Re: shared DLL VS static Link, Are they different?

From:

"Giovanni Dicanio" <gdicanio@_NOSPAM_email_DOT_it>

Newsgroups:

microsoft.public.vc.mfc

Date:

Sun, 3 Aug 2008 18:39:08 +0200

Message-ID:

<OQ96QgY9IHA.3612@TK2MSFTNGP04.phx.gbl>

As Joe and Doug pointed out, that library has problems with DllMain and
thread local storage, etc.

I did some stepping in the code.

The problem is that that library is a mainly-C-interface library (even if
the sources are .cpp), and there is a structure called "CvContext" in that
library.

The authors provided internal functions like icvCreateContext(),
icvDestroyContext(), and icvGetContext() - see file cxcore\src\cxerror.cpp
(where DllMain is also defined).

The problem is that this "context" management is a real mess, and if you try
to log the calls to icvCreateContext() and the calls to icvDestroyContext(),
you will see that they do not match!

I saw that inserting a custom code in icvCreateContext() and
icvDestroyContext() bodies, this custom code basically uses
OutputDebugString to trace message like "icvCreateContext - CvContext
allocated at %08x <memory address here...>").

The problem is that icvGetContext is badly designed IMHO (with some
confusing #ifdef's, too), and there is a mix of TlsGetValue(), and also
calls to icvCreateContext from icvGetContext...

There are global variables defined in that DLL, and their constructor calls
icvGetContext - you can see that using call stack:
CvType constructor calls
cvGetErrStatus calls
icvGetContext

It may happen that icvGetContext is called like shown above *before* DllMain
(because the global variables are constructed before DllMain is called).

If you put breakpoints you can see how bad and non-linear the code flow is
to get these "CvContext"s...

And if you use proper tracing, you will see that *not all* the instances of
CvContext created on the heap using malloc() are properly destroyed!
(BTW: this library is a C++ library, but uses malloc instead of new...)

In general, I think that DllMain is a poor place to do initialization and
cleanup - better providing external public functions - and if there is some
"context" state to be shared by DLL functions, better passing it explicitly
(or use a C++ library, so the "state" is inside the class body).

A couple articles about DllMain...

http://blogs.msdn.com/oleglv/archive/2003/10/24/56141.aspx

http://blogs.msdn.com/larryosterman/archive/2004/04/23/118979.aspx

HTH,
Giovanni

"asm23" <asmwarrior@gmail.com> ha scritto nel messaggio
news:g73pir$kcu$1@aioe.org...

hi, everyone, I meet a strange problem:
I'm using VC6 and create a dialog based applications. And I want to add a
third part library( Open computer library--OpenCV AS DLL). I linked with
the cxcored.lib. My APP will call functions in cxcore001d.dll.

I only add two functions in the OnInitDialog function:
/////////////////////////////////////////////////////////////////
BOOL COpenCVdialogDlg::OnInitDialog()
{
    ......
    m_iplImage = cvCreateImage(ImgSize,IPL_DEPTH_8U,IMAGE_CHANNELS);
    cvReleaseImage(&m_iplImage);
    ......
}
////////////////////////////////////////////////////////////////
In the code above, I only create an Image and release it quickly, which
seems I do Nothing. ^_^ ,The two functions are exported from
cxcore100d.dll( associated with cxcored.lib)

Here comes my problem. When I build the APP with static MFC library, the
program works fine. But When I build the APP with "Using MFC as shared
DLL" option, there are many memory leak report when I exit the APP.

It is very strange that "using MFC as shared DLL" and "using MFC as a
static library" are much different? Otherwise, There are something wrong
with the source code generating cxcore100d.dll?

By the way, OpenCV is an open source library,so, I have the full source
code of "cxcore100d.dll" , I examine the code and can't find anything
wrong.
Here is its DLL Main entry:
////////////////////////////////////////////////////////////////
#if defined WIN32 || defined WIN64
BOOL WINAPI DllMain( HINSTANCE, DWORD fdwReason, LPVOID )
{
    CvContext *pContext;

    switch (fdwReason)
    {
    case DLL_PROCESS_ATTACH:
        g_TlsIndex = TlsAlloc();
        if( g_TlsIndex == TLS_OUT_OF_INDEXES ) return FALSE;
        //break;

    case DLL_THREAD_ATTACH:
        pContext = icvCreateContext();
        if( pContext == NULL)
            return FALSE;
        TlsSetValue( g_TlsIndex, (LPVOID)pContext );
        break;

    case DLL_THREAD_DETACH:
        if( g_TlsIndex != TLS_OUT_OF_INDEXES )
        {
            pContext = (CvContext*)TlsGetValue( g_TlsIndex );
            if( pContext != NULL )
                icvDestroyContext( pContext );
        }
        break;

    case DLL_PROCESS_DETACH:
        if( g_TlsIndex != TLS_OUT_OF_INDEXES )
        {
            pContext = (CvContext*)TlsGetValue( g_TlsIndex );
            if( pContext != NULL )
                icvDestroyContext( pContext );
        }
        TlsFree( g_TlsIndex );
        break;
    default:
        ;
    }
    return TRUE;
}
////////////////////////////////////////////////////////////////
I was confused that why the "break" statement is commented after the
"case DLL_PROCESS_ATTACH:"?

Further more, the cvCreateImage and cvReleaseImage function in my APP is
regularly simple, they only allocate a piece of memory and delete it.

Thanks for reading my Post!

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