On October 13, 2023 11:54:46 AM PDT, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: >On Fri, 13 Oct 2023 at 05:18, Ramon de C Valle <rcvalle@xxxxxxxxxx> wrote: >> >> Both C and repr(C) Rust structs have this encoding, but I understand >> the problems with doing this in C since it doesn't have >> repr(transparent) structs so there would be a lot of casting back and >> forth. Maybe there is an alternative or this could be done for less >> used function pairs? > >We actually have some C variations of what I think people want to use >"repr(transparent) struct" for in Rust. > >Of course, that is depending on what kind of context you want to use >it for, and I might have lost some background. But I'm assuming you're >talking about the situation where you want to treat two or more types >as being "compatible" within certain contexts. > >There's the actual standard C "_Generic()" alternative, which allows >you to make macros etc that use different types transparently. > >It's not very widely used in the kernel, because we only fairly >recently moved to require recent enough compiler versions, but we do >use it now in a couple of places. > >And there's the much more traditional gcc extension in the form of the >__attribute__((__transparent_union__)) thing. In the kernel, that one >is even less used, and that one use is likely going away since the >need for it is going away. > >But while it's not standard C, it's actually been supported by >relevant compilers for much longer than "_Generic" has, and is >designed exactly for the "I have a function that can take arguments of >different types", either because the types are bitwise identical (even >if _conceptually_ not the same), or simply because you have a >different argument that describes the type (the traditional C union >model). > >I suspect, for example, that we *should* have used those transparent >unions for the "this function can take either a folio or a page" case, >instead of duplicating functions for the two uses. > >But probably because few people aren familiar with the syntax, that's >not what happened. > > Linus Transparent unions have been standard C since C99.
