---
c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
SPDX-License-Identifier: curl
Title: curl_ws_send
Section: 3
Source: libcurl
See-also:
  - curl_easy_getinfo (3)
  - curl_easy_perform (3)
  - curl_easy_setopt (3)
  - curl_ws_recv (3)
  - curl_ws_start_frame (3)
  - libcurl-ws (3)
Protocol:
  - WS
Added-in: 7.86.0
---

# NAME

curl_ws_send - send WebSocket data

# SYNOPSIS

~~~c
#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%

# EXAMPLE

~~~c
#include <string.h> /* for strlen */

int main(void)
{
  static const char *buffer = "PAYLOAD";
  size_t offset = 0;
  CURLcode result = 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(!result) {
    size_t sent;
    result = curl_ws_send(curl, buffer + offset,
                          strlen(buffer) - offset, &sent,
                          0, CURLWS_TEXT);
    offset += sent;

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

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

  curl_easy_cleanup(curl);
  return (int)result;
}
~~~

# %AVAILABILITY%

# 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.
