Libwebsockets is a simple-to-use, 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.
Users wanting a stable branch should follow v4.0-stable to get the most stable version at any given time.
See the changelog for information on the huge amount of new features in this release, and additional information below.
- NEW: Lws is now under the MIT license, see ./LICENSE for details - NEW: GLIB native event loop support, lws + gtk example - NEW: native lws MQTT client... supports client stream binding like h2 when multiple logical connections are going to the same endpoint over MQTT, they transparently and independently share the one connection + tls tunnel - NEW: "Secure Streams"... if you are making a device with client connections to the internet or cloud, this allows separation of the communications policy (endpoints, tls cert validation, protocols, etc) from the code, with the goal you can combine streams, change protocols and cloud provision, and reflect that in the device's JSON policy document without having to change any code. - NEW: lws_system: New lightweight and efficient Asynchronous DNS resolver implementation for both A and AAAA records, supports recursive (without recursion in code) lookups, caching, and getaddrinfo() compatible results scheme (from cache directly without per-consumer allocation). Able to perform DNS lookups without introducing latency in the event loop. - NEW: lws_system: ntpclient implementation with interface for setting system time via lws_system ops - NEW: lws_system: dhcpclient implementation - NEW: Connection validity tracking, autoproduce PING/PONG for protocols that support it if not informed that the connection has passed data in both directions recently enough - NEW: lws_retry: standardized exponential backoff and retry timing based around backoff table and lws_sul - NEW: there are official public helpers for unaligned de/serialization of all common types, see eh, lws_ser_wu16be() in include/libwebsockets/lws-misc.h - NEW: lws_tls_client_vhost_extra_cert_mem() api allows attaching extra certs to a client vhost from DER in memory - NEW: lws_system: generic blobs support passing auth tokens, per-connection client certs etc from platform into lws - NEW: public helpers to consume and produce ipv4/6 addresses in a clean way, along with lws_sockaddr46 type now public. See eg, lws_sockaddr46-based lws_sa46_parse_numeric_address(), lws_write_numeric_address() in include/libwebsockets/lws-network-helper.h - Improved client redirect handling, h2 compatibility - NEW: lwsac: additional features for constant folding support (strings that already are in the lwsac can be pointed to without copying again), backfill (look for gaps in previous chunks that could take a new use size), and lwsac_extend() so last use() can attempt to use more unallocated chunk space - NEW: lws_humanize: apis for reporting scalar quanties like 1234 as "1.234KB" with the scaled symbol strings passed in by caller - NEW: freertos: support lws_cancel_service() by using UDP pair bound to lo, since it doesn't have logical pipes - NEW: "esp32" plat, which implemented freertos plat compatibility on esp32, is renamed to "freertos" plat, targeting esp32 and other freertos platforms - NEW: base64 has an additional api supporting stateful decode, where the input is not all in the same place at the same time and can be processed incrementally - NEW: lws ws proxy: support RFC8441 - NEW: lws_spawn_piped apis: generic support for vforking a process with child wsis attached to its stdin, stdout and stderr via pipes. When processes are reaped, a specified callback is triggered. Currently Linux + OSX. - NEW: lws_fsmount apis: Linux-only overlayfs mount and unmount management for aggregating read-only layers with disposable, changeable upper layer fs - Improvements for RTOS / small build case bring the footprint of lws v4 below that of v3.1 on ARM - lws_tokenize: flag specifying # should mark rest of line as comment - NEW: minimal example for integrating libasound / alsa via raw file - lws_struct: sqlite and json / lejp translation now usable
Secure Streams is an optional layer above lws (
-DLWS_WITH_SECURE_STREAMS=1) that separates connectivity policy into a JSON document, which can be part of the firmware or fetched at boot time.
Code no longer deals with details like endpoint specification or tls cert stack used to validate the remote server, it's all specified in JSON, eg, see this example. Even the protocol to use to talk to the server, between h1, h2, ws or MQTT, is specified in the policy JSON and the code itself just deals with payloads and optionally metadata, making it possible to switch endpoints, update certs and even switch communication protocols by just editing the JSON policy and leaving the code alone.
Logical Secure Stream connections outlive any underlying lws connection, and support “nailed-up” connection reacquisition and exponential backoff management.
See ./lib/secure-streams/README.md and the related minimal examples for more details.
If you enable
-DLWS_ROLE_MQTT=1, lws can now support QoS0 and QoS1 MQTT client connections. See the examples at ./minimal-examples/mqtt-client
glib's event loop joins libuv, libevent and libev support in lws for both the
lws_context creating and owning the loop object for its lifetime, and for an already-existing “foreign loop” where the
lws_context is created, attaches, detaches, and is destroyed without affecting the loop.
This allows direct, lock-free integration of lws functionality with, eg, a GTK app's existing
GMainLoop / glib
g_main_loop. Just select
-DLWS_WITH_GLIB=1 at cmake time to enable. The -eventlib minimal examples also support --glib option to select using the glib loop at runtime.
There's also a gtk example that is built if lws cmake has
lws_systemhelper for attaching code to a single event loop from another thread
lws_system ops struct now has a member that enables other threads (in the same process) to request a callback they define from the lws event loop thread context as soon as possible. From here, in the event loop thread context, they can set up their lws functionality before returning and letting it operate wholly from the lws event loop. The original thread calling the api to request the callback returns immediately.
H2 clients and servers can now modulate RX flow control on streams precisely, ie, define the size of the first incoming data and hand out more tx credit at timing of its choosing to throttle or completely quench the remote server sending as it likes.
The only RFC-compatible way to acheive this is set the initial tx credit to 0 and set it explicitly when sending the headers... client code can elect to do this rather than automatically manage the credit by setting a new flag LCCSCF_H2_MANUAL_RXFLOW and indicating the initial tx credit for that stream in client connection info member manual_initial_tx_credit. A new public api lws_wsi_tx_credit() allows dynamic get and add to local and estimated remote peer credit for a connection. This api can be used without knowing if the underlying connection is h2 or not.
lws_system: DHCP client
DHCP client is now another network service that can be integrated into lws, with
LWS_WITH_SYS_DHCP_CLIENT at CMake. When enabled, the
lws_system state is held at
DHCP until at least one registered network interface acquires a usable set of DHCP information including ip, subnet mask, router / gateway address and at least one DNS server.
See the api-test-dhcp Minimal Example for how to use.
UDP support in lws has new helper that allow
lws_retry to be applied for retry, and the ability to synthesize rx and tx udp packetloss systemwide to confirm retry strategies. Since multiple transactions may be in flight on one UDP socket, the support relies on an
lws_sul in the transaction object to manage the transaction retries individually.
READMEs/README.udp.md for details.
lws_system: system state and notification handlers
Lws now has the concept of systemwide state held in the context... this is to manage that there may be multiple steps that need the network before it's possible for the user code to operate normally. The steps defined are
POLICY_INVALID. OPERATIONAL is the state where user code can run normally.
User and other parts of lws can hook notifier callbacks to receive and be able to veto system state changes, either definitively or because they have been triggered to perform a step asynchronously and will move the state on themselves when it completes.
By default just after context creation, lws attempts to move straight to OPERATIONAL. If no notifier interecepts it, it will succeed to do that and operate in a backwards-compatible way. Enabling various features like lws ntpclient also enable notifiers that hold progress at the related state until their operation completes successfully, eg, not able to enter
TIME_VALID until ntpclient has the time.
READMEs/README.lws_system.md for details.
lws_system: HAL ops struct
Lws allows you to define a standardized ops struct at context creation time so your user code can get various information like device serial number without embedding system-specific code throughout the user code. It can also perform some generic functions like requesting a device reboot.
READMEs/README.lws_system.md for details.
Optional lws system service enabled by cmake
-DLWS_WITH_SYS_NTPCLIENT intercepts the
TIME_VALID state and performs ntpclient to get the date and time before entering
TIME_VALID. This allows user code to validate tls certificates correctly knowing the current date and time by the time it reached OPERATIONAL.
Lws now allows you to apply a policy for how long a network connection may go without seeing something on it that confirms it‘s still valid in the sense of passing traffic cohernetly both ways. There’s a global policy in the context which defaults to 5m before it produces a PING if possible, and 5m10 before the connection will be hung up, user code can override this in the context, vhost (for server) and client connection info (for client).
lws_validity_confirmed(wsi) is provided so user code can indicate that it observed traffic that must mean the connection is passing traffic in both directions to and from the peer. In the absence of these confirmations lws will generate PINGs and take PONGs as the indication of validity.
lws_system: Async DNS support
Master now provides optional Asynchronous (ie, nonblocking) recursive DNS resolving. Enable with
-DLWS_WITH_SYS_ASYNC_DNS=1 at cmake. This provides a quite sophisticated ipv4 + ipv6 capable resolver that autodetects the dns server on several platforms and operates a UDP socket to its port 53 to produce and parse DNS packets from the event loop. And of course, it's extremely compact.
It broadly follows the getaddrinfo style api, but instead of creating the results on the heap for each caller, it caches a single result according to the TTL and then provides refcounted const pointers to the cached result to callers. While there are references on the cached result it can't be reaped.
READMEs/README.async-dns.md for detailed information on how it works, along with
api-tests/api-test-async-dns minimal example.
You can now opt to measure and store us-resolution statistics on effective latencies for client operations, and easily spool them to a file in a format suitable for gnuplot, or handle in your own callback. Enable
-DLWS_WITH_DETAILED_LATENCY=1 in cmake to build it into lws.
If you are concerned about operation latency or potential blocking from user code, or behaviour under load, or latency variability on specific platforms, you can get real numbers on your platform using this.
Timings for all aspects of events on connections are recorded, including the time needed for name resolution, setting up the connection, tls negotiation on both client and server sides, and each read and write.
READMEs/README.detailed-latency.md for how to use it.
Lws master now makes much better use of the DNS results for ipv4 and ipv6... it will iterate through them automatically making the best use it can of what's provided and attempting new connections for each potentially usable one in turn before giving up on the whole client connection attempt.
If ipv6 is disabled at cmake it can only use A / ipv4 records, but if ipv6 is enabled, it tries both; if only ipv6 is enabled it promotes ipv4 to ::ffff:18.104.22.168 IPv4-in-IPv6 addresses.
An internal union
lws_sockaddr46 that combines
struct sockaddr_in and
struct sockaddr_in6 is now public, and there are helpers that can parse (using
lws_tokenize) any valid numeric representation for ipv4 and ipv6 either into byte arrays and lengths, or directly to and from
Lws now supports the convention that half-closing an h2 http stream may make the stream ‘immortal’, in terms of not being bound by normal timeouts. For the client side, there's an api that can be applied to the client stream to make it transition to this “read-only” long poll mode.
READMEs/README.h2-long-poll.md for full details, including how to test it with the minimal examples.
H1 is not so simple to parse because the header length is not known until it has been fully parsed. The next header, or http body may be directly coalesced with the header as well. Lws has supported bulk h1 parsing from a buffer for a long time, but on clientside due to interactions with http proxying it had been stuck parsing the header bytewise out of the tls buffer. In master, everything now bulk parses from a buffer and uses a buflist to pass leftovers through the event loop cleanly.
Just before v3.2 there was a big refactor about how lws handles time. It now explicitly schedules anything that may happen in the future on a single, sorted linked-list, at us resolution. When entering a poll wait (or returning to an event lib loop) it checks the interval between now and the earliest event on the list to figure out how long to wait if there are no network events. For the event loop case, it sets a native event lib timer to enforce it.
READMEs/README.lws_sul.md for more details and a handy api where you can schedule your own arbitrary callbacks using this system.
Libwebsockets master is now under the MIT license. See ./LICENSE.
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 master: https://libwebsockets.org/lws-api-doc-master/html/index.html