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 callback function gets invoked by libcurl as soon as it has received header data. The
       header callback is 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.  The  provide
       header line is not null-terminated!

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

       Your  callback  should  return  the number of bytes actually taken care of. If that amount
       differs from the amount passed to your callback function, it signals an error condition to
       the library. This causes the transfer to get aborted and the libcurl function used returns
       CURLE_WRITE_ERROR.

       You can also abort the transfer by returning CURL_WRITEFUNC_ERROR. (7.87.0)

       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 is used instead. That is  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 is 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 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 gets  called  with  the
       server responses to the commands that libcurl sends.

       A more convenient way to get HTTP headers might be to use curl_easy_header(3).

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
       are  passed  to  the  header  callback  as  separate ones, although strictly they are just
       continuations of the previous lines.

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;
       }

       int main(void)
       {
         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)