Questions on epoll_wait()
 

Hi Folks,

Based on online tutorials and examples for epoll,

EPOLLIN - indicate available data.
EPOLLHUP - indicates a remote connection close.

Now in the below mentioned scenario, what would be the behavior of epoll_wait?

(1) epoll_wait returns for FD_X with EPOLLIN.
(2) In a while loop, I start reading the data.
(3) At the same time while I am executing read(Step 2), if remote end closes the connection for FD_X, Would I get a separate EPOLLHUP event, if I go back to wait on epoll_wait()? How do I detect this otherwise?

Based on written examples, I get an EIO error on read, if the remote end has closed the connection.

Is it possible to differenitate between an EIO error that occurs due to
(1) A remote connection close.
(2) Legitimate error in read.

Thanks for the help.

-Alphonse

Hey Alphonse,

to get more accurate error distinction, you can also set the EPOLLRDHUP flag using epoll_ctl().

When epoll_wait() returns, check for EPOLLRDHUP (peer disconnected), EPOLLHUP (unexpected close of socket), EPOLLERR (fd error condition), EPOLLIN (data available).

In the last case (only EPOLLIN set), invoke read().
If read() returns -1, check errno for specifics.
If read() returns 0, this would be quite a surprise.

Do you have a test program available to play with?

Kind regards
dmdeb