epoll_ctl opcode
file descriptor to register for events
events to register fd for
user data; the member of the data field of the created epoll_data_t instance that matches the type is set to it
0 on success or -1 on error. On error errno is set appropriately.
Calls epoll_ctl() using the current epoll file descriptor to modify the registration of fd for events with data as user data.
Creates the epoll_event_t instance passed to epoll_ctl() from events and data where the type of data must match one of the epoll_data_t members.
The current epoll file descriptor should have been sucessfully obtained by create() or epoll_create1() and not already been closed, otherwise epoll_ctl() will fail so that this method returns -1.