Libevent - an event notification library

Libevent - an event notification library

https://libevent.org/
https://libevent.org/libevent-book/
https://github.com/libevent/libevent

Libevent 是一个用C语言编写的、轻量级的开源高性能事件通知库，主要有以下几个亮点：

• 事件驱动（ event-driven），高性能；
• 轻量级，专注于网络，不如 ACE 那么臃肿庞大；
• 源代码相当精炼、易读；
• 跨平台，支持 Windows、 Linux、 *BSD 和 Mac Os；
• 支持多种 I/O 多路复用技术， epoll、 poll、 dev/poll、select 和 kqueue 等；
• 支持 I/O，定时器和信号等事件；注册事件优先级。

Introduce

The libevent API provides a mechanism to execute a callback function when a specific event occurs on a file descriptor or after a timeout has been reached. Furthermore, libevent also support callbacks due to signals or regular timeouts.

libevent is meant to replace the event loop found in event driven network servers. An application just needs to call event_dispatch() and then add or remove events dynamically without having to change the event loop.

Currently, libevent supports /dev/poll, kqueue(2), event ports, POSIX select(2), Windows select(), poll(2), and epoll(4). The internal event mechanism is completely independent of the exposed event API, and a simple update of libevent can provide new functionality without having to redesign the applications. As a result, Libevent allows for portable application development and provides the most scalable event notification mechanism available on an operating system. Libevent can also be used for multi-threaded applications, either by isolating each event_base so that only a single thread accesses it, or by locked access to a single shared event_base. Libevent should compile on Linux, *BSD, Mac OS X, Solaris, Windows, and more.

Libevent additionally provides a sophisticated framework for buffered network IO, with support for sockets, filters, rate-limiting, SSL, zero-copy file transmission, and IOCP. Libevent includes support for several useful protocols, including DNS, HTTP, and a minimal RPC framework.

Programs using libevent

https://libevent.org/

The usefulness of libevent API is demonstrated by the following applications:

• Chromium – Google's open-source web browser (uses Libevent)
• Memcached – a high-performance, distributed memory object caching system
• Transmission – a fast, easy, and free BitTorrent client
• NTP – the network time protocol that makes your clock right (uses Libevent in SNTP)
• tmux – A clean, modern, BSD-licensed terminal multiplexer, similar to GNU screen
• Tor – an anonymous Internet communication system.
• libevhtp – A fast and flexible replacement for libevent's http client/server API
• Prosody – A Jabber/XMPP server written in Lua
• PgBouncer – Lightweight connection pooler for PostgreSQL
• redsocks – a simple transparent TCP -> Socks5/HTTPS proxy daemon.
• Vomit – Voice Over Misconfigured Internet Telephones
• Crawl – A Small and Efficient HTTP Crawler
• Libio – an input/output abstraction library
• Honeyd – a virtual honeynet daemon – can be used to fight Internet worms.
• Fragroute – an IDS testing tool
• Nylon – nested proxy server
• Disconcert – a Distributed Computing Framework for Loosely-Coupled Workstations.
• Trickle – a lightweight userspace bandwidth shaper.
• watchcatd – software watchdog designed to take actions not as drastic as the usual solutions, which reset the machine.
• ScanSSH – a fast SSH server and open proxy scanner.
• Nttlscan – a network topology scanner for Honeyd.
• NetChat – a combination of netcat and ppp's chat.
• Io – a small programming language; uses libevent for network communication.
• Systrace – a system call sandbox.
• SpyBye – detect malware on web pages.
• GreenSQL – an SQL database firewall.
• dnsscan – a fast scanner for identifying open recursive dns resolvers
• Kargo Event – a PHP extension for libevent.
• Scytale – a database encryption tool.
• Coturn – Free open source implementation of TURN and STUN Server.

Working with events

https://libevent.org/libevent-book/Ref4_event.html

Libevent’s basic unit of operation is the event. Every event represents a set of conditions, including:

• A file descriptor being ready to read from or write to.
• A file descriptor becoming ready to read from or write to (Edge-triggered IO only).
• A timeout expiring.
• A signal occurring.
• A user-triggered event.

Events have similar lifecycles. Once you call a Libevent function to set up an event and associate it with an event base, it becomes initialized. At this point, you can add, which makes it pending in the base. When the event is pending, if the conditions that would trigger an event occur (e.g., its file descriptor changes state or its timeout expires), the event becomes active, and its (user-provided) callback function is run. If the event is configured persistent, it remains pending. If it is not persistent, it stops being pending when its callback runs. You can make a pending event non-pending by deleting it, and you can add a non-pending event to make it pending again.

#include <event2/event.h>

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A tiny introduction to asynchronous IO

https://libevent.org/libevent-book/01_intro.html