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Have app process end if another instance running...

This is a discussion on Have app process end if another instance running... within the Windows Programming forums, part of the Platform Specific Boards category; I'm looking for some C++ code or direction in writing some to close an app if it detects another instance ...

  1. #1
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    Have app process end if another instance running...

    I'm looking for some C++ code or direction in writing some to close an app if it detects another instance running.

    Our little update utility writes a log file, which I'm thinking of using as the clue. I was thinking that before the second instance attempts to write to the .log file, if the MyApp.log is opened, I could just skip out.

    How can I get that done with C++? I'm not a C++ guys, as you may have gathered from other posts, so if there is anything special I have to do to close an app (something needed other than a Me.Close type statement), please let me know.

    Any help GREATLY appreciated!!
    Last edited by Superfreak3; 03-07-2013 at 02:37 PM.

  2. #2
    Programming Wraith GReaper's Avatar
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    You can do a few hacks to make this happen using only C++, using file I/O. But what if someone makes a copy of the exe and runs it from a different directory? How will you know where all the other instances reside?
    On the other hand, all systems provide one way or another to do what you're asking. I certainly know that Windows & Linux do. No portable way though.
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  3. #3
    and the hat of wrongness Salem's Avatar
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    For which operating system?
    Then I can move this thread to the right place.
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  4. #4
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    Common solution for Win32:

    Method1.c

    This approach looks for another window with the same WNDCLASS name, and this obviously has some flaws/limitations: Your program may not have a window (maybe it's a service running in the background) or there might be another window from another program with the same name.

    If it's a no to both of the above, i'd say that would be the easiest method. But since im not clairvoyant and you keep refering to your program as an 'app', there is a fair chance you're not using Windows, in which case this solution is....suboptimal.
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    I think the most portable (at least for the large number of systems that support it) would be to try to bind to some fixed port on the local interface 127.0.0.1. sure, there's the chance of collision with other processes that may try to bind to that port, but for something that works across many platforms, I think this would be the most likely to succeed.
    Neo1 likes this.

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    It's a little widget that runs on Windows system startup to check if our Client app needs to be updated. I wouldn't worry too much about the .exe being copied and run elsewhere. The app just checks a local .ini and an .ini file on the server for versions to trigger the call to Winodws Installer packages (.msi, .msp). It runs silently - no interface.

    The reason I want to make this change is for some reason, on an end users XP systems, it seems that the widget is running twice. It looks like there are duplicate entries in the .log that is created. I'm just trying to prevent this from occuring.

    I was thinking that I could check to see if the .log file is open and if so, dump out.

    Again, I'm no C++ guy so take it easy on me

  7. #7
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    Quote Originally Posted by Superfreak3 View Post
    It's a little widget that runs on Windows system startup to check if our Client app needs to be updated. I wouldn't worry too much about the .exe being copied and run elsewhere. The app just checks a local .ini and an .ini file on the server for versions to trigger the call to Winodws Installer packages (.msi, .msp). It runs silently - no interface.

    The reason I want to make this change is for some reason, on an end users XP systems, it seems that the widget is running twice. It looks like there are duplicate entries in the .log that is created. I'm just trying to prevent this from occuring.

    I was thinking that I could check to see if the .log file is open and if so, dump out.

    Again, I'm no C++ guy so take it easy on me
    Something from the past, I first posted it over 10 years ago....

    You should just remove the messagebox bit and exit.

    Your app should call CloseHandle(hMapping) when it exists.


    Code:
    hMapping = CreateFileMapping(INVALID_HANDLE_VALUE, NULL,PAGE_READONLY, 0, 32, szAppName);
    
    if (hMapping)// Check to see if app is already running
    {
        if (GetLastError() == ERROR_ALREADY_EXISTS)
        {
    	sprintf(sBuffer,"%s is already running. Create another instance?",szAppName);
    	if(MessageBox(NULL, sBuffer,"Error",MB_ICONERROR| MB_YESNO)==IDNO)
               {
                    CloseHandle(hMapping);
    		ExitProcess(1);
               } 
        }
    } 
    else
    {
       MessageBox(NULL, "Error creating file mapping.", "Error",MB_ICONERROR| MB_OK);
       ExitProcess(1);
    }

    Other methods;

    How do you only allow one instance of an app?
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  8. #8
    Algorithm Dissector iMalc's Avatar
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    On Windows I would just use a mutex. If it can't get the mutex on startup then it should quit straight away.

    If you choose some other solution, make sure it is also one that doesn't end up with the problem where both copies see one-another running and decide to exit.
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  9. #9
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    Quote Originally Posted by iMalc View Post
    On Windows I would just use a mutex. If it can't get the mutex on startup then it should quit straight away.
    To add to this, it is best to pick a mutex with a unique name, to minimise chances of using a mutex that is used by another application (or, if you want to avoid accidents, that another person might guess). A name based on a GUID is handy for this.

    Theoretically, when your program is terminating, the mutex will be cleaned up. It is, however, a good idea to do that explicitly with a call of CloseHandle(). It takes little effort, and accounts for the fact that the difference between theory and practice is often the practice.
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  10. #10
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    I'd third what iMalc and Grumpy said. I'd use a mutex (Mutual Exclusion Synchronazation Object). I use those all the time to prevent two instances of a running program from accessing the same flat file not designed as multi-use. I name the mutex based on the file name.

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