[Haskell-cafe] Windows: openFile gives permission denied when file in use

[Antoine Latter]

On Wed, Dec 28, 2011 at 3:52 PM, Michael Snoyman <michael at snoyman.com> wrote:
> Hi all,
>
> I just received a bug report from a client that, when an input file is
> open in FrameMaker, my program gives a "permission denied error". This
> bug is reproducible with a simple Haskell program:
>
> import System.IO
>
> main = do
>    putStrLn "here1"
>    h <- openFile "filename.txt" ReadMode
>    putStrLn "here2"
>
> I tried writing a simple C program using fopen, and it ran just fine.
> Does anyone have experience with this issue, and know of a workaround?
>

When GHC opens files for reading, it asks windows to disallow write
access to the file. I'm guessing that Framemaker has the file open for
writing, so GHC can't get that permission.

I imagine that the Haskell runtime does this to make lazy-io less crazy.

Here's the call GHC makes:
https://github.com/ghc/packages-base/blob/0e1a02b96cfd03b8488e3ff4ce232466d6d5ca77/include/HsBase.h#L580

To open a file for reading in your C demo in a similar way you could
do something like:

fd = _sopen("file_name", _O_RDONLY | _O_NOINHERIT,_SH_DENYWR, 0);

Here "_SH_DENYWR" is telling windows to deny others from writing to this file.

Here's the msdn link for _sopen and _wsopen:
http://msdn.microsoft.com/en-us/library/w7sa2b22%28VS.80%29.aspx

I haven't tested any of that, but that should help you in reproducing
how GHC opens files for read on windows.

You should be able to use System.Win32.File.createFile with a share
mode of (fILE_SHARE_READ .|. fILE_SHARE_WRITE) and then wrangling a
Haskell Handle out of that, but I haven't tried it.

Or you could modify the call to _wsopen and FFI call that - it takes
fewer parameters and might be less confusing.

Antoine