WebSockets for C++

Version 0.1 alpha

Quick and dirty implementation of WebSocket server.

For testing purposes only.

Example

websocket::Server server;

// listen on 0.0.0.0:8888
// log errors to stderr
server.start("0.0.0.0", 8888, std::cerr); 

...

// check for a new event
websocket::Event event;
websocket::ConnectionId connId;
std::string message;
if (server.poll(event, connId, message) {
    switch (event) {
    case websocket::Event::NewConnection:
        std::cout << '#' << connId << " connected\n";
        break;
    case websocket::Event::Message:
        std::cout << '#' << connId << " says " << message << '\n';
        // send reply
        server.send(connId, message);
        break;
    case websocket::Event::Disconnect:
        std::cout << '#' << connId << " disconnected\n";
        break;
    }
}

...

// stop server
server.stop();
// destructor also can call stop(), but it's better to do it explicitly

Features and limitations

• Fragmented messages are not supported
• Client can't send a message longer than 125 bytes
• Server can't send a message longer than UINT32_MAX bytes
• Server doesn't validate client text frames.

Overview of the WebSocket protocol

Handshake

Client sends HTTP GET request

GET /some/path/with/optional?query HTTP/1.1
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13
... other fields ...

Server replies with

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=

and then they send data frames for each other

Client data frames

Data frames from client looks like this:

XX YY [LL LL | LL LL LL LL LL LL LL LL] KK KK KK KK [data]

where

• XX - byte with opcode and the "final fragment" flag, 0x81 for non-fragmented text frame.
• YY - byte with data length, high bit is set to 1, i.e. 0x85 for 5-byte data.
  If length is 126, next two bytes are real 16-bit length. If length is 127, next eight bytes are real 64-bit length.
• KK - four bytes of the masking key
• data - data bytes XORed with the masking key, data[i] ^ key[i % 4].

Example:

81 85 F0 F0 F0 F0 C1 C2 C3 C4 C5 -- text "12345"

Server data frames

Almost like client frames, but without masking.

XX LL [LL LL | LL LL LL LL LL LL LL LL] [data]

• XX - opcode and "final fragment" flag.
• LL - data length (0 - 125), 126 and 127 for extended length.
• data - data bytes.

Example:

81 05 31 32 33 34 35 -- text "12345"

License

websocket-cpp is placed in the public domain.

External links

• RFC 6455 - The WebSocket Protocol