In ba2555ef and a6f52b19 one-shot semantics were added to event manager in `base`. If my understanding of this code is correct, in this mode the event manager will use only notify the user of the first event on a registered fd after which point the fd will need to be re-registered to get another notification. It appears this was done to optimize the common case of file I/O where only a single event is needed This change lead to a regression[1] in Bas van Dijk's usb library under GHC 7.8. usb's use of the event manager requires that all events on an fd are reported until the fd is registered or else hardware events are lost. I'm a bit perplexed as to why the change was made in the way that it was. Making one-shot a event-manager-wide attribute seems to add a fair bit of complexity to the subsystem while breaking backwards compatibility with library code. Going forward library authors now need to worry about whether the system event manager is one-shot or not. Not only is this platform dependent but it seems that there is no way for a user to determine which semantics the system event handler uses. Is there a reason why one-shot wasn't exported as a per-fd attribute instead of per-manager? Might it be possible to back out this change and instead add a variant of `registerFd` which exposes one-shot semantics? Cheers, - Ben [1] https://github.com/basvandijk/usb/issues/7