__nocast vs __bitwise

__nocast warns about explicit or implicit casting to different types. HOWEVER, it doesn’t consider two 32-bit integers to be different types, so a __nocast int type may be returned as a regular int type and then the __nocast is lost.

So __nocast on integer types is usually not that powerful. It just gets lost too easily. It’s more useful for things like pointers. It also doesn’t warn about the mixing: you can add integers to __nocast integer types, and it’s not really considered anything wrong.

__bitwise ends up being a stronger integer separation. That one doesn’t allow you to mix with non-bitwise integers, so now it’s much harder to lose the type by mistake.

So the basic rule is:

  • __nocast on its own tends to be more useful for big integers that still need to act like integers, but you want to make it much less likely that they get truncated by mistake. So a 64-bit integer that you don’t want to mistakenly/silently be returned as int, for example. But they mix well with random integer types, so you can add to them etc without using anything special. However, that mixing also means that the __nocast really gets lost fairly easily.
  • __bitwise is for unique types that cannot be mixed with other types, and that you’d never want to just use as a random integer (the integer 0 is special, though, and gets silently accepted - it’s kind of like NULL for pointers). So gfp_t or the safe endianness types would be __bitwise: you can only operate on them by doing specific operations that know about that particular type.

Generally, you want __bitwise if you are looking for type safety. __nocast really is pretty weak.