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>
10
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);
18

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       very easy using this. buffer points to the delivered data, and the size
27       of that data is nitems; size is always 1. Do not assume that the header
28       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'll  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.
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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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