epoll is a scalable I/O event notification mechanism for Linux, first introduced in Linux 2.5.44 [1]. It is meant to replace the older POSIX select(2) and poll(2) system calls, to achieve better performance in more demanding applications, where the number of watched file descriptors is large (unlike the older system calls, which operate at O(n), epoll operates in O(1) [2]). epoll is similar to FreeBSD's kqueue, in that it operates on a configurable kernel object, exposed to user space as a file descriptor of its own.
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int epoll_create(int size);
Creates an epoll object and returns its file descriptor.
int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);
Controls (configures) which file descriptors are watched by this object, and for which events.
int epoll_wait(int epfd, struct epoll_event *events, int maxevents, int timeout);
Waits for any of the registered events, until at least one occurs or the timeout elapses.
epoll provides both edge-triggered and level-triggered modes. In edge-triggered mode, a call to epoll_wait will return only when a new event is enqueued with the epoll object, while in level-triggered mode, epoll_wait will return as long as the condition withholds.
For instance, if a pipe, registered with epoll, has received data, a call to epoll_wait will return, signaling the presence of data to be read. Suppose the reader only consumed part of data from the buffer. In level-triggered mode, further calls to epoll_wait will return immediately, as long as the pipe's buffer contains data to be read. In edge-triggered mode, however, epoll_wait will return only once new data is written to the pipe.