curl_ws_meta(3) Library Functions Manual curl_ws_meta(3)
NAME
curl_ws_meta - meta data WebSocket information
SYNOPSIS
#include <curl/curl.h>
const struct curl_ws_frame *curl_ws_meta(CURL *curl);
DESCRIPTION
When the write callback (CURLOPT_WRITEFUNCTION(3)) is invoked on re-
ceived WebSocket traffic, curl_ws_meta(3) can be called from within the
callback to provide additional information about the current frame.
This function only works from within the callback, and only when receiv-
ing WebSocket data.
This function requires an easy handle as input argument for libcurl to
know what transfer the question is about, but as there is no such
pointer provided to the callback by libcurl itself, applications that
want to use curl_ws_meta(3) need to pass it on to the callback on its
own.
struct curl_ws_frame
struct curl_ws_frame {
int age;
int flags;
curl_off_t offset;
curl_off_t bytesleft;
size_t len;
};
age This field specify the age of this struct. It is always zero for
now.
flags This is a bitmask with individual bits set that describes the
WebSocket data. See the list below.
offset When this chunk is a continuation of frame data already deliv-
ered, this is the offset into the final frame data where this
piece belongs to.
bytesleft
If this is not a complete fragment, the bytesleft field informs
about how many additional bytes are expected to arrive before
this fragment is complete.
len The length of the current data chunk.
FLAGS
The message type flags (CURLWS_TEXT/BINARY/CLOSE/PING/PONG) are mutually
exclusive.
CURLWS_TEXT
This is a message with text data. Note that this makes a differ-
ence to WebSocket but libcurl itself does not make any verifica-
tion of the content or precautions that you actually receive
valid UTF-8 content.
CURLWS_BINARY
This is a message with binary data.
CURLWS_CLOSE
This is a close message. No more data follows.
It may contain a 2-byte unsigned integer in network byte order
that indicates the close reason and may additionally contain up
to 123 bytes of further textual payload for a total of at most
125 bytes. libcurl does not verify that the textual description
is valid UTF-8.
CURLWS_PING
This is a ping message. It may contain up to 125 bytes of payload
text. libcurl does not verify that the payload is valid UTF-8.
Upon receiving a ping message, libcurl automatically responds
with a pong message unless the CURLWS_NOAUTOPONG or
CURLWS_RAW_MODE bit of CURLOPT_WS_OPTIONS(3) is set.
CURLWS_PONG
This is a pong message. It may contain up to 125 bytes of payload
text. libcurl does not verify that the payload is valid UTF-8.
CURLWS_CONT
Can only occur in conjunction with CURLWS_TEXT or CURLWS_BINARY.
This is not the final fragment of the message, it implies that
there is another fragment coming as part of the same message. The
application must reassemble the fragments to receive the complete
message.
Only a single fragmented message can be transmitted at a time,
but it may be interrupted by CURLWS_CLOSE, CURLWS_PING or
CURLWS_PONG frames.
PROTOCOLS
This functionality affects ws only
EXAMPLE
/* we pass a pointer to this struct to the callback */
struct customdata {
CURL *easy;
void *ptr;
};
static size_t writecb(char *buffer,
size_t size, size_t nitems, void *p)
{
struct customdata *c = (struct customdata *)p;
const struct curl_ws_frame *m = curl_ws_meta(c->easy);
printf("flags: %x\n", m->flags);
return 0;
}
int main(void)
{
CURL *curl = curl_easy_init();
if(curl) {
struct customdata custom;
custom.easy = curl;
custom.ptr = NULL;
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writecb);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, &custom);
curl_easy_perform(curl);
}
return 0;
}
AVAILABILITY
Added in curl 7.86.0
RETURN VALUE
This function returns a pointer to a curl_ws_frame struct with read-only
information that is valid for this specific callback invocation. If it
cannot return this information, or if the function is called in the
wrong context, it returns NULL.
SEE ALSO
curl_easy_getinfo(3), curl_easy_setopt(3), curl_ws_recv(3),
curl_ws_send(3), libcurl-ws(3)
libcurl 2026-07-12 curl_ws_meta(3)
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