C / C++ / MFC

23-Dec-13 5:42

Hello everybody, I'm new in this forum. I don't know where to place this thread, because it's about VB6 and C, but I think the problem is in the C code so here I am. I'm working with C with no special focus, so the solution can be in C++ too Smile | :)

Well, this is the problem. I have a VB6 program that simply calls a function when the Command1 is pressed. This one:

Public Function SampleSub(ByVal x As Integer, ByVal y As Integer) As Boolean
    MsgBox "Hello world!"
    SampleSub = True
End Function

I'm trying to call it from C like this:

C++

#include "main.h"

typedef BOOL (__stdcall *SampleSubPtr)(WORD, WORD);
static SampleSubPtr SampleSub;

DLL_EXPORT BOOL APIENTRY DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved)
{
    BOOL ret;

    if (fdwReason == DLL_PROCESS_ATTACH)
    {
            SampleSub = (SampleSubPtr)(0x00401B90);
            if (SampleSub == NULL)
                MessageBox(0, "SampleSub is NULL.", "M", 0);
            ret = SampleSub(5, 5);
    }

    return TRUE;
}

0x00401B90 is the pointer to the VB6 function. Then I place this DLL into the VB6 program memory space and the function is called. But it simply doesn't work. Debugging this with OllyDbg gives me this error when calling the function:

Access violation when reading [00000076]

I'm 100% sure that the function is at the address 0x00401B90. VB6 uses __stdcall and I use __stdcall from C. VB6 uses 16-bit integers and I use 16-bit integers (WORD) from C.

I can't solve this. Any ideas?

Thanks in advance! Smile | :)

David Crow23-Dec-13 5:44

David Crow

23-Dec-13 5:44

Is the SampleSub() function in a DLL?

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Arm5523-Dec-13 7:01

23-Dec-13 7:01

Hello. Thanks for the answer. No, the SampleSub() function is inside a VB6 program, an executable. Then I build a C DLL with the code above and inject it on the VB6 process. Sorry if I don't explain that very well.

Thanks again. Smile | :)

Richard Andrew x6423-Dec-13 9:13

23-Dec-13 9:13

How are you determining the address of the VB6 function?

The difficult we do right away...
...the impossible takes slightly longer.

Arm5523-Dec-13 10:08

23-Dec-13 10:08

Hi, thanks for your answer. I'm using:

AddressOf SampleSub

Also OllyDbg gives me that address.

Richard Andrew x6423-Dec-13 16:43

23-Dec-13 16:43

I would experiment with calling other VB6 functions to see if they give the same result. Especially try calling a function that takes no parameters to see if the problem might be the parameter passing.

Have you ever had success calling a VB6 function from C?

The difficult we do right away...
...the impossible takes slightly longer.

Arm5524-Dec-13 4:41

24-Dec-13 4:41

Hi, thanks for your answer. I tried calling a function without parameters and without return value and it doesn't work. This is the VB function now:

Public Sub SampleSub()
    MsgBox "Hello world!"
End Sub

And the new C prototype:

C++

typedef void (__stdcall *SampleSubPtr)(void);

But the same Access violation error (that says OllyDbg).

In the past I was able to call a VB6 function from C, that's why I'm so angry! Frown | :(

But I just can't remeber how I did it. I'm sure I did it like I'm doing now, but I don't know why it doesn't work Hmmm | :|

. Now SampleSub() is at 0x00401BC0.

Do you think that the way of compiling, the language (if C or C++) or something like that is causing errors? Because I'm 90% sure that the code is the same that the one I used in the past to call a VB6 function, so I need to think that I'm compiling wrong or something like that. I'm compiling like this (or that's what CodeBlocks does for me Poke tongue | ;-P

mingw32-g++.exe  -Wall -DBUILD_DLL -O2     -c "C:\Path\main.cpp" -o obj\Release\main.o

And using extern "C" in the main function.

Thanks again Smile | :)

modified 24-Dec-13 10:49am.

Richard Andrew x6424-Dec-13 6:51

24-Dec-13 6:51

I'm afraid I don't know what it could be. However, I had always believed that it was not possible to arbitrarily call into a VB6 executable.

I was under the impression that the old VB worked on a, sort of, virtual execution scheme, and the VB program was under the control of a supervisor thread.

I wish I could be more helpful!

The difficult we do right away...
...the impossible takes slightly longer.

Arm5524-Dec-13 7:11

24-Dec-13 7:11

Ok thank you very much, I'll keep trying and tell you if I could make the trick. If anybody have any idea please tell me!!

Thanks again! Smile | :)

Randor 24-Dec-13 18:12

Randor

24-Dec-13 18:12

Hmmm,

Btw, your original code has several bugs. The Visual Basic 6 Boolean is actually a VARIANT_BOOL[^] from the C++ programmers point of view. This is a difference of sizeof(short) versus sizeof(int). Also... most VB6 data types are actually VARIANTS ^

Best Wishes,
-David Delaune

[edit]
I remembered helping someone with this many years ago... searched a bit and found it:
VC++ & VB Inter Process Communication Plus Introduction To VB Subclassing[^]

Arm5525-Dec-13 9:25

25-Dec-13 9:25

Hi Randor, thank you very much for your reply. I'm actually just using a VB6 "function" with no return value and that takes no parameters. This one:

Public Sub SampleSub()
    MsgBox "Hello world!"
End Sub

With the typedef void (__stdcall *SampleSubPtr)(void); prototype. What should I use instead of void? I think it will be better to start calling a simple function with no parameters, that's why I delete the old function that returns a VB boolean and takes two Integers as it params. Poke tongue | ;-P

I checked out the link your provided but that's for a Variant type in VB6, do you mean I should use VARIANT in C++ for any VB6 type?

I was thinking... I always get the Access violation when reading... error, do you think that a type incompatibility can cause this?

What would you do in my place? This problem is driving me crazy! Frown | :(

Thanks again! Smile | :)

From where start?

Le@rner20-Dec-13 21:15

20-Dec-13 21:15

hi all,

please guide me and provide me a right direction.

currently I m using office automation and random password generation for this but is to time taking and slow process.

please help me to start for ms office file password recovery type application?

thanks in advance.

Richard MacCutchan20-Dec-13 22:05

20-Dec-13 22:05

Le@rner wrote:
please guide me and provide me a right direction.

Learn how to do basic research[^].

Veni, vidi, abiit domum

Le@rner20-Dec-13 22:34

20-Dec-13 22:34

I already do this this search list all recovery application.

not about the how to implement of code for this..

Richard MacCutchan20-Dec-13 22:40

20-Dec-13 22:40

Then I guess you need to try contacting one of the authors of those packages.

Veni, vidi, abiit domum

Le@rner20-Dec-13 22:44

20-Dec-13 22:44

really why those authors disclose their logics..

it means codeproject not help me for this...

Richard MacCutchan20-Dec-13 23:12

20-Dec-13 23:12

Le@rner wrote:
it means codeproject not help me for this...

No, it means we don't have the answer.

Veni, vidi, abiit domum

Le@rner20-Dec-13 23:58

20-Dec-13 23:58

can u please suggest me any other option or way to move on.

Richard MacCutchan21-Dec-13 0:18

how to detect ms office files are password protected?

21-Dec-13 0:18

The only thing I can suggest is more Googling, and maybe some initiative. You also need to consider why passwords exist; if it was easy to crack them then there would be no reason to use them in the first place.

Veni, vidi, abiit domum

Le@rner19-Dec-13 20:26

Re: how to detect ms office files are password protected?

19-Dec-13 20:26

hi all,

is there any option to know the office file is password protected or not?

thanks in advance.

Richard Andrew x6420-Dec-13 9:38

Re: how to detect ms office files are password protected?

20-Dec-13 9:38

Try opening it? Poke tongue | ;-P

The difficult we do right away...
...the impossible takes slightly longer.

Le@rner20-Dec-13 17:23

Re: how to detect ms office files are password protected?

20-Dec-13 17:23

I m using excel and word automation to open files is there any other method to chk file without opening the file is password protected or not.

coz other than excel and word files I m not able to open using automation....

and also if the file open function is inside the loop of random password generator and try with every new password to open the file the program becomes slower.

please help me,

Richard MacCutchan20-Dec-13 22:03