CURLOPT_HEADERFUNCTION(3) Library Functions Manual CURLOPT_HEADERFUNCTION(3)
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 proto-
type shown above.
This callback function gets invoked by libcurl as soon as it has re-
ceived 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 deliv-
ered 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_HEADER-
DATA(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 termina-
tor.
If this option is not set, or if it is set to NULL, but CURLOPT_HEADER-
DATA(3) is set to anything but NULL, the function used to accept re-
sponse data is used instead. That is the function specified with CUR-
LOPT_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 re-
sponse-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 call-
back 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).
SEE ALSO
CURLOPT_HEADERDATA(3), CURLOPT_WRITEFUNCTION(3), curl_easy_header(3)
libcurl 2026-07-12 CURLOPT_HEADERFUNCTION(3)
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