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

# NAME

curl_ws_meta - meta data WebSocket information

# SYNOPSIS

~~~c
#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 received
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 receiving
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.

WebSocket messages are split into *frames*. A WebSocket message can be made up
of an arbitrary number of frames. Each WebSocket frame payload can be up to
2^63-1 bytes. When libcurl delivers WebSocket data, it splits each frame into
*chunks*; each chunk is therefore part of a frame, or at most a full frame.

Each callback delivers data for a single chunk that is then part of a single
frame.

# struct curl_ws_frame

~~~c
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 delivered in a
previous callback, this is the offset into the complete frame payload where
this chunk's data belongs.

## `bytesleft`

When this is not a complete frame nor the last chunk for a frame, the
*bytesleft* field informs about how many additional bytes are expected to
arrive before this frame 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 difference to WebSocket
but libcurl itself does not make any verification 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 *frame* of the message, it implies that there is another
*frame* coming as part of the same message. The application must reassemble
the frames to receive the complete message.

Only a single multi-frame message can be transmitted at a time, but it may be
interrupted by CURLWS_CLOSE, CURLWS_PING or CURLWS_PONG frames.

# %PROTOCOLS%

# EXAMPLE

~~~c

/* 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", (unsigned int)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%

# 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.
