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?
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 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
substitutions to handle runtime adaptations via metadata. h1, h2, ws and mqtt
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
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|
|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 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.
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.
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: https://libwebsockets.org/lws-api-doc-main/html/index.html