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libcurl(3) libcurl libcurl(3)
NAME
libcurl-ws - WebSocket interface overview
DESCRIPTION
The WebSocket interface provides functions for receiving and sending
WebSocket data.
INCLUDE
You still only include <curl/curl.h> in your code.
SETUP
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 com-
munication 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 direc-
tions at the same time.
MESSAGES
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. Each frame in a
message can have a size up to 2^63 bytes.
libcurl delivers WebSocket data as frame fragments. It might send a
whole frame, but it might also deliver them in pieces depending on size
and network patterns. It makes sure to provide the API user about the
exact specifics about the fragment: type, offset, size and how much
data there is pending to arrive for the same frame.
A message has an unknown size until the last frame header for the mes-
sage has been received since only frames have set sizes.
Raw mode
libcurl can be told to speak WebSocket in "raw mode" by setting the
CURLWS_RAW_MODE bit to 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
assumes that the user of this knows WebSocket and can parse and figure
out the data all by itself.
This mode is intended for applications that already have a WebSocket
parser/engine that want to switch over to use libcurl for enabling Web-
Socket, but keep parts of the existing software architecture.
PING
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.
libcurl automatically responds to server PING messages with a PONG. It
does not send any PING messages automatically.
MODELS
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. Using a write callback with CURLOPT_WRITEFUNCTION(3) much like other
downloads for when the traffic is download oriented.
2. Using CURLOPT_CONNECT_ONLY(3) and use the WebSocket recv/send func-
tions.
Callback model
When a write callback is set and a WebSocket transfer is performed, the
callback is called to deliver all WebSocket data that arrives.
The callback can then call curl_ws_meta(3) to learn about the details
of the incoming data fragment.
CONNECT_ONLY model
By setting CURLOPT_CONNECT_ONLY(3) to 2L, the transfer only establishes
and setups the WebSocket communication and then returns control back to
the application.
Once such a setup has been successfully performed, the application can
proceed and use curl_ws_recv(3) and curl_ws_send(3) freely to exchange
WebSocket messages with the server.
AVAILABILITY
The WebSocket API was introduced as experimental in 7.86.0 and is still
experimental today.
It is only built-in if explicitly opted in at build time. We discourage
use of the WebSocket API in production because of its experimental
state. We might change API, ABI and behavior before this "goes live".
SEE ALSO
curl_easy_init(3), curl_ws_meta(3), curl_ws_recv(3), curl_ws_send(3),
CURLOPT_CONNECT_ONLY(3), CURLOPT_WRITEFUNCTION(3), CUR-
LOPT_WS_OPTIONS(3)
libcurl 8.4.0 September 26, 2023 libcurl(3)
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