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       CURLOPT_HEADERFUNCTION - callback that receives header data


       #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,


       Pass a pointer to your callback function, which should match the proto-
       type 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
       very easy using this. The size of the data pointed to by buffer is size
       multiplied  with nmemb. Do not assume that the header line is zero ter-

       The pointer named userdata is the one you set with the  CURLOPT_HEADER-
       DATA(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'll  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.

       If this option is not set, or if it is set to NULL, but CURLOPT_HEADER-
       DATA(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  head-
       ers  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  call-
       back  yourself  and  use  HTTP  status  lines,  for example, to delimit
       response boundaries.

       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


       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.




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


       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, "");

         curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, header_callback);





       Returns CURLE_OK



libcurl 7.61.1                  August 11, 2018      CURLOPT_HEADERFUNCTION(3)

curl 7.61.1 - Generated Thu Sep 6 07:55:52 CDT 2018
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