libcurl-ws(3) Library Functions Manual libcurl-ws(3) NAME libcurl-ws - WebSocket interface overview DESCRIPTION The WebSocket interface provides functions for receiving and sending Web- Socket 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 up- grading 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 Web- Socket" 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. EXTENSIONS The WebSocket protocol allows the client to request and negotiate exten- sions can add additional features and restrictions to the protocol. libcurl does not support the use of extensions and always sets up a connec- tion without them. 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. 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 lim- ited by the protocol and the number of fragments is not known until the fi- nal 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 inter- rupted 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 docu- mentations for further information. 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. 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. The automatic re- ply to PING messages can be disabled through CURLOPT_WS_OPTIONS(3). 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. CURLOPT_WRITEFUNCTION/CURLOPT_READFUNCTION model: Using a write callback with CURLOPT_WRITEFUNCTION(3) much like other downloads for when the traf- fic is download oriented. Using a read callback with CURLOPT_READFUNCTION(3) much like other uploads for sending WebSocket frames to the server. 2. CURLOPT_CONNECT_ONLY model: Using curl_ws_recv(3) and curl_ws_send(3) functions. CURLOPT_WRITEFUNCTION/CURLOPT_READFUNCTION 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 communi- cation 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 call- back is handed a pointer to the payload data as an argument and can call curl_ws_meta(3) to get relevant metadata. With libcurl 8.16.0 or later, sending of WebSocket frames via a CUR- LOPT_READFUNCTION(3) is supported. To use that on such a connection, register a callback via CURLOPT_READFUNCTION(3) and set CURLOPT_UP- LOAD(3) as well. Once, the WebSocket connection is established, your callback is invoked to get data to send. That data is sent in a CURLWS_BINARY frame with length of exactly the data returned. To send other frame types or longer frames, use curl_ws_start_frame(3) in the read callback. See the websocket-up- down example. When using curl_multi_perform(3) to drive transfers, more possibili- ties exist. The CURLOPT_READFUNCTION(3) may return CURL_READ- FUNC_PAUSE when it has no more data to send. Calling curl_easy_pause(3) afterwards resumes the upload and the read call- back is invoked again. 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 com- munication and then returns control back to the application. The ap- plication can then use curl_ws_recv(3) and curl_ws_send(3) to ex- change WebSocket messages with the server. RAW MODE 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 with- out 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 Web- Socket, and keep parts of the existing software architecture. SEE ALSO 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) libcurl 2026-08-27 libcurl-ws(3)
NAME | DESCRIPTION | INCLUDE | SETUP | EXTENSIONS | MESSAGES | PING | MODELS | RAW MODE | SEE ALSO
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