Exactly. IIRC the discussion was about a CGI. As ling as there is just one user - No Problem. But as soon as there is more than one chances are good that 2 users will click the button at the same time.

However. Neither of our solutions is a perfect solution. Just ideas.

If I had to calculate a new unique filename, I would still use $$ append a counter until I find a unique one. Additionally I would set a lock before searching for a free name.