apple doesn’t force it. If you just do `cc mything.c -o mything` you get a binary whose minimum required OS is the version of the SDK you

It’s entirely opt-in, Apple doesn’t force it. If you just do `cc mything.c -o mything` you get a binary whose minimum required OS is the version of the SDK you built it against, just as with any other UNIX-like OS. It’s just giving the developer the option to build something they know will run on an earlier version too. And since it was all initially done with the preprocessor rather than adding knowledge to the compilers, there’s no reason individual libraries can’t handle API versioning in exactly this way, including things like differing `ino_t` and `off_t` sizes.