Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages

  
 
  

home | help 
CURLOPT_HEADERFUNCTION(3)   Library Functions Manual   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 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 provided header line
     is not null-terminated. Do not modify the passed in buffer.

     The pointer named userdata is the one  you  set  with  the  CURLOPT_HEADER-
     DATA(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_HEADER-
     DATA(3) is set to anything but NULL, the function used to	accept	response
     data  is  used  instead. That is the function specified with CURLOPT_WRITE-
     FUNCTION(3), or if it is not specified or NULL - the default,  stream-writ-
     ing function.

     It is important to note that the callback is invoked for the headers of all
     responses	received  after initiating a request and not 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  re-
     sponse 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  re-
     ceived  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  men-
     tion 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 continuations of the previous lines.

DEFAULT
     Nothing.

PROTOCOLS
     This functionality affects ftp, http, imap, pop3 and smtp

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

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

	 curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, header_callback);

	 result = curl_easy_perform(curl);

	 curl_easy_cleanup(curl);
       }
     }

AVAILABILITY
     Added in curl 7.7.2

RETURN VALUE
     curl_easy_setopt(3) returns a CURLcode indicating success or error.

     CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
     libcurl-errors(3).

SEE ALSO
     CURLOPT_HEADERDATA(3), CURLOPT_WRITEFUNCTION(3), curl_easy_header(3)

libcurl 			   2026-06-11	       CURLOPT_HEADERFUNCTION(3)

Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=CURLOPT_HEADERFUNCTION&sektion=3&manpath=FreeBSD+Ports+15.1.quarterly>

home | help