CURLOPT_HEADERFUNCTION - callback that receives header data
size_t header_callback(char *buffer,
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HEADERFUNCTION,
Pass a pointer to your callback function, which should match the prototype shown
This function gets called by libcurl as soon as it has received header data. The
header callback will be called once for each header and only complete header
lines are passed on to the callback. Parsing headers is very easy using this.
The size of the data pointed to by buffer
. Do not assume that the header line is zero terminated! The
pointer named userdata
is the one you set with the
option. This callback function must return the
number of bytes actually taken care of. If that amount differs from the amount
passed in to your function, it'll signal an error to the library. This will
cause the transfer to get aborted and the libcurl function in progress will
A complete HTTP header that is passed to this function can be up to
If this option is not set, or if it is set to NULL, but
is set to anything but NULL, the function used to
accept response data will be used instead. That is, it will be the function
specified with CURLOPT_WRITEFUNCTION(3)
, or if it is not specified or
NULL - the default, stream-writing function.
It's important to note that the callback will be 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 will need
to collect headers in the callback yourself and use HTTP status lines, for
example, to delimit response boundaries.
When a server sends a chunked encoded transfer, it may contain a trailer. That
trailer is identical to a 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
For non-HTTP protocols like FTP, POP3, IMAP and SMTP this function will get
called with the server responses to the commands that libcurl sends.
Used for all protocols with headers or meta-data concept: HTTP, FTP, POP3, IMAP,
SMTP and more.
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;
CURL *curl = curl_easy_init();
curl_easy_setopt(curl, CURLOPT_URL, "http://example.com");
curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, header_callback);