Load Vb6 DLL Function

newbieQTDev

Hi.
I need to load a vb6 dll and run a function.
This is an example where the function return a int but i want return a vb6 string how i do?

#include <QCoreApplication>
#include <QLibrary>
#include <QDebug>

typedef int (*call_func)();

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QLibrary library( "C:\\Windows\\SysWOW64\\myDLL.dll" );
    library.load();
    if( !library.isLoaded() )
    {
        qDebug() << "Cannot load library.";
        return 0;
    }
    else qDebug()<<"I have load a library";
    call_func func = (call_func)library.resolve( "myIntFunction" );
    if( func)
    {
        qDebug()<<  func() << endl;
    } else qDebug() << "I not call function"<<endl;

    return a.exec();
}

Every help is appreciated. Thanks

mrjj

Hi
Since VB6 string is a vb type, im not sure you can use it directly.
https://stackoverflow.com/questions/5231879/vb6-how-to-pass-strings-to-a-dll-written-in-c-through-a-tlb-file

So it seems you should use LPSTR
and declare the stings in VB as
Dim file_name_in As String * 256

kshegunov

@mrjj said in Load Vb6 DLL Function:

So it seems you should use LPSTR

I find it hard to believe the function should return a char *, but it's certainly not impossible.

@newbieQTDev said in Load Vb6 DLL Function:

This is an example where the function return a int but i want return a vb6 string how i do?

You need to find what's the function prototype exactly, i.e. what is the C-style return type you should expect. I'm not into VB, but if it's LPSTR, as @mrjj mentions, then you're not only responsible to get the return value, but also probably to clean up the memory afterwards.

aha_1980

@newbieQTDev

just to add to @mrjj and @kshegunov, the interesting thing to know is the function signature, as typically written in a C header file.

is there any documentation for that library or did you write it yourself ?

kshegunov

Something which @aha_1980's comment reminded me of ... you could in principle decode the decorated name from the binary. I will explain how if it gets to this, however it's much simpler if you have docs or otherwise information what's the C equivalent of the VB types.

newbieQTDev

Thanks for all.
I will try to recover the documentation of the dll. I found some information about the conversion between the vb and c ++ data types.
The equivalent type should be BSTR.
Unraveling Strings in Visual C++
My work scenario is quite strange. I have to call in C ++ (QT) some functions of a dll written for vb6. I am migrating an application from vb6 to C ++ (QT). My vb6 application uses the dll. Then i will include the functionality of the dll in my C ++ application (QT)

newbieQTDev

As suggested by kshegunov I found the prototype of the function. this is

STDMETHOD(get_SoftwareCode)(/*[out, retval]*/ BSTR *pVal);

Which is the right method for the call?
Thanks

mrjj

Hi
Does the documentation mention who has ownership of the pVal?

Anyway, you can try
QString string("WHATEVER");
BSTR bstr = SysAllocString(string.utf16());
// use...
SysFreeString(bstr); // unless DLL owns it..

Notice this is native API and not Qt. So you might need extra includes
and/or link to libs.

Note that it seems to be other way. you call the DLL and it put data in the pVal.
In that case, you might need t know maximum it needs.

hskoglund

@mrjj For an [out] COM BSTR, the caller of the function (i.e. Qt, not VB6) assumes responsibility of the memory management for the BSTR MSDN doc

mrjj

@hskoglund
haha cool you answered. i was actually thinking of you as if anyone knew what vb6 likes in 2018
it might be you :)
Yes so get_SoftwareCode wants a buffer and the caller must free it.
Do you know a Qt way of constructing a BSTR besides the SysAllocString ?
Im afraid the details escapes me after a decade ;)

hskoglund

@mrjj Had a look inside Qt's sources, found this function in qt-everywhere-src-5.10.1/qtbase/src/plugins/platforms/windows/accessible/comutils.h:

BSTR QStringToBSTR(const QString &str)
{
    return SysAllocStringLen(reinterpret_cast<const OLECHAR *>(str.unicode()), UINT(str.length()));
}

mrjj

@hskoglund
Oh, good found. That should make it just work !

kshegunov

@newbieQTDev said in Load Vb6 DLL Function:

STDMETHOD(get_SoftwareCode)(/*[out, retval]*/ BSTR *pVal);

This is a method definition, according to MSDN, which means it applies to an object, so you also need to have the actual object to use it.

newbieQTDev

Hi kshegunov.
I am sorry.
I may not have explained my problem well.
I'm trying to use a dll that was written using COM technology. It was written in C but to be used in VB6.
The signature of the function I inserted in the post is one of the many methods that the dll provides. After your consideration maybe the code I used is not appropriate to my case?