Strings & DLLs

[Destined Soul]

Hi everyone.

I'm somewhat new to the area of DLLs, so I hope this doesn't seem too trivial.

I made a test DLL in C++ that outputs a character pointer (char*). I'm wondering if anyone knows how I could access this string in visual basic?

in C++, I have something like:

extern "C" __declspec (dllexport) __stdcall char* myFunc()
{
char* myPointer = new char[32];
myPointer = "Hello World";
return myPointer;
}

I know how to pass VB strings INTO a C++ dll, but I don't know how to retrieve them. Does anyone know how to do this?

Thanx in advance,

Destined

[denniswrenn]

Public Declare Function MyFunc Lib "dllname.dll" As String

MsgBox MyFunc

[parksie]

That's a huge memory leak.

http://www.parksie.net/StringDLL.zip

[Destined Soul]

True, parksie, it is a pretty big memory leak. It's been a long time since I've used C++, so my skills are quite rusty.

However, looking over your program, you seem to just modify the character pointer passed to the function - it doesn't return anything.

My C++ code is:

extern "C" __declspec (dllexport) __stdcall char* ReturnHello()
{
char* NewString = "hello world";
return NewString;
}

and my VB Code is:

Private Declare Function ReturnHello Lib "c:\test\DLL2\blah1.dll" Alias "ReturnHello@0" () As String

Private Sub Command1_Click()
Dim myStr As String
myStr = ReturnHello
MsgBox myStr
End Sub

The program crashes when I run it.

I'm using the Bloodshed C++ program/compiler & VB 6.0 Professional. I'm not sure if the C++ compiler is the issue, though.

Destined

[parksie]

char* NewString = "hello world";
return NewString;You can't guarantee the data will still exist (although it probably will ;)). My method is the same one as used in GetWindowText. The client provides a buffer and a length which the data is copied into, which is exactly the same effect as returning the data by value, if it weren't for VB and C++ having totally different string semantics.

[Destined Soul]

Ah.. okay. Now I see what you mean. Everytime VB tries to access this memory that has already been unallocated it crashes. It's too bad VB doesn't have a better implementation of pointers. :(

However, I think your earlier trick will work. Thanx parksie.

Destined

[Chris]

I post this question before, yet haven't found an answer for it. Hope can get somethings from here... :)

My question is, how to write a CALLBACK function? So straight forward :D :p

regards,

[jim mcnamara]

For code sample on static callback try:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vcsample98/html/vcsmpcallback.asp