On 11/03/2012 01:31, Volker Wysk wrote:
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.
Right. The problem with this: -- Blocks unsafeWithHandleFd stdout $ \fd -> putStrLn ("stdout: fd = " ++ show fd) is that unsafeWithHandleFd is holding the lock on stdout, while you try to write to it with putStrLn. The implementation of unsafeWithHandleFd could probably be fixed to avoid this - as you say, all it needs is to hold a reference to the Handle until the function has returned. The usual way to "hold a reference" to something is to use touch#.
However, I want to use it with stdin, stdout and stderr, only.
Is there some reason you can't just use 0, 1, and 2?
These three should never be garbage collected, should they? I think it would be safe to use unsafeWithHandleFd this way. Am I right?
I wouldn't do that, but you're probably right that it is safe right now. (but no guarantees that it will continue to work for ever.) Cheers, Simon