Multithreaded Qt Application Crashing Windows

Well The problem is quite complex. I don't know where to start. My Application runs 4 threads DGRDPClientThread.

a DGRDPClientThread has 1 DGRegion which has QList<DGRect*>.

at 1000 ms Interval DGRDPClientThread::tick() is invoked which calls _region->update()

void DGRegion::update() const{
    foreach(DGRect* rect, rects){
        if(rect->scan() != 0){
            mutex.lock();
            QByteArray buffer = rect->serialize();
            //socket->send(buffer);
            qDebug() << buffer;
            mutex.unlock();
        }
    }
} 

rect->scan() reads a rectangul开发者_开发百科ar area of screen in a char* buffer and then put it a QByteArray if current buffer is same as old buffer it retruns 0 othewise a non-zero value.

My Problem is: It runs properly and qDebug() prints buffer for few minutes. and then suddenly it stops. The whole Window freezes. and My application crashes. I don't think its due to memory leak. I deleteed whenever I called new and this is the only mutex in the whole app. Task Manager says it doesn't makes Huge Mem Use However while Freezing it takes Huge CPU

I don't know Why I cant debug this app. It says Application Started in debug Window. Its Its not started actually. and I dont see my threads in the thread drop down box ...

EDIT

int DGRect::scan(){
    HDC hdc=GetWindowDC(NULL);
    HWND win=WindowFromDC(hdc);

    HDC cdc=CreateCompatibleDC(hdc);
    HBITMAP temp=CreateCompatibleBitmap(hdc,width,height);
    PAINTSTRUCT ps;

    hdc=BeginPaint(win,&ps);
    HBITMAP oldb=(HBITMAP)SelectObject(cdc,temp);
    BitBlt(cdc,0,0,width,height,hdc,top,left,SRCCOPY);
    SelectObject(cdc,oldb);
    EndPaint(win,&ps);

    char* buff;
    buff = new char[size()];
    GetBitmapBits(temp,size(),buff);
    QByteArray returnBuff(buff, size());
    if(returnBuff != buffer){
        buffer.clear();
        buffer = returnBuff;
        delete[] buff;
        qDebug() << ">> \t\t\t\t UnMatched";
        return 1;
    }
    delete[] buff;
    qDebug() << ">> \t\t\t\t\t\t MATCHED";
    return 0;
}

QByteArray DGRect::serialize() const{
    QByteArray buff;
    QTextStream stream(&buff, QIODevice::ReadWrite);
    stream << row << col << left << top;
    //stream << buffer;
    //stream << qCompress(buffer, 8).toBase64();
    stream.flush();
    return QByteArray::number(row)+" "+QByteArray::number(col)+" "+QByteArray::number(left)+" "+QByteArray::number(top)+" "+QByteArray::number(buffer.size())+";";
    return buff;
}