Messages in this thread | | | Date | Mon, 01 Sep 2008 11:32:54 -0600 | From | Robert Hancock <> | Subject | Re: (more) epoll troubles |
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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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