# libevent — Event-Driven Network Programming Library

> A portable C library for scalable asynchronous I/O, providing event notification across epoll, kqueue, IOCP, and more, used by Memcached, Tor, and Chromium.

## Install

Save as a script file and run:

# libevent — Event-Driven Network Programming Library

## Quick Use
```bash
# Install
sudo apt install libevent-dev   # Debian/Ubuntu
brew install libevent           # macOS
```

```c
#include <event2/event.h>

void timeout_cb(evutil_socket_t fd, short what, void *arg) {
    printf("Timeout fired!\n");
}

int main() {
    struct event_base *base = event_base_new();
    struct timeval tv = {2, 0};  // 2 seconds
    struct event *ev = evtimer_new(base, timeout_cb, NULL);
    evtimer_add(ev, &tv);
    event_base_dispatch(base);
    event_free(ev);
    event_base_free(base);
    return 0;
}
```

## Introduction
libevent provides a mechanism to execute a callback function when a specific event occurs on a file descriptor or after a timeout. It is one of the oldest and most battle-tested event notification libraries in the C ecosystem, powering critical infrastructure like Memcached, Tor, and parts of Chromium.

## What libevent Does
- Abstracts OS-specific I/O multiplexing (epoll, kqueue, IOCP, /dev/poll, select) behind one API
- Provides buffered event I/O with automatic read/write watermarks (bufferevents)
- Offers HTTP server and client implementations for embedding lightweight web services
- Handles DNS resolution asynchronously without blocking the event loop
- Supports OpenSSL-based encrypted connections integrated with the bufferevent layer

## Architecture Overview
libevent centers on an `event_base` that manages a priority queue of pending events. When you add an event (I/O readiness, signal, or timer), the base registers it with the OS backend. The dispatch loop blocks until events fire, then invokes callbacks in priority order. Bufferevents layer read/write buffering on top of raw events, handling partial reads and flow control automatically.

## Self-Hosting & Configuration
- Build with CMake or autoconf: `mkdir build && cd build && cmake .. && make`
- Link against `libevent_core` (minimal), `libevent_extra` (HTTP, DNS), or `libevent_openssl`
- Configure the backend explicitly or let libevent auto-select the fastest available
- Set `EVENT_BASE_FLAG_NOLOCK` for single-threaded applications to avoid locking overhead
- Works on Linux, macOS, Windows, FreeBSD, Solaris, and most POSIX systems

## Key Features
- Proven at scale: Memcached serves millions of requests/sec using libevent
- Bufferevent abstraction handles partial I/O, TLS handshakes, and rate limiting
- Built-in HTTP 1.1 server suitable for embedding admin interfaces or health checks
- Thread-safe with explicit locking or lock-free single-threaded mode
- BSD licensed with over 20 years of production use

## Comparison with Similar Tools
- **libuv** — newer, better Windows support, powers Node.js; libevent has richer built-in protocols
- **libev** — faster on Unix but no Windows IOCP support and fewer built-in features
- **Boost.Asio** — C++ only with heavy template usage; libevent is plain C
- **io_uring (liburing)** — Linux-only, lower-level; libevent is cross-platform
- **epoll/kqueue directly** — maximum control but platform-specific and more code

## FAQ
**Q: Should I use libevent or libuv for a new project?**
A: libuv has a more modern API and better Windows integration. libevent is better if you need the built-in HTTP server, DNS resolver, or OpenSSL bufferevent.

**Q: Is libevent thread-safe?**
A: Yes, when compiled with threading support. Each thread should have its own event_base, or you can use `event_base_loopbreak()` / `event_active()` to communicate across threads.

**Q: Can I handle thousands of concurrent connections?**
A: Yes. libevent with epoll or kqueue scales to hundreds of thousands of connections. Memcached regularly handles 100K+ concurrent connections per instance.

**Q: Does libevent support HTTP/2?**
A: The built-in HTTP module supports HTTP/1.1 only. For HTTP/2, pair libevent with nghttp2 or use a different HTTP stack.

## Sources
- https://github.com/libevent/libevent
- https://libevent.org/doc/

---
Source: https://tokrepo.com/en/workflows/asset-cdf0e4d1
Author: Script Depot