As Jon first suggested, you can try creating a mutex. Call CreateMutex. If you get a non-null handle back, then call GetLastError. It will tell you whether you were the one who created the mutex or whether the mutex was already open before (Error_Already_Exists). Note that it is not necessary to acquire ownership of the mutex. The mutex is not being used for mutual exclusion. It's being used because it is a named kernel object. An event or semaphore could work, too.
The mutex technique gives you a Boolean answer: Yes, there is another instance, or no, there is not.
You frequently want to know more than just that. For instance, you might want to know the handle of the other instance's main window so you can tell it to come to the foreground in place of your other instance. That's where a memory-mapped file can come in handy; it can hold information about the first instance so later instances can refer to it.
Be careful when choosing the name of the mutex. Read the documentation carefully, and keep in mind that some characters (such as backslash) are not allowed in some OS versions, but are required for certain features in other OS versions.
Also remember the problem of other users. If your program could be run via remote desktop or fast user switching, then there could be other users already running your program, and you might not really want to restrict the current user from running your program. In that case, don't use a global name. If you do want to restrict access for all users, then make sure the mutex object's security attributes are such that everyone will be able to open a handle to it. Using a null pointer for the lpSecurityAttributes parameter is not sufficient for that; the "default security descriptor" that MSDN mentions gives full access to the current user and no access to others.
You're allowed to edit the DPR file of your program. That's usually a good place to do this kind of thing. If you wait until the OnCreate event of one of your forms, then your program already has a bit of momentum toward running normally, so it's clumsy to try to terminate the program at that point. Better to terminate before too much UI work has been done. For example:
var
mutex: THandle;
mutexName: string;
begin
mutexName := ConstructMutexName();
mutex := CreateMutex(nil, False, PChar(mutexName));
if mutex = 0 then
RaiseLastOSError; // Couldn't open handle at all.
if GetLastError = Error_Already_Exists then begin
// We are not the first instance.
SendDataToPreviousInstance(...);
exit;
end;
// We are the first instance.
// Do NOT close the mutex handle here. It must
// remain open for the duration of your program,
// or else later instances won't be able to
// detect this instance.
Application.Initialize;
Application.CreateForm(...);
Application.Run;
end.
There's a question of when to close the mutex handle. You don't have to close it. When your process finally terminates (even if it crashes), the OS will automatically close any outstanding handles, and when there are no more handles open, the mutex object will be destroyed (thus allowing another instance of your program to start and consider itself to be the first instance).
But you might want to close the handle anyway. Suppose you chose to implement the SendDataToPreviousInstance function I mentioned in the code. If you want to get fancy, then you could account for the case that the previous instance is already shutting down and is unable to accept new data. Then you won't really want to close the second instance. The first instance could close the mutex handle as soon as it knows it's shutting down, in effect becoming a "lame duck" instance. The second instance will try to create the mutex handle, succeed, and consider itself the real first instance. The previous instance will close uninterrupted. Use CloseHandle to close the mutex; call it from your main form's OnClose event handler, or wherever else you call Application.Terminate, for example.
