[SOLVED] VB6: Single-instance application across all user sessions

VB6: Single-instance application across all user sessions

I have an application that needs to be a single-instance app across all user sessions on a Windows PC. My research thus far has centered around using a mutex to accomplish this, but I am having an issue that I am not sure is really an issue, this is really a best-practice question I believe.

Here's the code first of all:

Private Const AppVer = "Global\UNIQUENAME" ' This is not what i am using but the name is unique

Public Sub Main()

    Dim mutexValue As Long

    mutexValue = CreateMutex(ByVal 0&, 1, AppVer)
    If (Err.LastDllError = ERROR_ALREADY_EXISTS) Then
        SaveTitle$ = App.Title
        App.Title = "... duplicate instance."
        MsgBox "A duplicate instance of this program exists."
        CloseHandle mutexValue
        Exit Sub
    End If
    ' Else keep on truckin'

Now, based on this article I believe I understand that by passing the NULL pointer to the CreateMutex function as I am above I'm basically assigning whatever security descriptor is associated with the currently logged in user.

If that means what I think it does (I may need more guidance here) that tells me that other users who log in will not be able to "see" the mutex created under the original user's session, nor will they be able to create a mutex with the same name.

Now, emperical evidence seems to back this up. I used a message box to pop the "LastDLLError" I was receiving, and when another user attempted to launch the application (while it was already running under another user account) I would receive an ERROR_ACCESS_DENIED code. I am OK with testing against this along with the ERROR_ALREADY_EXISTS code and just exiting on either/or. However, this feels sort of hackish and I'm wondering if someone can suggest an alternative. The "right" thing to do seems to be to pass the proper pointer to the CreateMutex function such that any user has the proper permissions to view any existing mutexes (mutices?), but I'm not so sure this is possible without the currently logged in user being an admin (which is unacceptible). Any assistance/guidance is greatly appreciated. Thanks in advance!

Solution

I was looking for a similar solution in VB6 late last year. At the time I was unable to find any examples of VB6 apps communicating across the user boundary, so I had to write my own.

See: Interprocess Communication via Semaphores

You can use the class to create and check for a global semaphore which will tell you if your app is already running under any user. I didn't look at the Mutex APIs but their usage is very similar. The GetSecurityDescriptor function is what you'll want to transpose if you've already got some Mutex code written.