如何确保对象只有一个线程

Question

I have a following code:

class Service {
public:
    void start(); // creates thread which creates window and goes to message loop
    void stop();  // sends WM_CLOSE message to thread window
private:
    HANDLE hMutex;
    HANDLE hThread;
    HWND   hWindow;
};

I want my class to perform behaviour like this:

Service obj;
obj.start(); // new thread
obj.start(); // do nothing because thread already exists

Now I'm stuck on question which handle to cover by mutex:

void Service::start() {
    // check if service is already running
    if (/*!!*/) // what should I check: (hThread != NULL)? or (hWindow != NULL)?
        return; // no need to create another thread
    else
        hThread = CreateThread(...);
}

Answer 1

You can control the state of the thread handle hThread , if it's signaled then it means that the thread is terminated:

DWORD result = WaitForSingleObject( hThread, 0);

if (result == WAIT_OBJECT_0) {
    // the signal is sent and therefore the thread has been terminated
}
else {
    // the signal is not sent and hThread is alive
}

Note that the second parameter is the timeout and need to be set to zero for non-blocking call.

You can inspect the detailed reference

Answer 2

You can simply check if hThread is a valid handle or not:

if (hThread != NULL)
    return;
else
    hThread = CreateThread(...);

CreateThread returns a valid handle if the thread is successfully created, so make sure you have proper handling after your call to CreateThread . You will also need to ensure you initialise hThread to NULL in your constructor:

Service::Service() : hMutex(NULL), hThread(NULL) etc...
{
}

If you were using std::thread instead, you could simply check whether the thread was joinable :

class Service
{
public:
    void start();
private:
    std::thread thread;
};
Service::start()
{
    if (thread.joinable())
        return;
}

Answer 3

I would take the RAII way and create the thread in the constructor and close it in the destructor. The main advantages are that you can't forget to close the service(Even when there's an exception), and that the constructor can only be called once and only by one thread at a time.