This implements a file descriptor used for event notification that
behaves like the eventfd in Linux.
The eventfd supports nonblocking operation by setting the EFD_NONBLOCK
flag and semaphore operation by settings the EFD_SEMAPHORE flag.
The major use case for this is when using poll() and the sockets that
you poll are dynamic. When a new socket needs to be added to the poll,
there must be some way to wake the thread and update the pollfds before
calling poll again. One way to solve it is to have a timeout set in the
poll call and only update the pollfds during a timeout but that is not
a very nice solution. By instead including an eventfd in the pollfds,
it is possible to wake the polling thread by simply writing to the
eventfd.
Signed-off-by: Tobias Svehagen <tobias.svehagen@gmail.com>
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