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ADD: MuPDF v1.26.7: the MuPDF source as downloaded by a default build of PyMuPDF 1.26.4. The directory name has changed: no version number in the expanded directory now.
author Franz Glasner <fzglas.hg@dom66.de>
date Mon, 15 Sep 2025 11:43:07 +0200
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23 .TH CURLOPT_HEADERFUNCTION 3 "17 Jun 2014" "libcurl 7.37.0" "curl_easy_setopt options"
24 .SH NAME
25 CURLOPT_HEADERFUNCTION \- callback that receives header data
26 .SH SYNOPSIS
27 #include <curl/curl.h>
28
29 size_t header_callback(char *buffer,
30 size_t size,
31 size_t nitems,
32 void *userdata);
33
34 CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HEADERFUNCTION, header_callback);
35 .SH DESCRIPTION
36 Pass a pointer to your callback function, which should match the prototype
37 shown above.
38
39 This function gets called by libcurl as soon as it has received header
40 data. The header callback will be called once for each header and only
41 complete header lines are passed on to the callback. Parsing headers is very
42 easy using this. The size of the data pointed to by \fIbuffer\fP is \fIsize\fP
43 multiplied with \fInitems\fP. Do not assume that the header line is zero
44 terminated!
45
46 The pointer named \fIuserdata\fP is the one you set with the
47 \fICURLOPT_HEADERDATA(3)\fP option.
48
49 This callback function must return the number of bytes actually taken care of.
50 If that amount differs from the amount passed in to your function, it'll signal
51 an error to the library. This will cause the transfer to get aborted and the
52 libcurl function in progress will return \fICURLE_WRITE_ERROR\fP.
53
54 A complete HTTP header that is passed to this function can be up to
55 \fICURL_MAX_HTTP_HEADER\fP (100K) bytes and includes the final line terminator.
56
57 If this option is not set, or if it is set to NULL, but
58 \fICURLOPT_HEADERDATA(3)\fP is set to anything but NULL, the function used to
59 accept response data will be used instead. That is, it will be the function
60 specified with \fICURLOPT_WRITEFUNCTION(3)\fP, or if it is not specified or
61 NULL - the default, stream-writing function.
62
63 It's important to note that the callback will be invoked for the headers of
64 all responses received after initiating a request and not just the final
65 response. This includes all responses which occur during authentication
66 negotiation. If you need to operate on only the headers from the final
67 response, you will need to collect headers in the callback yourself and use
68 HTTP status lines, for example, to delimit response boundaries.
69
70 For an HTTP transfer, the status line and the blank line preceding the response
71 body are both included as headers and passed to this function.
72
73 When a server sends a chunked encoded transfer, it may contain a trailer. That
74 trailer is identical to an HTTP header and if such a trailer is received it is
75 passed to the application using this callback as well. There are several ways
76 to detect it being a trailer and not an ordinary header: 1) it comes after the
77 response-body. 2) it comes after the final header line (CR LF) 3) a Trailer:
78 header among the regular response-headers mention what header(s) to expect in
79 the trailer.
80
81 For non-HTTP protocols like FTP, POP3, IMAP and SMTP this function will get
82 called with the server responses to the commands that libcurl sends.
83 .SH LIMITATIONS
84 libcurl does not unfold HTTP "folded headers" (deprecated since RFC 7230). A
85 folded header is a header that continues on a subsequent line and starts with
86 a whitespace. Such folds will be passed to the header callback as a separate
87 one, although strictly it is just a continuation of the previous line.
88 .SH DEFAULT
89 Nothing.
90 .SH PROTOCOLS
91 Used for all protocols with headers or meta-data concept: HTTP, FTP, POP3,
92 IMAP, SMTP and more.
93 .SH EXAMPLE
94 .nf
95 static size_t header_callback(char *buffer, size_t size,
96 size_t nitems, void *userdata)
97 {
98 /* received header is nitems * size long in 'buffer' NOT ZERO TERMINATED */
99 /* 'userdata' is set with CURLOPT_HEADERDATA */
100 return nitems * size;
101 }
102
103 CURL *curl = curl_easy_init();
104 if(curl) {
105 curl_easy_setopt(curl, CURLOPT_URL, "http://example.com");
106
107 curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, header_callback);
108
109 curl_easy_perform(curl);
110 }
111 .fi
112 .SH AVAILABILITY
113 Always
114 .SH RETURN VALUE
115 Returns CURLE_OK
116 .SH "SEE ALSO"
117 .BR CURLOPT_HEADERDATA "(3), " CURLOPT_WRITEFUNCTION "(3), "