C++ using COM

From:
Aidal <no@address.com>
Newsgroups:
microsoft.public.vc.language
Date:
Mon, 11 Dec 2006 12:21:41 +0100
Message-ID:
<eAAaGaRHHHA.960@TK2MSFTNGP04.phx.gbl>
Hi NG.

I have a little problem with a C++ DLL.

I have a flow like this:
------------------------
SomeWin32App -> C++ DLL -> COM

When the App opens the C++ DLL (gets a handle for it) the DLL
instantiates an object of the COM component right away.

The idea is that the App opens the C++ DLL and calls one of it's
functions which then calls a function from the instantiated COM component.

Once the App is done with the C++ DLL it closes the handle for it.

All of this works fine...

The problem is that if the App:
-------------------------------
1) opens the dll (gets a handle for it)
2) calls a function in the C++ DLL.
3) calls a function in the C++ DLL.

It appears that between calls the C++ DLL creates new instances of the
COM component instead of reusing the one already instantiated.

I can see this because one of the tasks of the COM component is to use FTP.

i.e.
- open a connection with some login info
- change to some dir
- upload a file
- download a file
- close the connection

If the App uses the C++ DLL to first (1) open a ftp connection and then
  (2) uplaod a file, then (2) will fail because it doesn't have an open
connection eventhough it was just created in step (1).

The COM component works as supposed because I've tested it in many ways
and an object is able to perform all kinds of FTP commands once a
connection is opened.

It's the C++ DLL that doesn't seem to reuse the instantiated object of
the COM class for some reason.

Here is some of the code:
(Try to ignore any code looking anciant like FAR PASCAL because it's
required by the App.)
------------------------------------------
char* buffer = 0;
const int STRMAX = 255;

MyInterop::IFtpCmdParserPtr pFtpCmdParser;

#ifdef __cplusplus
extern "C"
{
    __declspec(dllexport) char* FAR PASCAL FtpCommand(long, unsigned char*);
}
#endif

BOOL APIENTRY DllMain(HANDLE hModule, DWORD ul_reason_for_call, LPVOID
lpReserved)
{
     CoInitialize(NULL);

    HRESULT hRes = pFtpCmdParser.CreateInstance(MyInterop::CLSID_FtpCmdParser);

    switch (ul_reason_for_call)
    {

        case DLL_THREAD_ATTACH:
        case DLL_THREAD_DETACH:
            return 1;

        case DLL_PROCESS_ATTACH:

            buffer = (char*)malloc(STRMAX* sizeof(char));

            if(buffer == NULL)
            {
                CoUninitialize ();
                return 0;
            }
            else
            {
                return 1;
            }

        case DLL_PROCESS_DETACH:
            free(buffer);
            return 0;
     }
     CoUninitialize ();
    return 0;
}

_declspec(dllexport) char* FAR PASCAL FtpCommand(long dummy, char* args)
{
    USES_CONVERSION;

    _bstr_t bstrB(args);

    BSTR b = bstrB.copy();

    BSTR* bp = &b;

    pFtpCmdParser->ParseCommand(dummy, bp);

    char* char_p = W2A(c);

    return strcpy(buffer, char_p);
}
------------------------------------------
The code works fine but as described each call to FtpCommand appears to
be using their own instance of the COM class which wasn't the idea.

If anyone has experience with this and have ideas as to why this is
happening, it would be much appreciated.

PS. I'm no C++ wonder, so try to keep any reply without too many C++
specific terms unless acompanied by some links, thanks :)
(I'm a C# kinda guy myself).

/Aidal

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