CURLOPT_HEADERFUNCTION(3)

1CURLOPT_HEADERFUNCTION(3)  curl_easy_setopt options  CURLOPT_HEADERFUNCTION(3)
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NAME

6       CURLOPT_HEADERFUNCTION - callback that receives header data
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SYNOPSIS

9       #include <curl/curl.h>
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11       size_t header_callback(char *buffer,
12                              size_t size,
13                              size_t nitems,
14                              void *userdata);
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16       CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HEADERFUNCTION,
17                                 header_callback);
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DESCRIPTION

20       Pass a pointer to your callback function, which should match the proto‐
21       type shown above.
22
23       This function gets called by libcurl as soon as it has received  header
24       data.  The header callback will be called once for each header and only
25       complete header lines are passed on to the callback. Parsing headers is
26       easy  to  do  using this callback. buffer points to the delivered data,
27       and the size of that data is nitems; size is always 1.  Do  not  assume
28       that the header line is null-terminated!
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30       The  pointer named userdata is the one you set with the CURLOPT_HEADER‐
31       DATA(3) option.
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33       This callback function must return the number of bytes  actually  taken
34       care  of.   If  that  amount  differs from the amount passed in to your
35       function, it will signal an error to the library. This will  cause  the
36       transfer  to  get aborted and the libcurl function in progress will re‐
37       turn CURLE_WRITE_ERROR.
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39       A complete HTTP header that is passed to this function  can  be  up  to
40       CURL_MAX_HTTP_HEADER  (100K) bytes and includes the final line termina‐
41       tor.
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43       If this option is not set, or if it is set to NULL, but CURLOPT_HEADER‐
44       DATA(3)  is  set  to anything but NULL, the function used to accept re‐
45       sponse data will be used instead. That is,  it  will  be  the  function
46       specified  with  CURLOPT_WRITEFUNCTION(3), or if it is not specified or
47       NULL - the default, stream-writing function.
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49       It's important to note that the callback will be invoked for the  head‐
50       ers  of  all responses received after initiating a request and not just
51       the final response. This includes all responses which occur during  au‐
52       thentication  negotiation.  If  you need to operate on only the headers
53       from the final response, you will need to collect headers in the  call‐
54       back  yourself  and  use HTTP status lines, for example, to delimit re‐
55       sponse boundaries.
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57       For an HTTP transfer, the status line and the blank line preceding  the
58       response body are both included as headers and passed to this function.
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60       When  a  server  sends  a  chunked  encoded  transfer, it may contain a
61       trailer. That trailer is identical to an HTTP  header  and  if  such  a
62       trailer is received it is passed to the application using this callback
63       as well. There are several ways to detect it being a trailer and not an
64       ordinary header: 1) it comes after the response-body. 2) it comes after
65       the final header line (CR LF) 3) a Trailer: header  among  the  regular
66       response-headers mention what header(s) to expect in the trailer.
67
68       For non-HTTP protocols like FTP, POP3, IMAP and SMTP this function will
69       get called with the server  responses  to  the  commands  that  libcurl
70       sends.
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LIMITATIONS

73       libcurl  does  not  unfold  HTTP "folded headers" (deprecated since RFC
74       7230). A folded header is a header that continues on a subsequent  line
75       and  starts  with a whitespace. Such folds will be passed to the header
76       callback as a separate one, although strictly it is just a continuation
77       of the previous line.
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DEFAULT

80       Nothing.
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PROTOCOLS

83       Used  for  all  protocols with headers or meta-data concept: HTTP, FTP,
84       POP3, IMAP, SMTP and more.
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EXAMPLE

87       static size_t header_callback(char *buffer, size_t size,
88                                     size_t nitems, void *userdata)
89       {
90         /* received header is nitems * size long in 'buffer' NOT ZERO TERMINATED */
91         /* 'userdata' is set with CURLOPT_HEADERDATA */
92         return nitems * size;
93       }
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95       CURL *curl = curl_easy_init();
96       if(curl) {
97         curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
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99         curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, header_callback);
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101         curl_easy_perform(curl);
102       }
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AVAILABILITY

105       Always
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RETURN VALUE

108       Returns CURLE_OK
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