Pointers in Labview calls to Win32 API

Question

I want to  show a browse for folder dialog on the screen with the help of a call to shell32.dll
function :
LPITEMIDLIST SHBrowseForFolder(      
    LPBROWSEINFO lpbi
browseinfo looks like this:
typedef struct _browseinfo {
    HWND hwndOwner;
    LPCITEMIDLIST pidlRoot;
    LPTSTR pszDisplayName;
    LPCTSTR lpszTitle;
    UINT ulFlags;
    BFFCALLBACK lpfn;
    LPARAM lParam;
    int iImage;
} BROWSEINFO, *PBROWSEINFO, *LPBROWSEINFO;
Following the instructions in the help file of labview, every variable should be converted to a string and concatenated to obtain a binary structure comparable with the wanted structure  Problem  is that there are pointers inside this structure. Pointers to string should first be resolved. How can I get pointers to strings etc.?
Or even a solution for passing such a structure.

Answer 1

vlipje wrote:
I want to  show a browse for folder dialog on the screen with the help of a call to shell32.dll function :   LPITEMIDLIST SHBrowseForFolder(      
    LPBROWSEINFO lpbi
);   browseinfo looks like this:   typedef struct _browseinfo {
    HWND hwndOwner;
    LPCITEMIDLIST pidlRoot;
    LPTSTR pszDisplayName;
    LPCTSTR lpszTitle;
    UINT ulFlags;
    BFFCALLBACK lpfn;
    LPARAM lParam;
    int iImage;
} BROWSEINFO, *PBROWSEINFO, *LPBROWSEINFO;   Following
the instructions in the help file of labview, every variable should be
converted to a string and concatenated to obtain a binary
structure comparable with the wanted structure  Problem  is
that there are pointers inside this structure. Pointers to string
should first be resolved. How can I get pointers to strings etc.? Or even a solution for passing such a structure.
Unless
you are a total C crack (and then you wouldn't have asked here I
think), forget about doing this in LabVIEW alone. There are several
things here why it really will be difficult or even impossible to get
away here without a wrapper DLL.
1) LPCITEMIDLIST is a dynamically allocated
special structure used by shell32 to represent paths. To create such
PIDLs you have to call at least one more shell32 API and afterwards one
to dispose of it. The structure of a PIDL is not really documented so
even if you wanted you couldn't create it on your own in LabVIEW.
2) BFFCALLBACK is a callback function, that
means a function pointer. While with lots of magic and all kinds of
very involved tricks you could create and manage all kinds of pointers
in LabVIEW by calling numerous extra API functions through the Call
Library Node, this is something you can't really create in LabVIEW
without creating an extra VI that is placed into a DLL and some very
special code. Unless you know function pointers in C to a very intimate
level this simply is not possible and I'm not going to try to explain
how it could be done anyhow since writing that would give an essay of
many pages and still be not understandable unless you really know a lot
about C.
3) LPARAM is a
pointer you use to pass context specific user data to the function
which is then passed back to your callback function as one of the
parameters.
So simply said if you can't or don't want to create a wrapper DLL then
you simply don't want to deal with these difficulties on a LabVIEW
diagram level and if you know how to create a wrapper DLL you will
certainly choose for that solution as it is a lot simpler now and
especially in the long run.
Rolf Kalbermatter
Rolf Kalbermatter
CIT Engineering Netherlands
a division of Test & Measurement Solutions