C / C++ / MFC

17-Jan-07 5:44

Hi,
I have a doubt regarding memory sharing in a COM object.
Background: Recently we shifted to VS2005. Hence the dll written in VC6 is no longer directly useful. I might have to write a COM wrapper to access this dll. The application would be working with huge files and I want only a single copy maintained across my application. The dll should actually read the file and after the error checking send the a pointer of the memory to my application.
My question:
1) Is it possible to do it with COM?
2) What would happen if my COM object memory is on a remote server?
3) Does the COM object copy the memory and send it to the requested application or does it just pass the reference.
My intention is to maintain a single place of memory where the file data would be read and used.
Please let me know how can I achieve this through COM. Any links would be useful.

Regards,
Arti Gujare

led mike17-Jan-07 5:53

17-Jan-07 5:53

ArtiGujare wrote:
Recently we shifted to VS2005. Hence the dll written in VC6 is no longer directly useful.

I don't understand that? Your old C++ code should be able to still do the following in a VS2005 C++ project.

ArtiGujare wrote:
The dll should actually read the file and after the error checking send the a pointer of the memory to my application.

led mike

jayart17-Jan-07 6:00

17-Jan-07 6:00

When I said we shifted to VS2005 I meant that the application using the dll was previosly written in VC6 and dll too was written in VC6. Now the application has to be written in VS2005 and still use the VC6 dll. My typical use case is:
1) Application send the file name to the dll.
2) dll reads the file at that mentioned filepath and does some file checks
3) if the file is in expected format, the dll returns the pointer pointing to the file data to the applictaion.
I hope it looks clear now.

led mike17-Jan-07 6:23

17-Jan-07 6:23

I see no information in there that would keep you from accomplishing that using VS2005 C++ projects.

led mike

jayart17-Jan-07 6:43

17-Jan-07 6:43

What about :
2) What would happen if my COM object memory is on a remote server?
3) Does the COM object copy the memory and send it to the requested application or does it just pass the reference.

led mike17-Jan-07 6:57

17-Jan-07 6:57

For (3): You cannot pass a reference to memory across machines, that makes no sense at all. Therefore the memory is copied.

I have no idea why you are fixated on COM. Nothing in your requirments demands using COM. Without a requirement I would not choose to include the complexity and overhead associated with COM. In other words I would only use COM as a last resort.

led mike

jayart17-Jan-07 7:11

17-Jan-07 7:11

Hi Led,
I tried adding the VC6dll in my VS2005App using "Add Reference". I could not add it. I got the message
"A reference to "<dllpath>" could not be added. Please make sure that the file is accesible, and that it is a valid assembly or COM component"
I googled around a bit with this message and I found that I cannot simply use the VC6dll in my VS2005App. Many links suggested that I might have to wrap the dll in a COM component in order to use it. Hence the COM usage. Otherwise if there is an alternative to bypass COM wrapping I would be more than happy to do the same.
Any suggestions?

Thanks and Regards,
Arti Gujare

Mark Salsbery17-Jan-07 8:48

17-Jan-07 8:48

Is the DLL already implementing a COM object? If so then it should work no problem.
I thought one of the ideas behind com was portable objects.

If it's not a com DLL then there's no "Add Reference" that will load the DLL. You need to link
to it (implicitely or explicitely using LoadLibrary() or similar). If it uses MFC you may have
problems. If it uses the CRT you may have problems as well since you'll have mixed versions
between the app and the DLL.

Mark

led mike18-Jan-07 4:37

18-Jan-07 4:37

ArtiGujare wrote:
I tried adding the VC6dll in my VS2005App using "Add Reference".

"Add Reference" is for adding .NET assemblies in a "Managed" Project. You are in the C++ forum indicating that your project is a "Native" project. Therefore you do not add a reference to the DLL. You link to it using a .Lib file just as before. Using a DLL from a C++ project is pretty basic stuff, why are you having problems with this? Perhaps if you give us more information about your level of experience we might provide more targeted information.

led mike

Problem loading DLL

Eikthrynir17-Jan-07 5:44

Eikthrynir

17-Jan-07 5:44

Hi!
Does anyone know why would an extension DLL not load when using the following code?

if ( AfxLoadLibrary( "MyDLL.dll" ) == NULL )
  AfxMessageBox( "Error loading library." );

So, MyDLL.dll is an extension DLL. AfxLoadLibrary always returns NULL, even though the calling process and MyDLL.dll are in the same directory.

Any help will be greatly appreciated!
Thanks in advance!

Mark Salsbery17-Jan-07 5:51

17-Jan-07 5:51

What does your DllMain look like in the extension DLL? Is initialization failing there?

The app and the DLL are using the DLL version of MFC, correct?

Mark

Eikthrynir17-Jan-07 5:56

Eikthrynir

17-Jan-07 5:56

This is how my DllMain looks like:

extern "C" int APIENTRY
DllMain(HINSTANCE hInstance, DWORD dwReason, LPVOID lpReserved)
{
	// Remove this if you use lpReserved
	UNREFERENCED_PARAMETER(lpReserved);

	if (dwReason == DLL_PROCESS_ATTACH)
	{
		TRACE0("MYDLL.DLL Initializing!\n");

		// Extension DLL one-time initialization
		if (!AfxInitExtensionModule(MyDLL, hInstance))
			return 0;

		new CDynLinkLibrary(MyDLL);
	}
	else if (dwReason == DLL_PROCESS_DETACH)
	{
		TRACE0("MYDLL.DLL Terminating!\n");
		// Terminate the library before destructors are called
		AfxTermExtensionModule(MyDLL);
	}
	return 1;   // ok
}

I even put these lines of code right at the begining of DllMain:

CFile log;
log.Open( "log.001", CFile::modeCreate | CFile::modeWrite, NULL );
log.Write( "@", 1 );
log.Close( );

I ran the program using the new version of mydll.dll and NO log.001 was created.
So DllMain does not get executed.

Mark Salsbery17-Jan-07 6:03

17-Jan-07 6:03

You shouldn't need to write a log file. "MYDLL.DLL Initializing!" should show up in your debug
output window. You could put a breakpoint in there as well.

It's a mystery Smile | :)

Have you tried David Crow's suggestion?

Eikthrynir17-Jan-07 5:59

Eikthrynir

17-Jan-07 5:59

Problem solved!!!!!!!!!!
MyDLL.dll was using another dll which did not exist.

Thank you for your time!

Mark Salsbery17-Jan-07 6:04

17-Jan-07 6:04

Eikthrynir wrote:
MyDLL.dll was using another dll which did not exist.

Cool