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. 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
       _HEADERDATA&section=3">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'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 and includes the final line
       terminator.

       If this option is not set, or if it is set to NULL, but
       _HEADERDATA&section=3">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 _WRITEFUNCTION&section=3">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),

libcurl 7.77.0                 November 4, 2020      CURLOPT_HEADERFUNCTION(3)