Hi This is an addition to my previous post. This modified version of main seems to work: main = do fd <- unsafeWithHandleFd stdin return putStrLn ("stdin: fd = " ++ show fd) fd <- unsafeWithHandleFd stdout return putStrLn ("stdout: fd = " ++ show fd) The way I understand it, unsafeWithHandleFd's job is to keep a reference to the hande, so it won't be garbage collected, while the action is still running. Garbage collecting the handle would close it, as well as the underlying file descriptor, while the latter is still in use by the action. This can't happen as long as use of the file descriptor is encapsulated in the action. This encapsulation can be circumvented by returning the file descriptor, and that's what the modified main function above does. This should usually never be done. However, I want to use it with stdin, stdout and stderr, only. These three should never be garbage collected, should they? I think it would be safe to use unsafeWithHandleFd this way. Am I right? unsafeWithHandleFd is still broken (see previous message), but for my purposes it wouldn't necessarily need to be fixed. Happy hacking Volker Wysk