MutEx

=Allowing only one instance of your program=
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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 [[@http://msdn.microsoft.com/en-us/library/ms684266(VS.85).aspx|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 [[@http://msdn.microsoft.com/en-us/library/ms682411(VS.85).aspx|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.

Here is the code to do all of this.
[[code format="lb"]]
    ' inherit the default security attributes
    lpMutexAttributes = _NULL

    ' optain initial ownership by setting this to 1
    bInitialOwner = 1

    ' use a unique name for the mutex
    lpName$ = "LBMutExDemo"

    calldll #kernel32, "CreateMutexA",_
        lpMutexAttributes as ulong,_
        bInitialOwner     as long, _
        lpName$           as ptr,_
        hMutex            as ulong
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===The GetLastError function===
The firstsecond function is the one we use to create a mutext,check if our mutext is already in use, which is the [[@http://msdn.microsoft.com/en-us/library/ms679360(VS.85).aspx|GetLastError Function]]
It receivesreturns an error indicating that the security attributes, initial owner and name of our mutex object to create.is already present, which is ERROR_ALREADY_EXISTS = 183.

WeIf the mutex already exists, we do not want to grant other processes or threads access to our mutex, so we use NULL for the security attributes.
We wantprogram to benotify the initial owneruser 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.this situation and end gracefully.

Here is the code to do all of this.
[[code format="lb"]]
    ' inheritget the default security attributes
    lpMutexAttributes = _NULLlast error, so we know if there is already an instance running
    calldll #kernel32, "GetLastError",_
        LastError as ulong

    ' optain initial ownership by setting this to 1
    bInitialOwnercheck if there is already an instance running (ERROR_ALREADY_EXISTS = 1183)
    if LastError = 183 then
        calldll #kernel32, "CloseHandle",_
            hMutex as ulong,_
            result as long

    ' use a unique name for the mutex
    lpName$ = "LBMutExDemo"    ' exit if there is already an instance running
        notice "Exiting!"; chr$(13);_
            "Another instance of this program is already running!"
        end
    end if
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    calldll #kernel32, "CreateMutexA",_
        lpMutexAttributes as ulong,_
        bInitialOwner     as long, _
        lpName$           as ptr,_
        hMutex            as ulong
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==Third Part Title==
Text here.
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'code here
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