libwebsockets/minimal-examples-lowlevel

minimal-examples-lowlevel

These are the traditional lws low-level, wsi-based examples.

./minimal-examples contains higher-level, Secure Stream based examples for a growing subset of these cases. New users may find it easier to use those higher- level examples for the (common) cases they cover.

These lowlevel apis are not going anywhere, the higher level stuff uses them to get stuff done itself.

name	demonstrates
client-server	Minimal examples providing client and server connections simultaneously
crypto	Minimal examples related to using lws crypto apis
dbus-server	Minimal examples showing how to integrate DBUS into lws event loop
http-client	Minimal examples providing an http client
http-server	Minimal examples providing an http server
raw	Minimal examples related to adopting raw file or socket descriptors into the event loop
secure-streams	Minimal examples related to the Secure Streams client api
ws-client	Minimal examples providing a ws client
ws-server	Minimal examples providing a ws server (and an http server)

FAQ

Getting started

Build and install lws itself first (note that after installing lws on *nix, you need to run ldconfig one time so the OS can learn about the new library. Lws installs in /usr/local by default, Debian / Ubuntu ldconfig knows to look there already, but Fedora / CentOS need you to add the line /usr/local/lib to /etc/ld.so.conf and run ldconfig)

Then start with the simplest:

http-server/minimal-http-server

Why are most of the sources split into a main C file file and a protocol file?

Lws supports three ways to implement the protocol callback code:

• you can just add it all in the same source file

• you can separate it as these examples do, and #include it into the main sources

• you can build it as a standalone plugin that is discovered and loaded at runtime.

The way these examples are structured, you can easily also build the protocol callback as a plugin just with a different CMakeLists.txt... see https://github.com/warmcat/libwebsockets/tree/master/plugin-standalone for an example.

Why would we want the protocol as a plugin?

You will notice a lot of the main C code is the same boilerplate repeated for each example. The actual interesting part is in the protocol callback only.

Lws provides (-DLWS_WITH_LWSWS=1) a generic lightweight server app called 'lwsws' that can be configured by JSON. Combined with your protocol as a plugin, it means you don't actually have to make a special server "app" part, you can just use lwsws and pass per-vhost configuration from JSON into your protocol. (Of course in some cases you have an existing app you are bolting lws on to, then you don't care about this for that particular case).

Because lwsws has no dependency on whatever your plugin does, it can mix and match different protocols randomly without needing any code changes. It reduces the size of the task to just writing the code you care about in your protocol handler, and nothing else to write or maintain.

Lwsws supports advanced features like reload, where it starts a new server instance with changed config or different plugins, while keeping the old instance around until the last connection to it closes.

I get why there is a pss, but why is there a vhd?

The pss is instantiated per-connection. But there are almost always other variables that have a lifetime longer than a single connection.

You could make these variables "filescope" one-time globals, but that means your protocol cannot instantiate multiple times.

Lws supports vhosts (virtual hosts), for example both https://warmcat.com and https://libwebsockets are running on the same lwsws instance on the same server and same IP... each of these is a separate vhost.

Your protocol may be enabled on multiple vhosts, each of these vhosts provides a different vhd specific to the protocol instance on that vhost. For example many of the samples keep a linked-list head to a list of live pss in the vhd... that means it's cleanly a list of pss opened on that vhost. If another vhost has the protocol enabled, connections to that will point to a different vhd, and the linked-list head on that vhd will only list connections to his vhost.

The example "ws-server/minimal-ws-server-threads" demonstrates how to deliver external configuration data to a specific vhost + protocol combination using code. In lwsws, this is simply a matter of setting the desired JSON config.