---
name: "curl_ws_send"
section: "3"
platform: "linux"
package: "curl"
version: "8.23.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "macos"]
url: "https://new.carta.tech/curl_ws_send"
---

# curl\_ws\_send(3)

send WebSocket data

## 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)](/curl_ws_meta/3.md)*.

If *CURLWS\_RAW\_MODE* is enabled in *[CURLOPT\_WS\_OPTIONS(3)](/CURLOPT_WS_OPTIONS/3.md)*, 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)](/curl_ws_meta/3.md)* 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)
{
  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

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)](/libcurl-errors/3.md)*. If *[CURLOPT\_ERRORBUFFER(3)](/CURLOPT_ERRORBUFFER/3.md)* was set with *[curl\_easy\_setopt(3)](/curl_easy_setopt/3.md)* 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)](/libcurl-errors/3.md)* man page for the full list with descriptions.

## SEE ALSO

[**curl\_easy\_getinfo**(3)](/curl_easy_getinfo/3.md), [**curl\_easy\_perform**(3)](/curl_easy_perform/3.md), [**curl\_easy\_setopt**(3)](/curl_easy_setopt/3.md), [**curl\_ws\_recv**(3)](/curl_ws_recv/3.md), [**curl\_ws\_start\_frame**(3)](/curl_ws_start_frame/3.md), [**libcurl-ws**(3)](/libcurl-ws/3.md)