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

       CURLOPT_HEADERDATA(3), CURLOPT_WRITEFUNCTION(3),