Calling c++ dll's from visual basic

Hi,

I was given a dll function,

extern "C" {
DWORD MAGOCRTG_API InsertOCRTag( char   *lpTiffFileName,
                                                                 char   *lpTextFileName,
                                                                 DWORD  dwPagePointer);
}

The lpTextFileName is a string, lpTiffFileName is also a string and dwPagePointer should always be zero

I am supposed to be calling the above function from my vb application.

This is what I did.  Created a new vb project, added a module, and this line of code

Public declare function Merge Lib "C:\Program Files\GaussInterprise\DocView\MagOCRTg.dll" Alias "InsertOCRTag" (ByRef lpTiffFileName as String, ByRef lpTextFileName as String,ByRef dwpagepointer as integer") As Integer.

and inside a new form i m calling the function MERGE(X,Y,Z)

I know the part I am messing up is the datatypes.....Can anyone tell me how I can change my api call??   And wats DWORD, its not a structure , is it a "C" datatype ? Or is it something else I need to do apart from this?

Any help will be greatly appreciated

Thanks

-Paulin
shahpaulinAsked:
Who is Participating?

[Product update] Infrastructure Analysis Tool is now available with Business Accounts.Learn More

x
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

AzraSoundCommented:
DWORD is a double word, you can just use a Long in VB.  Declare it as ByVal instead of ByRef (its not a pointer).

Regarding strings, thats not the recommended way of passing strings from VB to a C dll, but it sounds like you are stuck with it.  I dont think you can just pass a VB string into a char * data type.  However, you should be able to pass a byte array that contains the ascii values of the characters of the string. This question always seems to come up, and I always forget what is finally implemented regarding the char *.   haha
shahpaulinAuthor Commented:
Hi Azra,

Thanks for your reply.  
Didn't changed nething, jst changed the integer to long.  And it worked.
Seems char * or string isn't a problem or atleast for now =), coz the
function is doing wat it is supposed to.

-Paulin
g0rathCommented:
Confir with this website

http://www.flipcode.com/articles/article_vbdlls.shtml

Strings aren't really that difficult after you understand that Microsft hides the details.

You should be using BSTR which is compatible with visual basic, when you typecast to a char *, the object will hide the details and give you a string to use inside your program.

But to pass a string back to Visual basic you cannot send a char *, you have to allocate new memory and return a BSTR.

BSTR __declspec(dllexport) CALLBACK StringExample(BSTR stringVar)
{
      LPSTR buffer;
      buffer = (LPSTR)stringVar;
      ::MessageBox(NULL,buffer,"in C++",0);
      buffer = _strrev(buffer);
      return(SysAllocString(stringVar));
}


This is from the example website above....I wrote a wrapper around my existing C/C++ functions specifically for a VB interface, so that my functions do not change.

by the way the LPSTR is Long Pointer to a String or rather a char *
the BSTR is actually a wchar_t * since it allows for I18N for windows...there are a series of functions that you can use to move them around from wide to narrow strings. For more info checkout the MSDN site.
Determine the Perfect Price for Your IT Services

Do you wonder if your IT business is truly profitable or if you should raise your prices? Learn how to calculate your overhead burden with our free interactive tool and use it to determine the right price for your IT services. Download your free eBook now!

shahpaulinAuthor Commented:
Well I don't know what to do?  I found a solution by my self.
Azra's answer kinda helped......
ne suggestions?

can i award half the points to azra only?
AzraSoundCommented:
Post a question in Community Support with your request.

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
leonstrykerCommented:
No comment has been added lately, so it's time to clean up this TA.
I will leave a recommendation in the Cleanup topic area that this question is:

Answered  AzraSound
 
Please leave any comments here within the next seven days.

PLEASE DO NOT ACCEPT THIS COMMENT AS AN ANSWER!

leonstryker
EE Cleanup Volunteer
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
Microsoft Development

From novice to tech pro — start learning today.