What does select(2) do if you close(2) a file descriptor in a separate thread?

Question

What is the behavior of the select(2) function when a file descriptor it is watching for reading is closed by another thread?

From some cursory testing,

Answer 1

It's a little confusing what you're asking...

Select() should return upon an "interesting" change. If the close() merely decremented the reference count and the file was still open for writing somewhere then there's no reason for select() to wake up.

If the other thread did close() on the only open descriptor then it gets more interesting, but I'd need to see a simple version of the code to see if something's really wrong.