Only one instance of application.

This is a discussion on Only one instance of application. within the Windows Programming forums, part of the Platform Specific Boards category; I must forbid the running of another instance of my app if itsd already running. I tried if (hInstance == ...

  1. #1
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    Only one instance of application.

    I must forbid the running of another instance of my app if itsd already running. I tried if (hInstance == hPrevInstance) but that obviously didnt work.

    How can this be achieved?
    Thanks
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  2. #2
    Registered User Codeplug's Avatar
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    One way:
    Create a named event using CreateEvent(). When another instance of your app runs, you can detect if the event has been created.
    Code:
    #include <iostream>
    using namespace std;
    
    #include <windows.h>
    
    int main()
    {
        HANDLE appRunningEvent = CreateEventA(0, FALSE, FALSE, "MyApp");
        if (!appRunningEvent)
        {
            cerr << "CreateEvent() failed: " << GetLastError() << endl;
            return 1;
        }//if
    
        if (GetLastError() == ERROR_ALREADY_EXISTS)
        {
            cout << "An instance of this app is already running." << endl;
            return 2;
        }//if
    
        cout << "No other instances running." << endl;
        
        Sleep(30 * 1000);
    
        CloseHandle(appRunningEvent);
    
        return 0;
    }//main
    gg

  3. #3
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    Ah sounds logical to me! Cool, thanks. Is this thread safe, though?
    But what if the application crashes before the CloseHandle is called? No other application will be allowed to run, as the event will still exist?
    Last edited by aker_y3k; 06-01-2005 at 11:17 AM.
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  4. #4
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    When your applications exits (or crashes) windows will free up any resources used by your program.

  5. #5
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    Roger, it works, thanks a lot.
    Y3K Network http://www.y3knetwork.com
    Bringing the software of the future on the net.

  6. #6
    Registered User Codeplug's Avatar
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    >> Is this thread safe, though?
    Only one thread in your application should be doing this anyways. If two of your processes call CreateEvent() at the same time, one of them should always return ERROR_ALREADY_EXISTS.

    gg

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