Programming

Is select only supposed to report state changes on an FD's, state, whether or not it had pending input available in the first place? I've got a situation where select() repeatedly reports no FD's ready for stdin when there's lots of data available. And if it only reports changes, how is this useful? It's impossible to guarantee an FD has no pending data before you call select(), making its return value meaningless at worst and at best a crapshoot.

Around 22 years ago I had similar problems with select on HP-UX. I would have thought these bugs would be worked out by now.

The behavior seems to be standardized. The linux page even specifically mentions looking for state changes, rather than describing it as "pending data". Presumably there's a "proper" way to use it I hope, that's better than "wait for it to timeout and check all possible streams regardless".

---------- Post updated 02-10-10 at 12:56 AM ---------- Previous update was 02-09-10 at 11:58 PM ----------

I think I've found a way that works: Ignore select's return values, which only seem to report state changes, and look at the FD_ISSET values, which actually do seem to represent pending data.

---------- Post updated at 01:07 AM ---------- Previous update was at 12:56 AM ----------

That's not it either. But I finally found a clear, working example: You have to FD_SET the FD's you want every single loop, not just once; if data isn't available, select() will clear them for you.