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call C++ Dll

Posted on 2002-03-13
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Last Modified: 2008-02-01
Hi Experts,

I made a Dll in C++ and called it from VB:

in C++:
int main0(int argc, char *inpFileName, char *outFileName)

in VB:
Private Declare Function main0 Lib "test.dll" (ByVal argc As Long, ByVal inpFileName As String, ByVal outFileName As String) As Long
main0 3, App.Path & "\test.inp", App.Path & "\test.out"

when run the VB, got error:
Bad DLL calling convertion.

Can you tell me what I missed?
0
Comment
Question by:wellwet
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19 Comments
 
LVL 6

Expert Comment

by:xSinbad
Comment Utility
Try these;

Declare Sub main0 Lib "test.dll" ((ByVal argc As Long, ByVal inpFileName As String, ByVal
outFileName As String) As Long

call main0 (3, App.Path & "\test.inp", App.Path & "\test.out")

' OR this maybe

Private Declare Function main0 Lib "test.dll" (ByVal argc As Long, ByVal inpFileName As String, ByVal
outFileName As String) As Long


call main0 (3, App.Path & "\test.inp", App.Path & "\test.out")


Cheers
Marcus
0
 
LVL 6

Expert Comment

by:xSinbad
Comment Utility
Woops extra ( in the first one.
0
 
LVL 5

Expert Comment

by:ianouii
Comment Utility
Just open your VB project, go to reference, refer the VC++ dll.

use the normal

Dim objVC as YourDllName

from there, u can straight to use it.

objVC.classmember.

good luck. :>
0
 

Author Comment

by:wellwet
Comment Utility
To xSinbad:

Thank you....but
>Declare Sub main0 Lib "test.dll" ((ByVal argc As Long, ByVal inpFileName As String, ByVal
outFileName As String) As Long
How a Sub returns a Long?

---------------------------------

To ianouii:

Thank you....but
>go to reference, refer the VC++ dll
Are you sure? What you said is for an ActiveX Dll...
0
 
LVL 6

Expert Comment

by:xSinbad
Comment Utility
Woops.. remove the as long it is not required.
0
 
LVL 6

Expert Comment

by:Triskelion
Comment Utility
Did you try putting extern "C" in front of your function?

extern "C" int main0(int argc, char *inpFileName, char *outFileName)
0
 
LVL 3

Expert Comment

by:DennisL
Comment Utility
Shouldn't the last 2 arguments be passed ByRef?

In C++ you're expecting a pointer to a string.

Private Declare Function main0 Lib "test.dll" (ByVal argc As Long, ByRef inpFileName As String, ByRef
outFileName As String) As Long
0
 

Author Comment

by:wellwet
Comment Utility
Hi Triskelion,

>extern "C" int main0(int argc, char *inpFileName, char *outFileName)

Why we need that?
I used VC++ 6.0 to compile the source into DLL and it works for other simple examples....
0
 
LVL 6

Expert Comment

by:Triskelion
Comment Utility
That's something to try.

Also try adding
   as integer
to the end of your declare so it is an int instead of a long.

Private Declare Function main0 Lib "test.dll" (ByVal argc As Long, ByVal inpFileName As String, ByVal
outFileName As String) As Integer


DennisL >> Shouldn't the last 2 arguments be passed ByRef?

No... ByVal
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Author Comment

by:wellwet
Comment Utility
>it is an int instead of a long

Well.... should be 'As Long' in VB for 'int' in C++ Dll....

>Shouldn't the last 2 arguments be passed ByRef?
Well...you have never call a C++ Dll with 'As String' :-(
0
 

Author Comment

by:wellwet
Comment Utility
More information:

My code above works if I compile my VB stuff to an exe and then run it. I got the error when I run VB under the design window......
0
 
LVL 6

Expert Comment

by:Triskelion
Comment Utility
I just made a fresh DLL and called it from Excel.
If you test your DLL with Excel, do you get the same results?
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LVL 6

Accepted Solution

by:
DrDelphi earned 100 total points
Comment Utility
You have to use the STDCALL calling convention if you are targeting VB. The DLL has to change.

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Author Comment

by:wellwet
Comment Utility
Hi DrDelphi,

In fact, I tried that before you post and I wes going to close the question:-)
What I used is _stdcall and was successfull.
Can you tell me
1. What is the stdcall for
2. What is the difference between _stdcall and STDCALL?
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LVL 6

Expert Comment

by:DrDelphi
Comment Utility
_stdcall is correct. I only capitilized it for emphasis.

the calling convention has to do with how things get cleaned up and the order in which parameters are passed.

Good luck!!
0
 

Author Comment

by:wellwet
Comment Utility
To Triskelion:

Thanks a lot for your comments...I am going to give the points to DrDelphi :-(
0
 

Author Comment

by:wellwet
Comment Utility
To DrDelphi:

If we don't use _stdcall, thins will not cleaned and the parameters will not in order?

Why my original code (without _stdcall) works fine if I compile my VB stuff to an exe and then run it. I got the error only when I run my VB project under the design window......
0
 
LVL 6

Expert Comment

by:DrDelphi
Comment Utility
I can't say for sure, but I suspect that because in the IDE there is an invisible instance of your application running (so that VB can debug it realtime),the DLL has already done what it needed to do once for the "invisible" app, so that it blows up on the "real" app. Of course, I don't know this for sure. I  DO know that the calling convention is the problem, however, having been bitten a time or two myself by the same thing.


Good luck!!
 
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Author Comment

by:wellwet
Comment Utility
Thans a lot for the helps from all experts.
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