Public member not initialized

Question

I am making a client server application, with the server having a GUI. I am using Qt. For communication I am using pipes.

I have divided the server application into a backend, and a GUI. The backend has a PipeServer class, and in the GUI, I have overriden functions like onReceiveMessage etc.

Everything worked fine until I decided to add a std::queue as a base class member.

At the start of the application, I get an exception, and upon inspection it seems that my queue does not start with 0 elements. In fact it seems like the queue is not initialized at all. There are 2 possibilites: it could be because I the GUI class inherits 2 classes, and somehow the second base class, which is my PipeServer does not properly initialize its members, or it could be because the pipeServerGUI object is moved to a different thread by QT.

Any ideas on how I could solve this?

Relevant code:

class HookServer
{
    PIPEINST Pipe[INSTANCES]; 
    HANDLE hEvents[INSTANCES]; 

    VOID DisconnectAndReconnect(DWORD); 
    BOOL ConnectToNewClient(HANDLE, LPOVERLAPPED); 
    VOID GetAnswerToRequest(LPPIPEINST); 

public:
    std::queue<std::string> messages;

    int init(std::string pipename);
    int run();
    virtual void onNewConnection() {};
    virtual void onReceiveMessage(std::string message) {};
};

class HookServerGUI : public QObject, public HookServer
{
    Q_OBJECT
    void onReceiveMessage(std::string message);
    void onNewConnection();

    public slots:

    void doWork() {
        init("\\\\.\\pipe\\hookpipe");
        run();
    }

    signals:
    void signalGUI(QString message);
};

    //GUIServerCreation

    QThread *thread = new QThread;
    HookServerGUI* worker = new HookServerGUI;
    QObject::connect(worker,SIGNAL(signalGUI(const QString&)),this,SLOT(processMessage(const QString&)));

    worker->moveToThread(thread);
    thread->start();



    QMetaObject::invokeMethod(worker, "doWork", Qt::QueuedConnection);

EDIT:

The exception is a access violation exception. It happens in this part of code:

VOID HookServer::GetAnswerToRequest(LPPIPEINST pipe)
{
    onReceiveMessage(pipe->chRequest);
    if(!messages.empty())
    {
        std::string s = messages.front();
        messages.pop();
        strcpy(pipe->chReply,s.c_str());
        pipe->cbToWrite = strlen(s.c_str()+1);
    }
}

Since messages.empty() return some huge number, it tries to read the first object and somehow fails.

There is also no PipeServerGUI constructor.

EDIT2:

I solved part of this problem by placing parenthesis after new HookServerGUI();

The problem is that still the function does not work, and throws a access violation exception. It happens on the front() line. When checked in a debugger, the function does have 1 element, so it is not because it is empty. Any ideas?

EDIT3:

With the second run, unfortunately the queue.size() is still incorrect. Seems like a data race to me.

Answer 1

The problems are in the code that you don't show, and it's a classic case of a memory bug, it looks like. Some code somewhere is writing on memory it doesn't own. Probably you have a bug in the way you use winapi. You need to create a minimal, self-contained test case.

I think you might be shooting yourself in the foot by not using QLocalSocket: on Windows, it's a named pipe - exactly what you want.

Besides, this is C++ code. There is no reason at all to put either PIPEINST or HANDLE into a raw C array. Use QVector or std::vector. Probably the rest of the code is full of C-isms like that, and something somewhere goes wrong.

I wouldn't discount a buffer overrun, since obviously you are ignoring the size of the buffer in PIPEINST from the - the strcpy can overrun the buffer. I'm also not sure that PIPEINST from the example code is using the same character type as what std::string::c_str() is returning.

Even if you wanted to implement your code using explicit pipes without QLocalSocket, you should still use C++, QString etc. and understand what's going on with your data.