Re: [Patch][RFC] epoll and half closed TCP connections

[Davide Libenzi]

On Sat, 12 Jul 2003, Eric Varsanyi wrote:

> > (Sorry, I missed this)
> > You can work that out very easily. When your read/write returns a lower
> > number of bytes, it means that it is time to stop processing this fd. If
> > events happened meanwhile, you will get them at the next epoll_wait(). If
> > not, the next time they'll happen. There's no blind spot if you follow
> > this simple rule, and you do not even have the extra syscall with EAGAIN.
>
> The scenario that I think is still uncovered (edge trigger only):
>
> User					Kernel
> --------				----------
> 					Read data added to socket
>
> 					Socket posts read event to epfd
>
> epoll_wait()				Event cleared from epfd, EPOLLIN
> 					  returned to user
>
> 					more read data added to socket
>
> 					Socket posts a new read event to epfd
>
> read() until short read with EAGAIN 	all data read from socket
>
> epoll_wait()				returns another EPOLLIN for socket and
> 					  clears it from epfd
>
> read(), returns 0 right away		socket buffer is empty

read will return -1 with errno=EAGAIN in that case, not zero.

- Davide