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Here another approach to check if a script was already started using atoms and semaphores. Atom: #include <MsgBoxConstants.au3> Global $iSingleton = Singleton() If Not $iSingleton Then Exit MsgBox($MB_TOPMOST, "Singleton Test", "Process is already running!") EndIf MsgBox($MB_TOPMOST, "Singleton Test", "Singleton atom initialized: " & $iSingleton) Singleton_Delete($iSingleton) ; #FUNCTION# ==================================================================================================================== ; Name ..........: Singleton ; Description ...: Checks if the script has been started already. ; Syntax ........: Singleton([$sOccurrenceName = @ScriptFullPath]) ; Parameters ....: $sOccurrenceName - [optional] a string value. Default is @ScriptFullPath. ; Return values .: If the function succeeds, the return value is the newly created atom or 0 else error is set and false is returned. ; Author ........: UEZ ; Modified ......: ; Remarks .......: If Singleton finds the atom it will return 0 and the atom token will be set to extended macro. It can be used to get the atom string using _WinAPI_AtomGlobalGetName. ; Related .......: ; Link ..........: https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-globalfindatomw ; https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-globaladdatomw ; Example .......: No ; =============================================================================================================================== Func Singleton($sOccurrenceName = @ScriptFullPath) Local $iFind = _WinAPI_AtomGlobalFind($sOccurrenceName) If @error Then Return SetError(1, 0, False) If $iFind Then Return SetExtended($iFind, 0) Local $iAtom = _WinAPI_AtomGlobalAdd($sOccurrenceName) If @error Then Return SetError(2, 0, False) Return $iAtom EndFunc ;==>Singleton ; #FUNCTION# ==================================================================================================================== ; Name ..........: Singleton_Delete ; Description ...: Deletes the atom generated by the first started script. ; Syntax ........: Singleton_Delete($iAtom) ; Parameters ....: $iAtom - an integer value which was generated by Singleton ; Return values .: True if successful else false. ; Author ........: UEZ ; Modified ......: ; Remarks .......: Don't forget to call Singleton_Delete before first started script ends. ; Related .......: ; Link ..........: https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-globaldeleteatom ; Example .......: No ; =============================================================================================================================== Func Singleton_Delete($iAtom) _WinAPI_AtomGlobalDelete($iAtom) If @error Then Return SetError(1, 0, False) Return True EndFunc ;==>Singleton_Delete ;internal functions Func _WinAPI_AtomGlobalFind($sAtomString) Local $aReturn = DllCall("kernel32.dll", "short", "GlobalFindAtomW", "wstr", $sAtomString) If @error Then Return SetError(1, 0, -1) Return $aReturn[0] EndFunc ;==>_WinAPI_AtomGlobalFind Func _WinAPI_AtomGlobalAdd($sAtomString) Local $aReturn = DllCall("kernel32.dll", "short", "GlobalAddAtomW", "wstr", $sAtomString) If @error Then Return SetError(1, 0, -1) Return $aReturn[0] EndFunc ;==>_WinAPI_AtomGlobalAdd Func _WinAPI_AtomGlobalDelete($nAtom) Local $aReturn = DllCall("kernel32.dll", "short", "GlobalDeleteAtom", "short", $nAtom) If @error Then Return SetError(1, 0, -1) Return $aReturn[0] = 0 EndFunc ;==>_WinAPI_AtomGlobalDelete Func _WinAPI_AtomGlobalGetName($nAtom, $iBufferSize = 512) Local $tBufferAtom = DllStructCreate("wchar name[" & $iBufferSize & "]") Local $aReturn = DllCall("kernel32.dll", "uint", "GlobalGetAtomNameW", "short", $nAtom, "struct*", $tBufferAtom, "int", $iBufferSize) If @error Or Not $aReturn[0] Then Return SetError(1, 0, -1) Return $tBufferAtom.name EndFunc ;==>_WinAPI_AtomGlobalGetName Semaphore: #include <MsgBoxConstants.au3> #include <WinAPIError.au3> Global $iSingleton = Singleton("&]8h/x87</htFV4-K*&.b.w~") If Not $iSingleton Then Exit MsgBox($MB_TOPMOST, "Singleton Test", "Process is already running!") EndIf MsgBox($MB_TOPMOST, "Singleton Test", "Singleton Semaphore initialized: " & $iSingleton) ; #FUNCTION# ==================================================================================================================== ; Name ..........: Singleton ; Description ...: Checks if the script has been started already. ; Syntax ........: Singleton($sOccurrenceName) ; Parameters ....: $sOccurrenceName - a string value which will be used to create the semaphore handle. ; Return values .: True if Singleton started the first time. False if script was already started ; Author ........: UEZ ; Modified ......: ; Remarks .......: The system closes the handle automatically when the process terminates. The semaphore object is destroyed when its last handle has been closed. ; Related .......: ; Link ..........: https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-createsemaphorea ; Example .......: No ; =============================================================================================================================== Func Singleton($sOccurrenceName) If StringLen($sOccurrenceName) > 260 Then $sOccurrenceName = StringLeft($sOccurrenceName, 260) Local $aReturn = DllCall("kernel32.dll", "handle", "CreateSemaphoreA", "ptr", Null, "long", 0, "long", 1, "str", $sOccurrenceName) If @error Or Not $aReturn[0] Then Return SetError(1, 0, -1) Return SetExtended($aReturn[0], $aReturn[0] And _WinAPI_GetLastErrorMessage() = "The operation completed successfully.") EndFunc ;==>Singleton Just start the script twice to see if it works. The disadvantage of using atoms is that atoms have a memory that means when your app is crashing or you forgot to delete the atom then the atom does still have the $sOccurrenceName saved and thus Singleton will not work if you use the same same value for $sOccurrenceName. With semaphore you don't have this issue. Thanks to jj2007 and SARG.