NAME¶

curl_ws_start_frame - start a new WebSocket frame

SYNOPSIS¶

#include <curl/curl.h>
CURLcode curl_ws_start_frame(CURL *curl,


                             unsigned int flags,


                             curl_off_t frame_len);

DESCRIPTION¶

Add the WebSocket frame header for the given flags and length to the transfers send buffer for WebSocket encoded data. Intended for use in a CURLOPT_READFUNCTION(3) callback.

When using a CURLOPT_READFUNCTION(3) in a WebSocket transfer, any data returned by that function is sent as a CURLWS_BINARY frame with the length being the amount of data read.

To send larger frames or frames of a different type, call curl_ws_start_frame() from within the read function and then return the data belonging to the frame.

The function fails, if a previous frame has not been completely read yet. Also it fails in CURLWS_RAW_MODE.

The read function in libcurl usually treats a return value of 0 as the end of file indication and stops any further reads. This would prevent sending WebSocket frames of length 0.

If the read function calls curl_ws_start_frame() however, a return value of 0 is not treated as an end of file and libcurl calls the read function again.

FLAGS¶

Supports all flags documented in curl_ws_meta(3).

PROTOCOLS¶

This functionality affects ws only

EXAMPLE¶

#include <string.h> /* for strlen */
struct read_ctx {


  CURL *easy;


  char *message;


  size_t msg_len;


  size_t nsent;
};
static size_t readcb(char *buf, size_t nitems, size_t buflen, void *p)
{


  struct read_ctx *ctx = p;


  size_t len = nitems * buflen;


  size_t left = ctx->msg_len - ctx->nsent;


  CURLcode result;


  if(!ctx->nsent) {


    /* Want to send TEXT frame. */


    result = curl_ws_start_frame(ctx->easy, CURLWS_TEXT,


                                 (curl_off_t)ctx->msg_len);


    if(result) {


      fprintf(stderr, "error starting frame: %d\n", result);


      return CURL_READFUNC_ABORT;


    }


  }


  if(left) {


    if(left < len)


      len = left;


    memcpy(buf, ctx->message + ctx->nsent, len);


    ctx->nsent += len;


    return len;


  }


  return 0;
}
int main(void)
{


  CURL *easy;


  struct read_ctx rctx;


  CURLcode res;


  easy = curl_easy_init();


  if(!easy)


    return 1;


  curl_easy_setopt(easy, CURLOPT_URL, "wss://example.com");


  curl_easy_setopt(easy, CURLOPT_READFUNCTION, readcb);


  /* tell curl that we want to send the payload */


  memset(&rctx, 0, sizeof(rctx));


  rctx.easy = easy;


  rctx.message = "Hello, friend!";


  rctx.msg_len = strlen(rctx.message);


  curl_easy_setopt(easy, CURLOPT_READDATA, &rctx);


  curl_easy_setopt(easy, CURLOPT_UPLOAD, 1L);


  /* Perform the request, res gets the return code */


  res = curl_easy_perform(easy);


  /* Check for errors */


  if(res != CURLE_OK)


    fprintf(stderr, "curl_easy_perform() failed: %s\n",


            curl_easy_strerror(res));


  /* always cleanup */


  curl_easy_cleanup(easy);


  return 0;
}

AVAILABILITY¶

Added in curl 8.16.0

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)