io_eagain man page
io_eagain — tell io_wait that you got an EAGAIN
void io_eagain(int64 fd);
If io_wait() said that you can read from a descriptor, you try to read from it, and you get EAGAIN, you need to tell io_wait(). This is important for edge triggered event notification schemes like Linux 2.4's SIGIO, or you will lose events.
io_tryread(), io_trywrite() and io_sendfile() and iob_send() already take care of this for you.
This function is only of interest if you integrate io_wait() with legacy code that uses read/write directly.
io_canread(3), io_canwrite(3), io_fd(3), io_fd_canwrite(3).