Preventing multiple instances of an App

[Brad Kartchner]

In case it matters, I'm using Dev-C++ 4.9.8.7 (and LOVING it).

I'm looking for a way to prevent multiple instances of my application from running at the same time.  Instead of starting a new instance of an application if one is already running, I need the original window to popup automatically. 

I have found a few code examples for doing this, but most of them  require using MFC.  I did find an article on doing this on developerfusion titled "Avoiding Multiple Instances of an Application" (http://www.developerfusion.com/show/1716/), which has a very clear example using a named mutex to detect multiple instances, and a shared data segment to store the handle of the main window.  This is done with the following code:

#pragma comment(linker, "/SECTION:.shr,RWS")
#pragma data_seg(".shr")
HWND hGlobal = NULL;
#pragma data_seg

Using the code in this article works to the extent that a second instance of my application will NOT start, but I have been unable to have the original instance's window popup like it should.  I suspect it has to do with the #pragma directives (which I freely admit, I do NOT understand). 

Can anyone help me?  Is there another way to do this or am I out of luck?

[Nobody/Anonymous]

    // Verify that we are not already running...

        // Create mutex...
        CreateMutex(NULL, TRUE, "MyProgram");

            // Already running, terminate...
            if(GetLastError() == ERROR_ALREADY_EXISTS)
                ExitProcess(1);

=)

Kip

[Derek Baker]

I implemented this myself just a few days ago.

HANDLE Mutex = CreateMutex(NULL, TRUE, "DBClip");

       if (GetLastError() != ERROR_SUCCESS)
       {
        CloseHandle(Mutex);
         
        return 0;   
       }

Change DBClip to what ever you want, as long as it's likely to be unique.

I put this at the start of WinMain, so the return terminates the program.

Derek

[Derek Baker]

Two points:

The CloseHandle(Mutex); in my mine isn't really necessary, as Windows will delete the mutex when the app terminates.

Some people say check for ERROR_SUCCESS as I do, rather than ERROR_ALREADY_EXISTS, for example: http://tinyurl.com/23fsr

Derek

[qWake]

Your application will register its own window class, right?  Then use something like this in WinMain:

HWND existingAppWindow = FindWindow("MyClassName", 0);
if (IsWindow(existingAppWindow)) {
    if (IsIconic(existingAppWindow)) {
        ShowWindow(existingAppWindow, SW_RESTORE);
    }
    BringWindowToTop(existingAppWindow);
    SetForegroundWindow(existingAppWindow);
}

qWake

[Derek Baker]

There are those that say use don't use FindWindow: http://tinyurl.com/2rz4o

Derek

[qWake]

Good point, I never thought of the race condition.  I shall change my evil ways (and a couple of program lines).

qWake

[Brad Kartchner]

So, does anyone know if it's possible to use a shared memory segment in Dev-C++, as the tutorial tries to do?

[Nobody/Anonymous]

Yes. Just use GlobalAlloc() for a structure of some sort.

Kip

[Liviu Nicolescu]

It seems that it's possible to use shared sections with gcc:

int foo __attribute__((section ("shared"), shared)) = 0;

quoted from

http://gcc.gnu.org/onlinedocs/gcc-3.4.0/gcc/Variable-Attributes.html#Variable%20Attributes

Liviu

[Brad Kartchner]

That's insanely usefull and exactly what I was looking for. I didn't even know that site existed.  Thanks to everyone for the responses!

[Nobody/Anonymous]

so cool!