WEB_SOCKET.md

Overview

WebSocket is a communications protocol that provides full-duplex (two-way) communication over a single TCP connection. Unlike HTTP, WebSocket is a distinct protocol designed for real-time, bidirectional data exchange. It was standardised by the IETF as RFC 6455 in 2011, and the WebSocket API in Web IDL is being standardised by the W3C.

Example

#include <map>
#include <chrono>
#include <string>
#include <cstring>
#include <memory>
#include <utility>
#include <cstdlib>
#include <restbed>
#include <system_error>
#include <openssl/sha.h>
#include <openssl/hmac.h>
#include <openssl/evp.h>
#include <openssl/bio.h>
#include <openssl/buffer.h>

using namespace std;
using namespace restbed;
using namespace std::chrono;

shared_ptr< Service > service = nullptr;

map< string, shared_ptr< WebSocket > > sockets = { };

string base64_encode( const unsigned char* input, int length )
{
    BIO* bmem, *b64;
    BUF_MEM* bptr;

    b64 = BIO_new( BIO_f_base64( ) );
    bmem = BIO_new( BIO_s_mem( ) );
    b64 = BIO_push( b64, bmem );
    BIO_write( b64, input, length );
    ( void ) BIO_flush( b64 );
    BIO_get_mem_ptr( b64, &bptr );

    char* buff = ( char* )malloc( bptr->length );
    memcpy( buff, bptr->data, bptr->length - 1 );
    buff[ bptr->length - 1 ] = 0;

    BIO_free_all( b64 );

    return buff;
}

multimap< string, string > build_websocket_handshake_response_headers( const shared_ptr< const Request >& request )
{
    auto key = request->get_header( "Sec-WebSocket-Key" );
    key.append( "258EAFA5-E914-47DA-95CA-C5AB0DC85B11" );

    std::byte hash[ SHA_DIGEST_LENGTH ];
    SHA1( reinterpret_cast< const unsigned char* >( key.data( ) ), key.length( ), (unsigned char*)hash );

    multimap< string, string > headers;
    headers.insert( make_pair( "Upgrade", "websocket" ) );
    headers.insert( make_pair( "Connection", "Upgrade" ) );
    headers.insert( make_pair( "Sec-WebSocket-Accept", base64_encode( (unsigned char*)hash, SHA_DIGEST_LENGTH ) ) );

    return headers;
}

void ping_handler( void )
{
    for ( auto entry : sockets )
    {
        auto key = entry.first;
        auto socket = entry.second;

        if ( socket->is_open( ) )
        {
            socket->send( WebSocketMessage::PING_FRAME );
        }
        else
        {
            socket->close( );
        }
    }
}

void close_handler( const shared_ptr< WebSocket > socket )
{
    if ( socket->is_open( ) )
    {
        auto response = make_shared< WebSocketMessage >( WebSocketMessage::CONNECTION_CLOSE_FRAME, Bytes( { std::byte(10), std::byte(00) } ) );
        socket->send( response );
    }

    const auto key = socket->get_key( );
    sockets.erase( key );

    fprintf( stderr, "Closed connection to %s.\n", key.data( ) );
}

void error_handler( const shared_ptr< WebSocket > socket, const error_code error )
{
    const auto key = socket->get_key( );
    fprintf( stderr, "WebSocket Errored '%s' for %s.\n", error.message( ).data( ), key.data( ) );
}

void message_handler( const shared_ptr< WebSocket > source, const shared_ptr< WebSocketMessage > message )
{
    const auto opcode = message->get_opcode( );

    if ( opcode == WebSocketMessage::PING_FRAME )
    {
        auto response = make_shared< WebSocketMessage >( WebSocketMessage::PONG_FRAME, message->get_data( ) );
        source->send( response );
    }
    else if ( opcode == WebSocketMessage::PONG_FRAME )
    {
        // Ignore PONG_FRAME.
        //
        // Every time the ping_handler is scheduled to run, it fires off a PING_FRAME to each
        // WebSocket. The client, if behaving correctly, will respond with a PONG_FRAME.
        //
        // On each occasion the underlying TCP socket sees any packet data transfer, whether
        // a PING, PONG, TEXT, or BINARY... frame. It will automatically reset the timeout counter
        // leaving the connection active; see also Settings::set_connection_timeout.
        return;
    }
    else if ( opcode == WebSocketMessage::CONNECTION_CLOSE_FRAME )
    {
        source->close( );
    }
    else if ( opcode == WebSocketMessage::BINARY_FRAME )
    {
        // We don't support binary data.
        auto response = make_shared< WebSocketMessage >( WebSocketMessage::CONNECTION_CLOSE_FRAME, Bytes( { std::byte(10), std::byte(03) } ) );
        source->send( response );
    }
    else if ( opcode == WebSocketMessage::TEXT_FRAME )
    {
        auto response = make_shared< WebSocketMessage >( *message );
        response->set_mask( 0 );

        for ( auto socket : sockets )
        {
            auto destination = socket.second;
            destination->send( response );
        }

        const auto key = source->get_key( );
        fprintf( stderr, "Received message '%.*s' from %s\n",
                (int) message->get_data( ).size( ),
                (const char*) message->get_data( ).data( ),
                key.data( )
        );
    }
}

void get_method_handler( const shared_ptr< Session > session )
{
    const auto request = session->get_request( );
    const auto connection_header = request->get_header( "connection" );

    if ( connection_header.find( "upgrade" ) not_eq string::npos )
    {
        if ( request->get_header( "upgrade" ) == "websocket" )
        {
            const auto headers = build_websocket_handshake_response_headers( request );

            session->upgrade( SWITCHING_PROTOCOLS, headers, [ ]( const shared_ptr< WebSocket > socket )
            {
                if ( socket->is_open( ) )
                {
                    socket->set_close_handler( close_handler );
                    socket->set_error_handler( error_handler );
                    socket->set_message_handler( message_handler );

                    socket->send( "Welcome to Corvusoft Chat!", [ ]( const shared_ptr< WebSocket > socket )
                    {
                        const auto key = socket->get_key( );
                        sockets.insert( make_pair( key, socket ) );

                        fprintf( stderr, "Sent welcome message to %s.\n", key.data( ) );
                    } );
                }
                else
                {
                    fprintf( stderr, "WebSocket Negotiation Failed: Client closed connection.\n" );
                }
            } );

            return;
        }
    }

    session->close( BAD_REQUEST );
}

int main( const int, const char** )
{
    auto resource = make_shared< Resource >( );
    resource->set_path( "/chat" );
    resource->set_method_handler( "GET", get_method_handler );

    auto settings = make_shared< Settings >( );
    settings->set_port( 1984 );

    service = make_shared< Service >( );
    service->publish( resource );
    service->start( settings );

    return EXIT_SUCCESS;
}

Build

$ clang++ -std=c++20 -o example example.cpp -l restbed -l ssl -l crypto

Execution

$export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/local/lib $ ./example

Client

<!DOCTYPE HTML>
<html>
  <head>
    <style>
      html, body {
        margin: 0;
      }

      ul {
        list-style: none;
      }

      li {
         text-align: left;
      }

      input {
        width: 40%;
        height: 40px;
        line-height: 40px;
        margin-bottom: 10px;
      }

      a {
        margin: 10px;
      }

      .disabled {
        color: #000;
        pointer-events: none;
      }

      #controls {
        width: 100%;
        bottom: 10%;
        position: absolute;
        text-align: center;
      }
    </style>
    <script type="text/javascript">
      function on_return_submit( evt )
      {
         if ( window.restbed.ws === null || window.restbed.ws.readyState !== window.restbed.ws.OPEN )
         {
            return;
         }

         if( evt && evt.keyCode == 13 )
         {
            var message = document.getElementById( "message" );
            window.restbed.ws.send( message.value );

            message.value = "";
         }
      }

      function toggle_control_access( )
      {
         var open = document.getElementById( "open" );
         open.disabled = !open.disabled;

         var message = document.getElementById( "message" );
         message.disabled = !message.disabled;

         var close = document.getElementById( "close" );
         close.className = ( close.className === "disabled" ) ? "" : "disabled";
      }

      function add_message( message )
      {
         var li = document.createElement( "li" );
         li.appendChild( document.createTextNode( "> " + message ) );

         var ul = document.getElementById( "messages" );
         ul.appendChild( li );
      }

      function open( )
      {
         if ( "WebSocket" in window )
         {
            var ws = new WebSocket( "ws://localhost:1984/chat" );

            ws.onopen = function( )
            {
               add_message( "Established connection." );

               toggle_control_access( );
            };

            ws.onmessage = function( evt )
            {
               add_message( evt.data );
            };

            ws.onclose = function( evt )
            {
               add_message( "Connection closed with RFC6455 code " + evt.code + "." );

               toggle_control_access( );
            };

            ws.onerror = function( evt )
            {
               add_message( "Error: socket connection interrupted." );
            };

            window.restbed.ws = ws;
         }
         else
         {
            alert( "WebSockets NOT supported by your Browser!" );
         }
      }

      function close( )
      {
         window.restbed.ws.close( );
      }

      ( function( )
      {
         window.restbed = { ws: null };
      } )( );
    </script>
  </head>
  <body>
     <div>
       <ul id="messages"></ul>
       <div id="controls">
         <input id="message" type="text" onKeyPress="return on_return_submit( event )" disabled/>
         <div>
           <a id="open" href="javascript:open( )">Open Chat</a>
           <a id="close" href="javascript:close( )" class="disabled">Close Chat</a>
         <div>
       </div>
     </div>
  </body>
</html>