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CURLOPT_HEADERFUNCTION(3) curl_easy_setopt options CURLOPT_HEADERFUNCTION(3)
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 above.
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. buffer points to the delivered data, and the size
of that data is nitems; size is always 1. Do not assume that the header
line is null-terminated!
The pointer named userdata is the one you set with the
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
CURL_MAX_HTTP_HEADER (100K) bytes and includes the final line
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 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
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 will
get called with the server responses to the commands that libcurl
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 will be passed to the header
callback as a separate one, although strictly it is just a continuation
of the previous line.
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, "https://example.com");
curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, header_callback);
libcurl 7.79.1 November 4, 2020 CURLOPT_HEADERFUNCTION(3)