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Title: Epoll  
Author: World Heritage Encyclopedia
Language: English
Subject: Kqueue, Linux kernel, Select (Unix), Input/output completion port, Evdev
Collection: Events (Computing), Linux Kernel Features
Publisher: World Heritage Encyclopedia


epoll is a Linux kernel system call, a scalable I/O event notification mechanism, first introduced in Linux kernel 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 in O(n) time, epoll operates in O(1) time[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.


  • API 1
  • Triggering modes 2
  • See also 3
  • References 4
  • External links 5


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 viz. 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.[3]

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.

See also


  1. ^ "epoll(7) - Linux manual page". 2012-04-17. Retrieved 2014-03-01. 
  2. ^ Oleksiy Kovyrin (2006-04-13). "Using epoll() For Asynchronous Network Programming". Retrieved 2014-03-01. 
  3. ^ Love, Robert (2013). Linux System Programming (Second Edition ed.). O’Reilly. pp. 97, 98.  

External links

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