[Mingw-users] [mingw - StupidTechiQuestions] Calling a Mingw DLL from Visual Basic

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By: mattkg0

Hi Guys,

I’m trying to build a DLL in Mingw that I can call from Visual Basic 6 however
VB requires that that the calling convention used in the DLL is stdcall and
that’s where I’m getting stuck. I’ve been searching the web all day looking
for an answer but unfortunately any stuff I dig up that seems remotely relevant
is just horribly confusing to me.

Anyhow I have my source file for the DLL (TestDLL.c) which contains:

#ifdef BUILD_DLL
#define DLLCALL __declspec(dllexport)
#else
#define DLLCALL __declspec(dllimport)
#endif

DLLCALL char TestCall1()
{
	return(1);
}

DLLCALL char TestCall2(char Num1)
{
	return(Num1);
}

DLLCALL char TestCall3(char Num1, char Num2)
{
	char Num3;

	return(Num2 - Num1);
}

I build the DLL with:

gcc -c -DBUILD_DLL -o TestDLL.o TestDLL.c
dllwrap --output-lib=libTestDLL.a --dllname=TestDLL.dll --driver-mame=gcc
TestDLL.o

In my VB project I have all the “Declare Function” statements in a module which
I thought would mean that I don’t need to worry about def files or import libs
(maybe that’s where I’m going wrong)? The external functions are declared as:

Declare Function TestCall1 Lib "C:\mingw\source\Test DLL Call\TestDLL.dll" ()
As Byte
Declare Function TestCall2 Lib "C:\mingw\source\Test DLL Call\TestDLL.dll" (Num1
As Byte) As Byte
Declare Function TestCall3 Lib "C:\mingw\source\Test DLL Call\TestDLL.dll" (Num1
As Byte, Num2 As Byte) As Byte

Now TestCall1 works fine when I call it from the VB application (most probably
because it doesn’t require any arguments) but when I call either TestCall2 or
TestCall3 I get the following VB error:

Run-time error ‘49’: Bad DLL Calling Convention

Ok, maybe the line:  

#define DLLCALL __declspec(dllexport) 

should be: 

#define DLLCALL __declspec(dllexport) __stdcall

But when I recompile the DLL and run the VB app again I get another error this
time:

Run-time error ‘453’: Can’t find DLL entry point TestCall2 in C:\mingw\source\Test
DLL Call\TestDLL.dll

I’ve read the documentation and FAQ on the mingw site but I still don’t know
where to go from here, should I be building the DLL with:

gcc –shared –o TestDLL.dll TestDLL.c
–Wl,--output-def,TestDLL.def,--out-implib,libTestDLL.a

Instead of using dllwrap? Do I need to create a def file by hand? I’ve created
a DLL and called it from another C application, but why is it so hard if I want
to mix languages. I would appreciate any help as I'm very close to flying into
a violent rage followed by tears.

Thanks,
Matthew.


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