Libwebsockets

Libwebsockets is a simple-to-use, MIT-license, pure C library providing client and server
for http/1, http/2, websockets, MQTT and other protocols in a security-minded,
lightweight, configurable, scalable and flexible way. It's easy to build and
cross-build via cmake and is suitable for tasks from embedded RTOS through mass
cloud serving.

80+ independent minimal examples for
various scenarios, CC0-licensed (public domain) for cut-and-paste, allow you to get started quickly.

There are a lot of READMEs on a variety of
topics.

We do a huge amount of CI testing per push, currently 495 builds on
27 platforms.