I must call a VB function from a C++ DLL. The only way I found is with the
AddressOf method. But when I try to use the function in my C++ DLL, it
crashes (permorfed an illegal operation) and my VB program must close.

Here's how I do it (dummy example):

declare sub "DLL_Function" lib "DLLCpp.DLL" (VBFn as long)
(...)
'Function call
DLL_Function AddressOf(BoxFn)
(...)

(in my main module)

Sub BoxFn(Msg as string)
    Msgbox Msg
end sub

The problem is not in VB (I create an executable file and it works well)
The problem is not in passing the function to the DLL.
The problem comes when I try to use the function in my DLL (it seems like
the DLL can't find the function and then crashes...)

Here's the C++ part:

typedef void __stdcall Fn(char* Msg);

void __stdcall DLL_Function(Fn* VbFunction) {
    (...)
    VbFunction("Hello world");    // Crashes here
    (...)

Your VB Function is expecting a VB String (a BSTR*),
containing a UNICODE string. You're passing a
static character pointer.

If you change the prototype and pass VB a BSTR*,
it should work. You can make this easier in the long run
by making the VB portion an ActiveX server.
That will save a lot of the work, since VC
knows how to read type libraries.

sub BoxFn()
    Msgbox "Hello World"
end sub

so that no strings can interfere in the communication process. It crashes
again. So it's not a string problem (I made other tests and VB converts
char* from C++ to strings in VB).

So my problem remains unsolved, ans it IS a communication problem. It seems
like AddressOf doesn't return the correct value, or it returns a value that
C++ cannot recognize.

The only other possibility I can think of, is the declaration of the
DLLFunction inside the VB program:

declare sub "DLL_Function" lib "DLLCpp.DLL" (VBFn as long)

maybe VBFn must be something else than a long??? (ByVal long?)
(VBFn is what eventually contains the address of BoxFn)

Thanks for your time!

Take a look at these two KB articles -- they might help you out. Doing callbacks
is rather hard, but it's not impossible at all.

http://support.microsoft.com/support/kb/articles/q181/5/78.asp
http://support.microsoft.com/support/kb/articles/q171/7/29.asp

--
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Please post/reply to the newsgroup(s)

Klaus H. Probst
http://members.xoom.com/kprobst/
~~~~~~~~~~~~~~~~~~~~~~~~~~~

--
Jonathan Wood
SoftCircuits Programming
http://www.softcircuits.com

DBL_FindRec(DBL_HTABLE hTable,DBL_HREC hRec, DBL_U8* searchMethod);

hTable = handle to the table
hRec = Record to be searched for
searchmethod = a operator (= < >)

Thanks.
Richard.

~~~~~~~~~~~~~~~~~~~~~~~~~~~
Please post/reply to the newsgroup(s)

Klaus H. Probst
http://members.xoom.com/kprobst/
~~~~~~~~~~~~~~~~~~~~~~~~~~~

HOWTO: Call C Functions That Use the _cdecl Calling Convention
http://support.microsoft.com/support/kb/articles/Q153/5/86.asp

--
Eduardo A. Morcillo
New: Read Office document properties with VB code.
Free ActiveX controls, TypeLibs & Code:
http://www.geocities.com/SiliconValley/Foothills/9940

declare sub "DLL_Sub" lib "DLLCpp.DLL" (ByVal VBFn as long)
(...)
'C++ Sub call
DLL_Sub AddressOf(BoxFn)

(in my main module)

Sub BoxFn()
    Msgbox "Hello World"
end sub

Here's the C++ part:

void __stdcall DLL_Sub(long VbFnAddr) {
    typedef void (__stdcall *VbFn) (void);
    VbFn VbFunction = (VbFn) VbFnAddr;
    VbFunction();    // Crashes here

One of them is to test the API calls within C++ dlls only... to see if it is
not a problem with windows' DLLs (installation problem...)

Any suggestions?

Thanks.

~~~~~~~~~~~~~~~~~~~~~~~~~~~
Klaus H. Probst
http://members.xoom.com/kprobst/
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Ok. Thank you all for your help. You know, when stupidity reached my mind I
learned the word Sorry. Now I can't even say I'm sorry without feeling
cheap.

Here's the error (and the REAL syntax of my app):

declare function "RUN" lib "DLL_LINKER" (ByVal BoxFn as long) _
                                                                          as
string
(...)
Dim Result as string
Result = RUN(AddressOf(BoxFn))

(...)

and in my main module:

sub BoxFn()
    Msgbox "Hello World"
end sub

The problem was in the declare line: I was using the same name for the
argument AND for my sub!!

As I said... stupidity reached my mind... ;-))

Thanks again!

--
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Please post/reply to the newsgroup(s)

Klaus H. Probst
http://members.xoom.com/kprobst/
~~~~~~~~~~~~~~~~~~~~~~~~~~~

~~~~~~~~~~~~~~~~~~~~~~~~~~~
Klaus H. Probst
http://members.xoom.com/kprobst/
~~~~~~~~~~~~~~~~~~~~~~~~~~~

"NOTE: An .EXE file created in Visual Basic will allow you to call a DLL
function that has been declared with the _cdecl calling convention without an
error. It is only when you try to call such a function when running a program
from the Visual Basic IDE, that Visual Basic generates the following error:

Run-time Error '49':
Bad DLL Calling Convention

The fact that the EXE version allows you to call such functions has been
confirmed to be a bug by Microsoft. You should not rely on this behavior as this
might change in future versions of Visual Basic."

--
Eduardo A. Morcillo
New: Read Office document properties with VB code.
Free ActiveX controls, TypeLibs & Code:
http://www.geocities.com/SiliconValley/Foothills/9940