What is select supposed to do if you close a monitored fd?

I can test this to find the behavior but that's not the point. In my answer to another question, a commenter recommended closing a monitored fd from another thread to wake up select. Another commenter couldn't find a reference to this behavior in the standard, and I can't find one either.

Can someone provide a pointe开发者_C百科r to the standard on this behavior?

From the description of select in "The Open Group Base Specifications Issue 7":

A descriptor shall be considered ready for reading when a call to an input function with O_NONBLOCK clear would not block, whether or not the function would transfer data successfully. (The function might return data, an end-of-file indication, or an error other than one indicating that it is blocked, and in each of these cases the descriptor shall be considered ready for reading.)

So, I would say this method is portable.