epoll

epoll is a Linux kernel system call, a scalable I/O event notification mechanism, first introduced in Linux kernel 2.5.44. Its function is to monitor multiple file descriptors to see if I/O is possible on any of them. 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 in O(n) time, epoll operates in O(1) time). 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.

API

int epoll_create1(int flags);

Creates an epoll object and returns its file descriptor. The flags parameter allows epoll behavior to be modified. It has only one valid value, EPOLL_CLOEXEC. 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.

Triggering modes

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.

Critique

Bryan Cantrill thinks epoll has 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 critique is addressed by EPOLLONESHOT which was added in Linux 2.6.2 released on February, 2004.

See also

• Input/output completion port (IOCP)
• kqueue
• libevent

External links

• How to use epoll? A complete example in C
• epoll manpage
• epoll patch