1--- 2c: Copyright (C) Daniel Stenberg, <daniel@haxx.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) 11Protocol: 12 - HTTP 13 - FTP 14 - POP3 15 - IMAP 16 - SMTP 17--- 18 19# NAME 20 21CURLOPT_HEADERFUNCTION - callback that receives header data 22 23# SYNOPSIS 24 25~~~c 26#include <curl/curl.h> 27 28size_t header_callback(char *buffer, 29 size_t size, 30 size_t nitems, 31 void *userdata); 32 33CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HEADERFUNCTION, 34 header_callback); 35~~~ 36 37# DESCRIPTION 38 39Pass a pointer to your callback function, which should match the prototype 40shown above. 41 42This callback function gets invoked by libcurl as soon as it has received 43header data. The header callback is called once for each header and only 44complete header lines are passed on to the callback. Parsing headers is easy 45to do using this callback. *buffer* points to the delivered data, and the 46size of that data is *nitems*; *size* is always 1. The provide header 47line is not null-terminated! 48 49The pointer named *userdata* is the one you set with the 50CURLOPT_HEADERDATA(3) option. 51 52Your callback should return the number of bytes actually taken care of. If 53that amount differs from the amount passed to your callback function, it 54signals an error condition to the library. This causes the transfer to get 55aborted and the libcurl function used returns *CURLE_WRITE_ERROR*. 56 57You can also abort the transfer by returning CURL_WRITEFUNC_ERROR. (7.87.0) 58 59A complete HTTP header that is passed to this function can be up to 60*CURL_MAX_HTTP_HEADER* (100K) bytes and includes the final line terminator. 61 62If this option is not set, or if it is set to NULL, but 63CURLOPT_HEADERDATA(3) is set to anything but NULL, the function used to 64accept response data is used instead. That is the function specified with 65CURLOPT_WRITEFUNCTION(3), or if it is not specified or NULL - the 66default, stream-writing function. 67 68It is important to note that the callback is invoked for the headers of all 69responses received after initiating a request and not just the final 70response. This includes all responses which occur during authentication 71negotiation. If you need to operate on only the headers from the final 72response, you need to collect headers in the callback yourself and use HTTP 73status lines, for example, to delimit response boundaries. 74 75For an HTTP transfer, the status line and the blank line preceding the response 76body are both included as headers and passed to this function. 77 78When a server sends a chunked encoded transfer, it may contain a trailer. That 79trailer is identical to an HTTP header and if such a trailer is received it is 80passed to the application using this callback as well. There are several ways 81to detect it being a trailer and not an ordinary header: 1) it comes after the 82response-body. 2) it comes after the final header line (CR LF) 3) a Trailer: 83header among the regular response-headers mention what header(s) to expect in 84the trailer. 85 86For non-HTTP protocols like FTP, POP3, IMAP and SMTP this function gets called 87with the server responses to the commands that libcurl sends. 88 89A more convenient way to get HTTP headers might be to use 90curl_easy_header(3). 91 92# LIMITATIONS 93 94libcurl does not unfold HTTP "folded headers" (deprecated since RFC 7230). A 95folded header is a header that continues on a subsequent line and starts with 96a whitespace. Such folds are passed to the header callback as separate ones, 97although strictly they are just continuations of the previous lines. 98 99# DEFAULT 100 101Nothing. 102 103# EXAMPLE 104 105~~~c 106static size_t header_callback(char *buffer, size_t size, 107 size_t nitems, void *userdata) 108{ 109 /* received header is nitems * size long in 'buffer' NOT ZERO TERMINATED */ 110 /* 'userdata' is set with CURLOPT_HEADERDATA */ 111 return nitems * size; 112} 113 114int main(void) 115{ 116 CURL *curl = curl_easy_init(); 117 if(curl) { 118 curl_easy_setopt(curl, CURLOPT_URL, "https://example.com"); 119 120 curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, header_callback); 121 122 curl_easy_perform(curl); 123 } 124} 125~~~ 126 127# AVAILABILITY 128 129Always 130 131# RETURN VALUE 132 133Returns CURLE_OK 134