Tribblix: manual page: CURLOPT

CURLOPT_HEADERFUNCTION(3) Introduction to Library Functions

NAME

CURLOPT_HEADERFUNCTION - callback that receives header data

SYNOPSIS

#include <curl/curl.h>

size_t header_callback(char *buffer,
size_t size,
size_t nitems,
void *userdata);

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HEADERFUNCTION,
header_callback);

DESCRIPTION

Pass a pointer to your callback function, which should match the
prototype shown above.

This callback function gets invoked by libcurl as soon as it has
received header data. The header callback is called once for each
header and only complete header lines are passed on to the callback.
Parsing headers is easy to do using this callback. buffer points to
the delivered data, and the size of that data is nitems; size is
always 1. The provided header line is not null-terminated. Do not
modify the passed in buffer.

The pointer named userdata is the one you set with the
CURLOPT_HEADERDATA(3) option.

Your callback should return the number of bytes actually taken care
of. If that amount differs from the amount passed to your callback
function, it signals an error condition to the library. This causes
the transfer to get aborted and the libcurl function used returns
CURLE_WRITE_ERROR.

You can also abort the transfer by returning CURL_WRITEFUNC_ERROR.
(7.87.0)

A complete HTTP header that is passed to this function can be up to
CURL_MAX_HTTP_HEADER (100K) bytes and includes the final line
terminator.

If this option is not set, or if it is set to NULL, but
CURLOPT_HEADERDATA(3) is set to anything but NULL, the function used
to accept response data is used instead. That is the function
specified with CURLOPT_WRITEFUNCTION(3), or if it is not specified or
NULL - the default, stream-writing function.

It is important to note that the callback is invoked for the headers
of all responses received after initiating a request and not just the
final response. This includes all responses which occur during
authentication negotiation. If you need to operate on only the
headers from the final response, you need to collect headers in the
callback yourself and use HTTP status lines, for example, to delimit
response boundaries.

For an HTTP transfer, the status line and the blank line preceding
the response body are both included as headers and passed to this
function.

When a server sends a chunked encoded transfer, it may contain a
trailer. That trailer is identical to an HTTP header and if such a
trailer is received it is passed to the application using this
callback as well. There are several ways to detect it being a trailer
and not an ordinary header: 1) it comes after the response-body. 2)
it comes after the final header line (CR LF) 3) a Trailer: header
among the regular response-headers mention what header(s) to expect
in the trailer.

For non-HTTP protocols like FTP, POP3, IMAP and SMTP this function
gets called with the server responses to the commands that libcurl
sends.

A more convenient way to get HTTP headers might be to use
curl_easy_header(3).

LIMITATIONS

libcurl does not unfold HTTP "folded headers" (deprecated since RFC
7230). A folded header is a header that continues on a subsequent
line and starts with a whitespace. Such folds are passed to the
header callback as separate ones, although strictly they are just
continuations of the previous lines.

DEFAULT

Nothing.

PROTOCOLS

This functionality affects ftp, http, imap, pop3 and smtp

EXAMPLE

static size_t header_callback(char *buffer, size_t size,
size_t nitems, void *userdata)
{
/* received header is nitems * size long in 'buffer' NOT ZERO TERMINATED */
/* 'userdata' is set with CURLOPT_HEADERDATA */
return nitems * size;
}

int main(void)
{
CURL *curl = curl_easy_init();
if(curl) {
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");

curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, header_callback);

curl_easy_perform(curl);
}
}

AVAILABILITY

Added in curl 7.7.2

RETURN VALUE

curl_easy_setopt(3) returns a CURLcode indicating success or error.

CURLE_OK (0) means everything was OK, non-zero means an error
occurred, see libcurl-errors(3).