How to block running two instances of the same program?

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There are several methods you can use to accomplish only allowing one instance of your application:

Method 1: Global synchronization object or memory

It’s usually done by creating a named global mutex or event. If it is already created, then you know the program is already running.

For example in windows you could do:

    #define APPLICATION_INSTANCE_MUTEX_NAME "{BA49C45E-B29A-4359-A07C-51B65B5571AD}"

    //Make sure at most one instance of the tool is running
    HANDLE hMutexOneInstance(::CreateMutex( NULL, TRUE, APPLICATION_INSTANCE_MUTEX_NAME));
    bool bAlreadyRunning((::GetLastError() == ERROR_ALREADY_EXISTS));
    if (hMutexOneInstance == NULL || bAlreadyRunning)
    {
        if(hMutexOneInstance)
        {
            ::ReleaseMutex(hMutexOneInstance);
            ::CloseHandle(hMutexOneInstance);
        }
        throw std::exception("The application is already running");
    }

Method 2: Locking a file, second program can’t open the file, so it’s open

You could also exclusively open a file by locking it on application open. If the file is already exclusively opened, and your application cannot receive a file handle, then that means the program is already running. On windows you’d simply not specify sharing flags FILE_SHARE_WRITE on the file you’re opening with CreateFile API. On linux you’d use flock.

Method 3: Search for process name:

You could enumerate the active processes and search for one with your process name.

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