curl_ws_send(3)

NAME
curl_ws_send - send WebSocket data

SYNOPSIS
#include <curl/curl.h>

CURLcode curl_ws_send(CURL *curl, const void *buffer, size_t buflen,
                      size_t *sent, curl_off_t fragsize,
                      unsigned int flags);

DESCRIPTION
Send the specific message chunk over an established WebSocket connection. buffer must point to a valid memory location containing (at least) buflen bytes of payload memory.

sent is set to the number of payload bytes actually sent. If the return value is CURLE_OK but sent is less than the given buflen, libcurl was unable to consume the complete payload in a single call. In this case the application must call this function again until all payload is processed. buffer and buflen must be updated on every following invocation to only point to the remaining piece of the payload.

fragsize should always be set to zero unless a (huge) frame shall be sent using multiple calls with partial content per call explicitly. In that case you must set the CURLWS_OFFSET bit and set the fragsize as documented in the section on CURLWS_OFFSET below.

flags must contain at least one flag indicating the type of the message. To send a fragmented message consisting of multiple frames, additionally set the CURLWS_CONT bit in all frames except the final one. The appropriate message type bit should be set in every frame of a fragmented message without exemption. Omitting the message type for continuation frames of a fragmented message is only supported for backwards compatibility and highly discouraged.

For more details on the supported flags see below and in curl_ws_meta(3).

If CURLWS_RAW_MODE is enabled in CURLOPT_WS_OPTIONS(3), the flags argument should be set to 0.

Warning: while it is possible to invoke this function from a callback, such a call is blocking in this situation, e.g. only returns after all data has been sent or an error is encountered.

FLAGS
Supports all flags documented in curl_ws_meta(3) and additionally the following flags.

CURLWS_OFFSET
    The provided data is only a partial frame and there is more coming in a following call to curl_ws_send(). When sending only a piece of the frame like this, the fragsize must be provided with the total expected frame size in the first call and must be zero in all subsequent calls.

PROTOCOLS
This functionality affects ws only

EXAMPLE
#include <string.h> /* for strlen */

int main(void)
{
  const char *buffer = "PAYLOAD";
  size_t offset = 0;
  CURLcode res = CURLE_OK;
  CURL *curl = curl_easy_init();

  curl_easy_setopt(curl, CURLOPT_URL, "wss://example.com/");
  curl_easy_setopt(curl, CURLOPT_CONNECT_ONLY, 2L);
  /* start HTTPS connection and upgrade to WSS, then return control */
  curl_easy_perform(curl);

  while(!res) {
    size_t sent;
    res = curl_ws_send(curl, buffer + offset, strlen(buffer) - offset, &sent,
                       0, CURLWS_TEXT);
    offset += sent;

    if(res == CURLE_OK) {
      if(offset == strlen(buffer))
        break; /* finished sending */
    }

    if(res == CURLE_AGAIN)
      /* in real application: wait for socket here, e.g. using select() */
      res = CURLE_OK;
  }

  curl_easy_cleanup(curl);
  return (int)res;
}

AVAILABILITY
Added in curl 7.86.0

RETURN VALUE
This function returns a CURLcode indicating success or error.

CURLE_OK (0) means everything was OK, non-zero means an error occurred, see libcurl-errors(3). If CURLOPT_ERRORBUFFER(3) was set with curl_easy_setopt(3) there can be an error message stored in the error buffer when non-zero is returned.

Instead of blocking, the function returns CURLE_AGAIN. The correct behavior is then to wait for the socket to signal readability before calling this function again.

Any other non-zero return value indicates an error. See the libcurl-errors(3) man page for the full list with descriptions.

SEE ALSO
curl_easy_getinfo(3), curl_easy_perform(3), curl_easy_setopt(3), curl_ws_recv(3), libcurl-ws(3)