1---
2c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
3SPDX-License-Identifier: curl
4Title: CURLOPT_HEADERFUNCTION
5Section: 3
6Source: libcurl
7See-also:
8  - CURLOPT_HEADERDATA (3)
9  - CURLOPT_WRITEFUNCTION (3)
10  - curl_easy_header (3)
11---
12
13# NAME
14
15CURLOPT_HEADERFUNCTION - callback that receives header data
16
17# SYNOPSIS
18
19~~~c
20#include <curl/curl.h>
21
22size_t header_callback(char *buffer,
23                       size_t size,
24                       size_t nitems,
25                       void *userdata);
26
27CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HEADERFUNCTION,
28                          header_callback);
29~~~
30
31# DESCRIPTION
32
33Pass a pointer to your callback function, which should match the prototype
34shown above.
35
36This callback function gets invoked by libcurl as soon as it has received
37header data. The header callback is called once for each header and only
38complete header lines are passed on to the callback. Parsing headers is easy
39to do using this callback. *buffer* points to the delivered data, and the
40size of that data is *nitems*; *size* is always 1. The provide header
41line is not null-terminated!
42
43The pointer named *userdata* is the one you set with the
44CURLOPT_HEADERDATA(3) option.
45
46Your callback should return the number of bytes actually taken care of. If
47that amount differs from the amount passed to your callback function, it
48signals an error condition to the library. This causes the transfer to get
49aborted and the libcurl function used returns *CURLE_WRITE_ERROR*.
50
51You can also abort the transfer by returning CURL_WRITEFUNC_ERROR. (7.87.0)
52
53A complete HTTP header that is passed to this function can be up to
54*CURL_MAX_HTTP_HEADER* (100K) bytes and includes the final line terminator.
55
56If this option is not set, or if it is set to NULL, but
57CURLOPT_HEADERDATA(3) is set to anything but NULL, the function used to
58accept response data is used instead. That is the function specified with
59CURLOPT_WRITEFUNCTION(3), or if it is not specified or NULL - the
60default, stream-writing function.
61
62It is important to note that the callback is invoked for the headers of all
63responses received after initiating a request and not just the final
64response. This includes all responses which occur during authentication
65negotiation. If you need to operate on only the headers from the final
66response, you need to collect headers in the callback yourself and use HTTP
67status lines, for example, to delimit response boundaries.
68
69For an HTTP transfer, the status line and the blank line preceding the response
70body are both included as headers and passed to this function.
71
72When a server sends a chunked encoded transfer, it may contain a trailer. That
73trailer is identical to an HTTP header and if such a trailer is received it is
74passed to the application using this callback as well. There are several ways
75to detect it being a trailer and not an ordinary header: 1) it comes after the
76response-body. 2) it comes after the final header line (CR LF) 3) a Trailer:
77header among the regular response-headers mention what header(s) to expect in
78the trailer.
79
80For non-HTTP protocols like FTP, POP3, IMAP and SMTP this function gets called
81with the server responses to the commands that libcurl sends.
82
83A more convenient way to get HTTP headers might be to use
84curl_easy_header(3).
85
86# LIMITATIONS
87
88libcurl does not unfold HTTP "folded headers" (deprecated since RFC 7230). A
89folded header is a header that continues on a subsequent line and starts with
90a whitespace. Such folds are passed to the header callback as separate ones,
91although strictly they are just continuations of the previous lines.
92
93# DEFAULT
94
95Nothing.
96
97# PROTOCOLS
98
99Used for all protocols with headers or meta-data concept: HTTP, FTP, POP3,
100IMAP, SMTP and more.
101
102# EXAMPLE
103
104~~~c
105static size_t header_callback(char *buffer, size_t size,
106                              size_t nitems, void *userdata)
107{
108  /* received header is nitems * size long in 'buffer' NOT ZERO TERMINATED */
109  /* 'userdata' is set with CURLOPT_HEADERDATA */
110  return nitems * size;
111}
112
113int main(void)
114{
115  CURL *curl = curl_easy_init();
116  if(curl) {
117    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
118
119    curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, header_callback);
120
121    curl_easy_perform(curl);
122  }
123}
124~~~
125
126# AVAILABILITY
127
128Always
129
130# RETURN VALUE
131
132Returns CURLE_OK
133