[Haskell-cafe] Opening the same file multiple times

[Simon Marlow]

On 12 December 2005 17:53, Donn Cave wrote:

> Quoth Einar Karttunen <ekarttun at cs.helsinki.fi>:
> ...
>> *** Exception: z: openFile: resource busy (file is locked)
> 
> Now that I have access to a platform where ghc builds, I can
> duplicate your results - and in case it helps, here's another
> work-around.  Apparently this "feature" uses POSIX filesystem
> locks, because it appears that we can exploit the brain-damaged
> semantics of that system:  when a process closes a file, all
> its locks are cleared - whether they were acquired through that
> file descriptor or another unrelated open.  It may be the first
> time anyone has ever benefited from this!
> 
> import IO
> 
> main = do
>         fc <- openFile "z" ReadMode   -- sacrificial handle
>         fr <- openFile "z" ReadMode
>         hClose fc
>         fa <- openFile "z" AppendMode
>         hPutStr fa "append this line\n"
>         hGetLine fr >>= print
> 
> If you may need to open more read handles later, you can add
> another extra close after the append handle.

Thank you (I suppose :-) for exposing a bug in GHC's file locking.  It
doesn't have anything to do with POSIX filesystem locks, FWIW, it's just
a bug, in some very old code.

Cheers,
	Simon