So far semaphore was used with possible values in range 0 to
UINT32_MAX. Each write resulted in semaphore increment. As an example
after two writes and single read eventfd counter was correctly zeroed,
but semaphore counter was not. This means that poll() signalled at this
stage POLLIN (semaphore counter was > 0), but it clearly should
not (eventfd counter == 0). Blocking version of read() was also
returning immediately, returning 0 as previous eventfd counter.
Change read_sem to be a binary semaphore, which counter represents
eventfd counter being zero (when semaphore counter == 0) or
non-zero (when semaphore counter == 1). Try to take the semaphore in
eventfd read() and decrement eventfd counter when semaphore was ready.
Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Search for unused eventfd object and just remember its instance in loop
body. Initialize object later, to make it distinct from "search
phase". This change is basically an improvement for readability.
Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
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>
