When tmpfile() creates a temporary file, how does it decide where to create it?

Is it always the same spot?
Is it following an environment variable?
Does it try different solutions, returning when one or the other succeeds?

Waiting, waiting...

With continued searching, I came across a Support.Microsoft.com page that stated that, though the literature would appear to indicate otherwise, tmpfile() always creates its file in the root folder of the current directory.

This was causing me the weirdest behavior. For security here where I work, C:\ is modifiable, but D:\ is read only.

The app I’m working on (which I inherited) creates a report by appending info to a tmpfile(). Sometimes this would fail, but sometimes it would succeed. It was apparently arbitrary, and the app gives no indication of what the problem is; it merely reports, “No report was created based on user input.”