Re: REQ: add a non-closing version of handleToFd
On 01/10/2011 12:31, Volker Wysk wrote:
Hello
The function to extract a file descriptor from a handle, handleToFd, closes
the handle as a side effect. This appears to be necessary in the general
case, otherwise the implementers wouldn't have made it this way.
But there are cases in which it isn't necessary to close the handle.
For instance, this little function needs to extract the file descriptor, but
there is no need to close, or change, the handle.
isatty :: Handle-- ^ Handle to check
- IO Bool -- ^ Whether the handle is connected to a terminal
isatty h = do
fd- handleToFd_noclose h
isterm- {# call isatty as hssh_c_isatty #}-- this is c2hs
((fromIntegral fd) :: CInt)
return (isterm /= (0::CInt))
handleToFd_noclose :: Handle - IO Fd
-- to be implemented
The isatty above can't be implemented with things the way they are. To me,
this is a serious drawback.
The cases, in which the handle needs to be closed, should be identified, and
documented. Then, a version of handleToFd, which doesn't close the handle can
be implemented.
There's another danger. A Handle has an associated finalizer that
closes the file descriptor when the Handle is no longer needed by the
program, so you can't make a handleToFd_noClose because the Handle might
be closed anyway. What you can do is make a withHandleFD:
withHandleFD :: Handle - (FD - IO a) - IO a
it's still quite dodgy, depending on what you do with the FD. Perhaps
it should be called unsafeWithHandleFD.
Anyway, patches gratefully accepted...
Cheers,
Simon
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Re: REQ: add a non-closing version of handleToFd
* Simon Marlow: There's another danger. A Handle has an associated finalizer that closes the file descriptor when the Handle is no longer needed by the program, so you can't make a handleToFd_noClose because the Handle might be closed anyway. Could you keep a reference to the Handle associated with the FD? Or can the auto-closing be moved to the FD, and the Handle could keep just a reference to it? What you can do is make a withHandleFD: withHandleFD :: Handle - (FD - IO a) - IO a it's still quite dodgy, depending on what you do with the FD. Perhaps it should be called unsafeWithHandleFD. And you can't really fix that with monadic regions, either. Handles (whether called Handle or FD) are the more fundamental abstraction in the sense that it's more efficient to go from handles to other abstractions rather than back. ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: REQ: add a non-closing version of handleToFd
On 6 October 2011 14:58, Simon Marlow marlo...@gmail.com wrote:
... What you can do is make a withHandleFD:
withHandleFD :: Handle - (FD - IO a) - IO a
it's still quite dodgy, depending on what you do with the FD. Perhaps it
should be called unsafeWithHandleFD.
Anyway, patches gratefully accepted...
Maybe something like this together with a big warning message
explaining the danger:
{-# LANGUAGE NamedFieldPuns #-}
module System.Posix.IO where
import Control.Concurrent.MVar (MVar)
unsafeWithHandleFd :: Handle - (Fd - IO a) - IO a
unsafeWithHandleFd h@(FileHandle _ m) f = unsafeWithHandleFd' h m f
unsafeWithHandleFd h@(DuplexHandle _ _ w) f = unsafeWithHandleFd' h w f
unsafeWithHandleFd' :: Handle - MVar Handle__ - (Fd - IO a) - IO a
unsafeWithHandleFd' h m f =
withHandle' unsafeWithHandleFd h m $ \h_@Handle__{haDevice} -
case cast haDevice of
Nothing - ioError (ioeSetErrorString (mkIOError IllegalOperation
unsafeWithHandleFd
(Just h) Nothing)
handle is not a file descriptor)
Just fd - do
x - f (Fd (FD.fdFD fd))
return (h_, x)
I'm not sure about the DuplexHandle case. I mimicked handleToFd by
only converting the write side but I have no idea why that is correct.
Bas
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REQ: add a non-closing version of handleToFd
Hello
The function to extract a file descriptor from a handle, handleToFd, closes
the handle as a side effect. This appears to be necessary in the general
case, otherwise the implementers wouldn't have made it this way.
But there are cases in which it isn't necessary to close the handle.
For instance, this little function needs to extract the file descriptor, but
there is no need to close, or change, the handle.
isatty :: Handle-- ^ Handle to check
- IO Bool -- ^ Whether the handle is connected to a terminal
isatty h = do
fd - handleToFd_noclose h
isterm - {# call isatty as hssh_c_isatty #}-- this is c2hs
((fromIntegral fd) :: CInt)
return (isterm /= (0::CInt))
handleToFd_noclose :: Handle - IO Fd
-- to be implemented
The isatty above can't be implemented with things the way they are. To me,
this is a serious drawback.
The cases, in which the handle needs to be closed, should be identified, and
documented. Then, a version of handleToFd, which doesn't close the handle can
be implemented.
Cheers
Volker
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