curl_ws_send(3) Library Functions Manual 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 connec-
tion. 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 invoca-
tion 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 appro-
priate 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 compati-
bility 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 argu-
ment 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 com-
ing 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 be-
havior is then to wait for the socket to signal readability before call-
ing this function again.
Any other non-zero return value indicates an error. See the libcurl-er-
rors(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)
libcurl 2026-07-12 curl_ws_send(3)
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