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.TH CURLOPT_HEADERFUNCTION 3 "September 16, 2020" "libcurl 7.73.0" "curl_easy_setopt options"

.SH NAME
CURLOPT_HEADERFUNCTION \- callback that receives header data
.SH 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);
.SH DESCRIPTION
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. \fIbuffer\fP points to the delivered data, and the size of
that data is \fInitems\fP; \fIsize\fP is always 1. Do not assume that the
header line is null-terminated!

The pointer named \fIuserdata\fP is the one you set with the
\fICURLOPT_HEADERDATA(3)\fP 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 return \fICURLE_WRITE_ERROR\fP.

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

If this option is not set, or if it is set to NULL, but
\fICURLOPT_HEADERDATA(3)\fP 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 \fICURLOPT_WRITEFUNCTION(3)\fP, 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.

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 sends.
.SH 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 will be passed to the header callback as a separate
one, although strictly it is just a continuation of the previous line.
.SH DEFAULT
Nothing.
.SH PROTOCOLS
Used for all protocols with headers or meta-data concept: HTTP, FTP, POP3,
IMAP, SMTP and more.
.SH EXAMPLE
.nf
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();
if(curl) {
  curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");

  curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, header_callback);

  curl_easy_perform(curl);
}
.fi
.SH AVAILABILITY
Always
.SH RETURN VALUE
Returns CURLE_OK
.SH "SEE ALSO"
.BR CURLOPT_HEADERDATA "(3), " CURLOPT_WRITEFUNCTION "(3), "