libwebsockets

Language: C

Networking / WebSocket

libwebsockets was created by Andy Green to provide a fast, lightweight, and flexible WebSocket library in C. It is widely used in IoT, embedded systems, and web applications where low-level, high-performance WebSocket support is required.

libwebsockets is a lightweight C library for building WebSocket servers, clients, and HTTP(S) servers. It supports asynchronous, event-driven communication over WebSocket and HTTP protocols.

Installation

linux: sudo apt install libwebsockets-dev
mac: brew install libwebsockets
windows: Build from source: https://libwebsockets.org

Usage

libwebsockets provides APIs for creating WebSocket clients and servers, handling events like connections, messages, and disconnections, and integrating with HTTP and SSL/TLS for secure communication.

Creating a simple WebSocket server

#include <libwebsockets.h>
#include <string.h>
#include <signal.h>

static int interrupted = 0;

static int callback_echo(struct lws *wsi, enum lws_callback_reasons reason, void *user, void *in, size_t len) {
    switch (reason) {
        case LWS_CALLBACK_RECEIVE:
            lws_write(wsi, (unsigned char *)in, len, LWS_WRITE_TEXT);
            break;
        default:
            break;
    }
    return 0;
}

int main() {
    struct lws_context_creation_info info;
    memset(&info, 0, sizeof(info));

    struct lws_protocols protocols[] = {
        { "echo-protocol", callback_echo, 0, 1024, },
        { NULL, NULL, 0, 0 }
    };

    info.port = 8080;
    info.protocols = protocols;

    struct lws_context *context = lws_create_context(&info);

    while (!interrupted) {
        lws_service(context, 1000);
    }

    lws_context_destroy(context);
    return 0;
}

Sets up a simple WebSocket echo server on port 8080 using libwebsockets.

Creating a simple WebSocket client

// Client code involves creating context, connecting to server using lws_client_connect_via_info,
// and handling events like LWS_CALLBACK_CLIENT_RECEIVE.

Using SSL/TLS

// Configure SSL options in lws_context_creation_info for secure WebSocket connections using certificates.

Handling multiple protocols

// Define multiple lws_protocols with different callbacks and attach them to the context.

Asynchronous messaging

// Use lws_callback_on_writable() to schedule asynchronous writes without blocking the event loop.

Error Handling

lws_create_context failed: Ensure port is free, protocols are defined correctly, and SSL/TLS paths are valid if used.
WebSocket connection failed: Check server availability, firewall, and network connectivity.
Memory allocation errors: Verify sufficient system memory and proper use of lws_context and buffers.

Best Practices

Use the event-driven API for non-blocking communication.

Handle all WebSocket and HTTP events in callbacks to maintain responsiveness.

Use SSL/TLS for secure connections where needed.

Predefine protocols to organize multiple types of WebSocket messages.

Clean up the context and connections properly to avoid memory leaks.

Official DocsGithub