How can I pass a file refnum into and out of external c++ code? What type does it convert to in c++?

Question

I am trying to write external c++ code that will read a file already opened by Labview and therefore assigned a refnum. (Unfortunately, I definately can't use the standard Labview vis.) However I am not sure what c++ type to use in order to manage the refnum.
All help and thoughts appreciated. Thanks,
Joanna

Answer 1

You could do ALL your file handling in C or C++ (MFC CFile for
example) and pass Microsoft file handles into and out of LabVIEW
instead of LabVIEW file references into and out of C. This may be an
easier way to attack the problem.
You could create a DLL in MSVC that exports a FileOpen function, a
FileClose function and a FileRead and/or FileWrite Function and then
call that DLL from place to place as required in your code.
It would help us if you would explain what kind of data you are trying
to read or write and what the application is. Is it binary data?
text files? Do you need some special Win32 file system feature like
file mapped memory? I guess what I am asking is what is your
motivation for doing file handling in C or C++?
Douglas De Clue
LabVIEW developer
[email protected]
"Rolf" wrote in message news:...
> A LabVIEW file refnum is an internal Magic Cookie to LabVIEW and there is no
> way to directly extract the actual information assigned to that Magic
> Cookie.
> However the CIN Reference Manual describes one function which allows to
> retrieve a lower level "File" handle and at least on Windows 32 bit and
> LabVIEW
> from version 5 up to and including 6.1 this "File" LabVIEW datatype directly
> maps
> to the Win32 API "FILE" Handle.
>
> The function to use is
>
> MgErr FRefNumToFD(LVRefNum refNum, File *fdp);
>
> It is declared in extcode.h or one of its dependant headers and exported
> from "labview.lib"
> all located in the cintools directory and you can link this lib also to a
> normal DLL project.
> However calling this DLL then from any other process than LabVIEW or a
> LabVIEW
> executable will not initialize the DLL anymore correctly.
>
> Your best option if you need to write in C(++) should be to use the LabVIEW
> file manager
> functions described on the External Code Manual (manual/lvexcode.pdf) on
> this File handle.
> If you need to use directly some Win32 API functions with it please note
> that although currently
> the "File" in the LabVIEW file manager functions matches the FILE in Windows
> 32 bit API
> functions this is an undocumented and hence unsupported feature. The next
> LabVIEW version
> may actually use a different datatype for its "File" parameter to the
> LabVIEW file manager calls
> and your use of assuming File == FILE may simply crash.
>
> Also operating on a file refnum in LabVIEW which has been accessed directly
> with Win API
> functions may result in strange behaviour such as the file read/write mark
> not being updated as
> you would maybe expect it.
>
> "Jo" wrote in message
> news:50650000000800000016520000-1023576873000@exch​ange.ni.com...
> > How can I pass a file refnum into and out of external c++ code? What
> > type does it convert to in c++?
> >
> > I am trying to write external c++ code that will read a file already
> > opened by Labview and therefore assigned a refnum. (Unfortunately, I
> > definately can't use the standard Labview vis.) However I am not sure
> > what c++ type to use in order to manage the refnum.
> > All help and thoughts appreciated. Thanks,
> > Joanna