epoll is a Linux kernel system call for a scalable I/O event notification mechanism, first introduced in version 2.5.44 of the Linux kernel mainline. Its function is to monitor multiple file descriptors to see whether I/O is possible on any of them. It is meant to replace the older POSIX
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 in O(n) time,
epoll operates in O(1) time).
epoll is similar to FreeBSD's
kqueue, in that it consists of a set of user-space functions, each taking a file descriptor argument denoting the configurable kernel object, against which they cooperatively operate.
epoll uses a red-black tree (RB-tree) data structure to keep track of all file descriptors that are currently being monitored.
int epoll_create1(int flags);
epoll object and returns its file descriptor. The
flags parameter allows epoll behavior to be modified. It has only one valid value,
epoll_create is an older variant of
epoll_create1 and is deprecated as of Linux kernel version 2.6.27 and glibc version 2.9.
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.
op can be ADD, MODIFY or DELETE.
int epoll_wait(int epfd, struct epoll_event *events, int maxevents, int timeout);
Waits for any of the events registered for with
epoll_ctl, until at least one occurs or the timeout elapses. Returns the occurred events in
events, up to
maxevents at once.
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 holds.
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.
Bryan Cantrill pointed out that
epoll had mistakes that could have been avoided, had it learned from its predecessors: input/output completion ports, event ports (Solaris) and kqueue. However, a large part of his criticism was addressed by
EPOLLONESHOT was added in version 2.6.2 of the Linux kernel mainline, released in February 2004.
EPOLLEXCLUSIVE was added in version 4.5, released in March 2016.