you could just add an additional integer to your filename, so it will be unique. if you start with 0 and go up, you even have the real order.

generate the filename, test if a file allready exists with this if not create one... you don't need locking for that.

Actually, you would need locking for that, unless you have an atomic test-and-create-file function. Otherwise, you could have two processes which end up in a race condition:

Process 1 checks for filename XXX-42, doesn't exist
Process 2 checks for filename XXX-42, doesn't exist
Process 1 creates filename XXX-42 and starts writing to it
Process 2 opens filename XXX-42 and starts writing to it
Process 1 finishes writing to XXX-42
Process 2 finishes writing to XXX-42
[download]

Then what's in the file? Who knows? Probably not what you want.