NAME

       CURLOPT_HEADERFUNCTION - callback that receives header data

SYNOPSIS

       #include <curl/curl.h>

       size_t header_callback(char *buffer,
                              size_t size,
                              size_t nitems,
                              void *userdata);

       CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HEADERFUNCTION,
                                 header_callback);

DESCRIPTION

       Pass a pointer to your callback function, which should match the prototype shown above.

       This  function  gets  called by libcurl as soon as it has received header data. The header
       callback will be called once for each header and only complete header lines are passed  on
       to  the  callback. Parsing headers is easy to do using this callback. buffer points to the
       delivered data, and the size of that data is nitems; size is always 1. Do not assume  that
       the header line is null-terminated!

       The pointer named userdata is the one you set with the CURLOPT_HEADERDATA(3) option.

       This  callback  function  must return the number of bytes actually taken care of.  If that
       amount differs from the amount passed in to your function, it will signal an error to  the
       library.  This will cause the transfer to get aborted and the libcurl function in progress
       will return CURLE_WRITE_ERROR.

       A complete HTTP header that is passed to this function can be up  to  CURL_MAX_HTTP_HEADER
       (100K) bytes and includes the final line terminator.

       If  this  option  is not set, or if it is set to NULL, but CURLOPT_HEADERDATA(3) is set to
       anything but NULL, the function used to accept response data will be  used  instead.  That
       is,  it  will  be  the  function  specified with CURLOPT_WRITEFUNCTION(3), or if it is not
       specified or NULL - the default, stream-writing function.

       It's important to note that the callback will be invoked for the headers of all  responses
       received  after  initiating  a  request and not just the final response. This includes all
       responses which occur during authentication negotiation. If you need to  operate  on  only
       the  headers  from  the  final  response, you will need to collect headers in the callback
       yourself and use HTTP status lines, for example, to delimit response boundaries.

       For an HTTP transfer, the status line and the blank line preceding the response  body  are
       both included as headers and passed to this function.

       When  a server sends a chunked encoded transfer, it may contain a trailer. That trailer is
       identical to an HTTP header and if such  a  trailer  is  received  it  is  passed  to  the
       application  using  this  callback  as  well.  There are several ways to detect it being a
       trailer and not an ordinary header: 1) it comes after the response-body. 2) it comes after
       the  final  header  line  (CR  LF) 3) a Trailer: header among the regular response-headers
       mention what header(s) to expect in the trailer.

       For non-HTTP protocols like FTP, POP3, IMAP and SMTP this function will  get  called  with
       the server responses to the commands that libcurl sends.

LIMITATIONS

       libcurl does not unfold HTTP "folded headers" (deprecated since RFC 7230). A folded header
       is a header that continues on a subsequent line and starts with a whitespace.  Such  folds
       will  be  passed  to the header callback as a separate one, although strictly it is just a
       continuation of the previous line.

DEFAULT

       Nothing.

PROTOCOLS

       Used for all protocols with headers or meta-data concept: HTTP, FTP, POP3, IMAP, SMTP  and
       more.

EXAMPLE

       static size_t header_callback(char *buffer, size_t size,
                                     size_t nitems, void *userdata)
       {
         /* received header is nitems * size long in 'buffer' NOT ZERO TERMINATED */
         /* 'userdata' is set with CURLOPT_HEADERDATA */
         return nitems * size;
       }

       CURL *curl = curl_easy_init();
       if(curl) {
         curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");

         curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, header_callback);

         curl_easy_perform(curl);
       }

AVAILABILITY

       Always

RETURN VALUE

       Returns CURLE_OK

SEE ALSO

       curl_easy_header(3), CURLOPT_HEADERDATA(3), CURLOPT_WRITEFUNCTION(3),