CURLOPT_HEADERFUNCTION(3)				     curl_easy_setopt options					 CURLOPT_HEADERFUNCTION(3)

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 very easy using this. The size of the data pointed to by  buf-
       fer  is	size  multiplied  with nmemb. Do not assume that the header line is zero 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  dif-
       fers  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_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.

       When  a server sends a chunked encoded transfer, it may contain a trailer. That trailer is identical to a 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 regu-
       lar 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.

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, "http://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),

libcurl 7.54.0							 February 03, 2016					 CURLOPT_HEADERFUNCTION(3)