Re: (more) epoll troubles

[Robert Hancock]

Robert Hancock wrote:
> Michael Noisternig wrote:
> > Hello,
> >
> > and sorry again if this is the wrong place to ask (again, please hint 
> > to me to an appropriate place to ask in that case).
> >
> > After experimenting with epoll edge-triggered mode I am clueless why 
> > on a few occassions I seem to not get any input notification despite 
> > data is available.
> >
> > In detail: I have set up sockets with epoll events 
> > EPOLLET|EPOLLRDHUP|EPOLLIN. When I get EPOLLIN for a socket, I read() 
> > as long as I get what I asked for, i.e. whenever read() returns either 
> > EAGAIN or less data than I asked for I take this as indication that I 
> > must wait for another EPOLLIN notification. However, this does not 
> > seem to work always.
> >
> > Here is some log from my program:
> >
> > 0x9e6b8a8: read not avail (1460/2048 read)
> > i.e. tried to read 2048 bytes, got 1460 -> assume must wait for 
> > EPOLLIN for more data to read
> > (note that the fd is always in the epoll set with 
> > EPOLLET|EPOLLRDHUP|EPOLLIN)
>
> It would likely be better to always continue trying to read until EAGAIN 
> is returned, even if the read returned less than the requested amount, 
> as implied here:
>
> http://linux.die.net/man/7/epoll
>
> "The function do_use_fd() uses the new ready file descriptor until 
> EAGAIN is returned by either read(2) or write(2). An event driven state 
> machine application should, after having received EAGAIN, record its 
> current state so that at the next call to do_use_fd() it will continue 
> to read(2) or write(2) from where it stopped before. "

Though, this is somewhat contradicted by the FAQ section:

"the condition that the read/write I/O space is exhausted can be 
detected by checking the amount of data read/write from/to the target 
file descriptor. For example, if you call read(2) by asking to read a 
certain amount of data and read(2) returns a lower number of bytes, you 
can be sure to have exhausted the read I/O space for such file descriptor."
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