Christian Brauner <brauner@xxxxxxxxxx> writes: >>>> + /// Prevent values of this type from being moved to a different task. >>>> + /// >>>> + /// This is necessary because the C FFI calls assume that `current` is set to the task that >>>> + /// owns the fd in question. >>>> + _not_send_sync: PhantomData<*mut ()>, >>> >>> I don't fully understand this. Can you explain in a little more detail >>> what you mean by this and how this works? >> >> Yeah, so, this has to do with the Rust trait `Send` that controls >> whether it's okay for a value to get moved from one thread to another. >> In this case, we don't want it to be `Send` so that it can't be moved to >> another thread, since current might be different there. >> >> The `Send` trait is automatically applied to structs whenever *all* >> fields of the struct are `Send`. So to ensure that a struct is not >> `Send`, you add a field that is not `Send`. >> >> The `PhantomData` type used here is a special zero-sized type. >> Basically, it says "pretend this struct has a field of type `*mut ()`, >> but don't actually add the field". So for the purposes of `Send`, it has >> a non-Send field, but since its wrapped in `PhantomData`, the field is >> not there at runtime. > > This probably a stupid suggestion, question. But while PhantomData gives > the right hint of what is happening I wouldn't mind if that was very > explicitly called NoSendTrait or just add the explanatory comment. Yes, > that's a lot of verbiage but you'd help us a lot. I suppose we could add a typedef: type NoSendTrait = PhantomData<*mut ()>; and use that as the field type. The way I did it here is the "standard" way of doing it, and if you look at code outside the kernel, you will also find them using `PhantomData` like this. However, I don't mind adding the typedef if you think it is helpful. Alice
