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CI status Coverity Scan Build Status CII Best Practices Codacy Badge Total alerts Language grade: C/C++ Language grade: JavaScript


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.

It supports a lot of lightweight ancilliary implementations for things like JSON, CBOR, JOSE, COSE, and supports OpenSSL and MbedTLS v2 and v3 out of the box for everything. It's very gregarious when it comes to event loop sharing, supporting libuv, libevent, libev, sdevent, glib and uloop, as well as custom event libs.

100+ 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 582 builds on 30 platforms. You can see the lws CI rack and read about how lws-based Sai is used to coordinate all the testing.



HTML + CSS + JPEG + PNG display stack in lws

Want to drive your EPD or TFT / OLED display using HTML + CSS? Only got an ESP32?

Want remote JPEGs, PNGs, HTML, RGBA composition, gamma, error diffusion if needed?

Realtime render into a line buffer because you don't have enough heap for a framebuffer?

Take a look here...

Perl binding for lws available

Thanks to Felipe Gasper, there's now a perl binding for lws available at metacpan, this uses the recent generic event loop support in lws to have lws as a guest on an existing perl event loop.

Lws examples switching to Secure Streams

Secure Streams direct

Secure Streams support in lws was introduced a couple of years ago, it's a higher-level interface to lws wsi-level apis that simplifies connectivity by segregating connection policy like protocol and endpoint information into a separate JSON policy file, and just having the code deal with payloads; as many details of the wire protocol as possible are hidden or moved to the policy, so user code is almost identical even if the wire protocol changes.

The user code just asks to create a SS by "streamtype name", it is created according to the details (protocol, endpoint, etc) under the same name in the policy.

Key policy entries like endpoint can contain ${metadata-name} string substitutions to handle runtime adaptations via metadata. h1, h2, ws and mqtt are supported.

As a layer on top of the wsi apis, SS provides a higher-level way to access the existing wsi-level capabilities, both kinds of API will remain supported. Secure Streams are longer-lived than a single wsi, so an SS can coordinate retries by itself. SS-based user code is typically significantly smaller and more maintainable than wsi layer.

In main branch I have moved the older examples into ./minimal-examples-lowlevel and am starting to port more cases from there into SS-based examples.

Comparison between wsi and SS level lws usage

Feature "low-level" wsi way Secure Streams way
Create context code same
Loop support, sul scheduler default, event libs same
Supports comms mode Client, Server, Raw same
Supports protocols h1, h2, ws, mqtt (client) same
TLS support mbedtls (including v3), openssl (including v3), wolfssl, boringssl, libressl same
Serializable, proxiable, muxable, transportable No Yes
Auto-allocated per-connection user object pss specified in lws_protocols Specified in ss info struct
Connection User API Protocol-specific lws_protocols cbs (> 100) SS API (rx, tx, state callbacks only)
Sending adaptation lws_callback_on_writeable() + WRITEABLE lws_ss_request_write() + tx() cb
Sending buffer User-chosen + malloc'd partial handling SS-provided, no partials
Create vhosts code JSON policy
TLS validation cert bundle or code JSON policy, or cert bundle
Connection retry / backoff code JSON policy, Auto
Nailing up code JSON policy, Auto
Endpoint and protocol details spread around the code JSON policy
Protocol selection, pipeline / stream sharing code JSON policy
ws subprotocol selection code JSON policy
ws binary / text code JSON policy
Protocol-specific metadata Protocol-specific apis in code (eg, lws_hdr) JSON policy, generic metadata apis in code
Connection validity rules struct JSON policy, Auto
Stream as Long Poll code JSON policy
Auth code JSON policy + automatic rotation if provider supported, else code

Serialized Secure Streams

Secure Streams direct

Secure Streams APIs are also serializable, the exact same client code can fulfil the connection directly in the same process as you would expect, or forward the actions, metadata and payloads to an SS Proxy that owns the policy over a Unix Domain or TCP socket connection to be fulfilled centrally. This allows, eg, h2 streams from different processes sharing a single connection.

Secure Streams direct

The serialized SS can also travel over generic transports like UART, an example is provided implementing the Binance example on an RPi Pico with a UART transport to a UART transport SS proxy, where the pico itself has no network stack, tls, compression or wss stack, but can send and receive to and from the endpoint as if it did.

The optional lws_trasport_mux is used to interpose between the UART transport and the SSPC layer, allowing a single pipe to carry many separate SS connections.

The user SS code is identical however it is transported, muxed and fulfilled.

v4.3 is released

See the changelog

Lws work retrospective

The initial commit for lws will have been 11 years ago come Oct 28 2021, it's been a lot of work. There are a total of 4.3K patches, touching 800KLOC cumulatively (this is not the size in the repo, but over the years, how many source lines were changed by patches).


Gratifyingly, it turns out over the years, ~15% of that was contributed by 404 contributors: that's not so bad. Thanks a lot to everyone who has provided patches.

Today at least tens of millions of devices and product features rely on lws to handle their communications including several from FAANG; Google now include lws as part of Android sources.


This is the libwebsockets C library for lightweight websocket clients and servers. For support, visit

and consider joining the project mailing list at

You can get the latest version of the library from git:

Doxygen API docs for development: