Need help calling a proprietary C DLL from Python

[Craig]

On Mar 20, 4:55 pm, "Chris Mellon" <arka... at gmail.com> wrote:
> On Thu, Mar 20, 2008 at 3:42 PM, Craig <craigm3... at gmail.com> wrote:
>
> > On Mar 20, 2:38 pm, Craig <craigm3... at gmail.com> wrote:
> >  > On Mar 20, 2:29 pm, sturlamolden <sturlamol... at yahoo.no> wrote:
>
> >  > > On 20 Mar, 19:09, Craig <craigm3... at gmail.com> wrote:
>
> >  > > The culprit i here:
>
> >  > > > Before - X = 0, CacheSize = 0, OpenMode = 3, vHandle = 0
>
> >  > > This binds these names to Python ints, but byref expects C types.
>
> >  > > Also observe that CacheSize and OpenMode should be c_short.
>
> >  > I changed CacheSize and OpenMode to c_short, and commented out that
> >  > line producing the "Before" message, and the output is the same.
>
> >  > Further "tinkering" revealed that it is the byref on the fName and pw
> >  > that are causing the error.
>
> >  > The entire problem appears to be around the production of a BSTR and
> >  > the passing of pointers (byref) to the BSTR.
>
> >  Can anyone shed some light on how to work with BSTR's?
>
> Since you didn't tell ctypes about the function signature, it assumes
> it returns int and gives you a python int. Provide a function
> signature (c_void_p should work for BSTR) for both functions and you
> should have an easier time of it.
>
>
>
> > --
> >  http://mail.python.org/mailman/listinfo/python-list


Thanks Chris.

I changed X to a c_short also, and modified the example as suggested:
X = c_short(0)
X = windll.vbis5032.VmxOpen(byref(c_void_p(fName)), byref(CacheSize),
byref(OpenMode), byref(vHandle), byref(c_void_p(pw)))
printf ("After - X = %d, CacheSize = %d, OpenMode = %d, vHandle = %d
\n", X, CacheSize, OpenMode, vHandle)

And, this is what printed:
After - X = 4325376, CacheSize = 0, OpenMode = 3, vHandle = 17586736

The printf shows that the values of X and vHandle have changed, but I
am not sure it is working properly as X should be a MUCH smaller
number (< 256). I would have expected a 0 (VIS_OK), 13 (VIS_DOS_ERROR)
or 14(VIS_DISK_ERROR) if it had worked.

Next, I changed:
fName = windll.oleaut32.SysAllocStringByteLen("u:\\msdb\\dcod\x00",
13)
so that I knew it would not find the file, and got:
After - X = 2555917, CacheSize = 0, OpenMode = 3, vHandle = 0

The vHandle staying 0 tells me that it did not find the file, but the
value of X seems random.

Based on the original C prototype, what am I doing wrong so that the
return from the call to vbis5032 is not "usable"?