Lightweight C library for HTML5 websockets
lws event library support

v4.0 and below

Before v4.1, lws allowed selecting some event library support for inclusion in the libwebsockets library

Option Feature

The user code can select by info->options flags at runtime which event loop it wants to use.

The only restriction is that libev and libevent can't coexist, because their header namespace conflicts.

v4.1 and above

Lws continues to support the old way described above, but there's an additional new cmake option that decides how they are built if any are selected, LWS_WITH_EVLIB_PLUGINS.

The old behaviour is set by LWS_WITH_EVLIB_PLUGINS=0, for UNIX platforms, this is set to 1 by default. This causes the enabled event lib support to each be built into its own dynamically linked plugin, and lws will bring in the requested support alone at runtime after seeing the info->options flags requested by the user code.

This has two main benefits, first the conflict around building libevent and libev together is removed, they each build isolated in their own plugin; the libwebsockets core library build doesn't import any of their headers (see below for exception). And second, for distro packaging, the event lib support plugins can be separately packaged, and apps take dependencies on the specific event lib plugin package, which itself depends on the libwebsockets core library. This allows just the needed dependencies for the packageset without forcing everything to bring everything in.

Separately, lws itself has some optional dependencies on libuv, if you build lwsws or on Windows you want plugins at all. CMake will detect these situations and select to link the lws library itself to libuv if so as well, independent of whatever is happening with the event lib support.

evlib plugin install

The produced plugins are named

event lib plugin name

The evlib plugins are installed alongside into the configured library dir, it's often /usr/local/lib/ by default on linux.

Lws looks for them at runtime using the build-time-configured path.

Component packaging

The canonical package name is libwebsockets, the recommended way to split the packaging is put the expected libs and pkgconfig in libwebsockets or libwebsockets-core, the latter is followed by the provided cmake, and produce an additional package per build event library plugin, named, eg libwebsockets-evlib_glib, which has a dependency on libwebsockets[-core].

Applications that use the default event loop can directly require libwebsockets[-core], and application packages that need specific event loop support can just require, eg, libwebsockets-evlib_glib, which will bring that in and the core lws pieces in one step. There is then no problem with multiple apps requiring different event libs, they will bring in all the necessary pieces which will not conflict either as packages or at runtime.


The cmake helper config LWS_WITH_DISTRO_RECOMMENDED is adapted to build all the event libs with the event lib plugin support enabled.