Getting the file descriptor of a handle, without closing it
pf3 at volker-wysk.de
Sun Mar 11 02:31:17 CET 2012
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
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
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.
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