Allowing only one instance of your program

- StPendl May 8, 2011

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The Basics

Every now and then one wants to make sure that only one instance of his application is running to prevent problems with shared resources when multiple instances are run concurrently.

This is called mut ually ex clusive access, in short Mut-Ex or mutex.

MSDN has a nice article about Mutex Objects , which you should read to get a general understanding of the subject.

The API functions involved

The CreateMutex function

The first function is the one we use to create a mutext, which is the CreateMutex Function
It receives the security attributes, initial owner and name of our mutex object to create.

We do not want to grant other processes or threads access to our mutex, so we use NULL for the security attributes.
We want to be the initial owner of the mutex to prevent subsequent instances of our program to be run, so we set that to TRUE = 1.
We select a unique name for our mutex to have it easily separated from other mutex objects.

In return we get the Windows handle of the mutex, so we can access it at a later stage.

Here is the code to do all of this.

 ' inherit the default security attributes 
 lpMutexAttributes = _NULL 
 
 ' optain initial ownership by setting this to 1 
 bInitialOwner = 1 
 
 ' use a unique name for the mutex mutex, maximum _MAX_PATH = 260 characters 
 lpName$ = "LBMutExDemo" 
 
 calldll #kernel32, "CreateMutexA",_ 
 lpMutexAttributes as ulong,_ 
 bInitialOwner as long,_ 
 lpName$ as ptr,_ 
 hMutex as ulong 

The GetLastError function

The second function is the one we use to check if our mutext is already in use, which is the GetLastError Function
It returns an error indicating that the mutex is already present, which is ERROR_ALREADY_EXISTS = 183.

If the mutex already exists, we do want our program to notify the user of this situation and end gracefully.

The call to CreateMutex has already created another handle to the mutex, so we need to close that handle too.

Here is the code to do all of this.

 ' get the last error, so we know if there is already an instance running 
 calldll #kernel32, "GetLastError",_ 
 LastError as ulong 
 
 ' check if there is already an instance running (ERROR_ALREADY_EXISTS = 183) 
 if LastError = 183 then 
 ' close the Windows handle obtained 
 calldll #kernel32, "CloseHandle",_ 
 hMutex as ulong,_ 
 result as long 
 
 ' exit if there is already an instance running 
 notice "Exiting!"; chr$(13);_ 
 "Another instance of this program is already running!" 
 end 
 end if 

The ReleaseMutex function

The third function is the one we use to free our mutext before we end the program, which is the ReleaseMutex Function
It removes the ownership from the mutex.

This function must be called at the end of your program to be able to debug and run your program from within the LB editor.
If it happens, that your program crashes, while the mutex is active, you need to restart the LB editor to free the mutex.

Here is the code to do all of this.

 ' free the mutex so it is no longer owned by this instance 
 calldll #kernel32, "ReleaseMutex",_ 
 hMutex as ulong,_ 
 result as long 

Third Part Title

The CloseHandle function

The fourth function is the one we use to close the handle to our mutext before we end the program, which is the CloseHandle Function
It removes the mutex entirely from the system.

This function must be called at the end of your program to be able to debug and run your program from within the LB editor.
If it happens, that your program crashes, while the mutex is active, you need to restart the LB editor to free the mutex.

Here is the code to do all of this.

 ' close the mutex handle to cleanup 
 calldll #kernel32, "CloseHandle",_ 
 hMutex as ulong,_ 
 result as long 

The full example

Text here.

 'code here