Originally posted by oiaohm
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- An epoll file descriptor has a private struct eventpoll that keeps track of which fd's are attached to this fd. struct eventpoll also has a wait queue that keeps track of all processes that are currently epoll_waiting on this fd. struct epoll also has a list of all file descriptors that are currently available for reading or writing.
- When you add a file descriptor to an epoll fd using epoll_ctl, epoll adds the struct eventpoll to that fd's wait queue. It also checks if the fd is currently ready for processing and adds it to the ready list, if so.
- When you wait on an epoll fd using epoll_wait, the kernel first checks the ready list, and returns immediately if any file descriptors are already ready. If not, it adds itself to the single wait queue inside struct eventpoll, and goes to sleep.
- When an event occurs on a socket that is being epoll()ed, it calls the epoll callback, which adds the file descriptor to the ready list, and also wakes up any waiters that are currently waiting on that struct eventpoll
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