Preventing program running more than once

[Martin Francom]

I have a program that I want to ensure only runs once. That is
only one copy is running on any of the network computers at any
one time. The program is always stored in the on only one of the
network computers and is alway in the same directory.
.
What I figured I would do, at the start up of the program, is
to open a file for exclusive access and have an error check to
see if the file can not be opened which would indicate that the
program is already running. If this occurs the program (second one)
will immediately end.
.
This approach seams to be working to prevent a network user from
running the program when someone else is running it.
.
Does anyone see any problems with this approach?

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[Roger Garstang]

Just when/if the app crashes or hangs the file may still be locked.

Still an alright method though. A better since it is networked
anyway is to setup a server app they connect to and perhaps only
one IP is allowed to run. Maybe even a time period that sends
a test to the app to see if it is still active too.

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If you aim at nothing...you will hit it.

[Trento Castricone]

FUNCTION PBMAIN
    LOCAL lpClassName AS ASCIIZ * 255
    LOCAL hMutex AS LONG
    '
    lpClassName = "PrintFileList"
    IF App_PrevInstance(lpClassName) THEN
         MSGBOX """Print Directory"" is already running !",,"Print File List Best"
         EXIT FUNCTION
    END IF
    hMutex = CreateMutex(BYVAL %Null, 0, lpclassName)
.
.
.
.

------------------
Trento Castricone
http://www.raineyday.com
mailto:[email protected][email protected]</A>

[Russ Srole]

I use Trento's method, however if one user is local & another logs on via Windows Remote desktop & tries to run you software, he/she will succeed. I'm looking into an alternate approach. If I make it work I'll post code.

Russ

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[Scott Turchin]

EDITED: OOps - saw you were lo oking for network use.
A Lock file is about the simplest way to go......
Maybe have it write a PID file like Apache does, put the PID inside or put the computername in a file.....

With the help of other coders in teh late 90's we put this together, uses a mutex.

Declare Function App_PrevInstance(lpclassName As Asciiz) As Long
'
'

Function PBmain() 
Local hMutex as Dword 'Or long works
szClassName = "MYAPP"
g_szMine is my appname
g_szCCS is my company name
If IsTrue App_PrevInstance(szClassName) Then
    MessageBox ByVal 0, "Another instance of " + g_szMine + " is already running!", ByVal StrPtr(g_szMine), ByVal %MB_ICONSTOP
    Exit Function
Else
    hMutex = CreateMutex(ByVal %Null, 0, szClassName)
End If
CloseHandle hMutex
End function


'
'
'
Function App_PrevInstance(lpclassName As Asciiz) As Long
Local hMutex As Long
hMutex = CreateMutex(ByVal %Null, 0, lpclassName)
If hMutex = 0 Then Exit Function ' Error in Mutex
If GetLastError = %ERROR_ALREADY_EXISTS Or FindWindow(lpClassName, "") > 0 Then Function = %TRUE
CloseHandle hMutex
End Function

------------------
Scott Turchin
MCSE, MCP+I
Computer Creations Software http://www.tngbbs.com/ccs

[This message has been edited by Scott Turchin (edited September 23, 2005).]