libcurl-ws - Man Page

The WebSocket interface provides functions for receiving and sending WebSocket data.

You still only include <curl/curl.h> in your code.

WebSocket is also often known as WebSockets, in plural. It is done by upgrading a regular HTTP(S) GET request to a WebSocket connection.

WebSocket is a TCP-like message-based communication protocol done over HTTP, specified in RFC 6455.

To initiate a WebSocket session with libcurl, setup an easy handle to use a URL with a "WS://" or "WSS://" scheme. "WS" is for cleartext communication over HTTP and "WSS" is for doing WebSocket securely over HTTPS.

A WebSocket request is done as an HTTP/1 GET request with an "Upgrade WebSocket" request header field. When the upgrade is accepted by the server, it responds with a 101 Switching and then the client can speak WebSocket with the server. The communication can happen in both directions at the same time.

The WebSocket protocol allows the client to request and negotiate extensions can add additional features and restrictions to the protocol.

libcurl does not support the use of extensions and always sets up a connection without them.

WebSocket communication is message based. That means that both ends send and receive entire messages, not streams like TCP. A WebSocket message is sent over the wire in one or more frames. A message which is split into several frames is referred to as a fragmented message and the individual frames are called fragments. Each frame (or fragment) in a message can have a size of up to 2^63 bytes and declares the frame size in the header. The total size of a message that is fragmented into multiple frames is not limited by the protocol and the number of fragments is not known until the final fragment is received.

Transmission of a frame must not be interrupted by any other data transfers and transmission of the different fragments of a message must not be interrupted by other user data frames. Control frames - Ping, PONG and CLOSE - may be transmitted in between any other two frames, even in between two fragments of the same user data message. The control frames themselves on the other hand must never be fragmented and are limited to a size of 125 bytes.

libcurl delivers WebSocket data as chunks of frames. It might deliver a whole frame as a single chunk, but it might also deliver it in several pieces depending on size and network patterns. See the individual API documentations for further information.

WebSocket is designed to allow long-lived sessions and in order to keep the connections alive, both ends can send PING messages for the other end to respond with a PONG. Both ends may also send unsolicited PONG messages as unidirectional heartbeat.

libcurl automatically responds to server PING messages with a PONG that echoes the payload of the PING message. libcurl does neither send any PING messages nor any unsolicited PONG messages automatically.

Because of the many different ways WebSocket can be used, which is much more flexible than limited to plain downloads or uploads, libcurl offers two different API models to use it:

1. CURLOPT_WRITEFUNCTION model: Using a write callback with CURLOPT_WRITEFUNCTION(3) much like other downloads for when the traffic is download oriented.

2. CURLOPT_CONNECT_ONLY model: Using curl_ws_recv(3) and curl_ws_send(3) functions.

CURLOPT_WRITEFUNCTION MODEL

CURLOPT_CONNECT_ONLY(3) must be unset or 0L for this model to take effect.

curl_easy_perform(3) establishes and sets up the WebSocket communication and then blocks for the whole duration of the connection. libcurl calls the callback configured in CURLOPT_WRITEFUNCTION(3), whenever an incoming chunk of WebSocket data is received. The callback is handed a pointer to the payload data as an argument and can call curl_ws_meta(3) to get relevant metadata.

CURLOPT_CONNECT_ONLY MODEL

CURLOPT_CONNECT_ONLY(3) must be 2L for this model to take effect.

curl_easy_perform(3) only establishes and sets up the WebSocket communication and then returns control back to the application. The application can then use curl_ws_recv(3) and curl_ws_send(3) to exchange WebSocket messages with the server.

libcurl can be told to speak WebSocket in "raw mode" by setting the CURLWS_RAW_MODE bit of the CURLOPT_WS_OPTIONS(3) option.

Raw WebSocket means that libcurl passes on the data from the network without parsing it, leaving that entirely to the application.

This mode is intended for applications that already have a WebSocket parser/engine and want to switch over to use libcurl for enabling WebSocket, and keep parts of the existing software architecture.

CURLOPT_CONNECT_ONLY(3), CURLOPT_WRITEFUNCTION(3), CURLOPT_WS_OPTIONS(3), curl_easy_init(3), curl_ws_meta(3), curl_ws_recv(3), curl_ws_send(3)

curl_ws_meta(3), curl_ws_recv(3), curl_ws_send(3), libcurl(3).