Adds a file descriptor to the set of file descriptors polled for this source.
This is usually combined with Source to add an event source. The event
source's check function will typically test the
revents field in the PollFD
struct and return true if events need to be processed.
Using this API forces the linear scanning of event sources on each main loop iteration. Newly-written event sources should try to use add_unix_fd instead of this API.
a PollFD structure holding information about a file descriptor to watch.