=head1 NAME

nbd_opt_extended_headers - request the server to enable extended
headers

=head1 SYNOPSIS

 #include <libnbd.h>

 int nbd_opt_extended_headers (
       struct nbd_handle *h
     );

=head1 DESCRIPTION

Request that the server use extended headers, by sending
C<NBD_OPT_EXTENDED_HEADERS>.  This can only be used if
L<nbd_set_opt_mode(3)> enabled option mode; furthermore, libnbd
defaults to automatically requesting this unless you use
L<nbd_set_request_extended_headers(3)> or
L<nbd_set_request_structured_replies(3)> prior to connecting.
This function is mainly useful for integration testing of corner
cases in server handling.

This function returns true if the server replies with success,
false if the server replies with an error, and fails only if
the server does not reply (such as for a loss of connection).
Note that some servers fail a second request as redundant;
libnbd assumes that once one request has succeeded, then
extended headers are supported (as visible by
L<nbd_get_extended_headers_negotiated(3)>) regardless if
later calls to this function return false.  If this function
returns true, the use of structured replies is implied.

=head1 RETURN VALUE

This call returns a boolean value.

=head1 ERRORS

On error C<-1> is returned.

Refer to L<libnbd(3)/ERROR HANDLING>
for how to get further details of the error.

The following parameters must not be NULL: C<h>.
For more information see L<libnbd(3)/Non-NULL parameters>.

=head1 HANDLE STATE

nbd_opt_extended_headers
can be called when the handle is in the following state:

 ┌─────────────────────────────────────┬─────────────────────────┐
 │ Handle created, before connecting   │ ❌ error                │
 │ Connecting                          │ ❌ error                │
 │ Connecting & handshaking (opt_mode) │ ✅ allowed              │
 │ Connected to the server             │ ❌ error                │
 │ Connection shut down                │ ❌ error                │
 │ Handle dead                         │ ❌ error                │
 └─────────────────────────────────────┴─────────────────────────┘

=head1 VERSION

This function first appeared in libnbd 1.18.

If you need to test if this function is available at compile time
check if the following macro is defined:

 #define LIBNBD_HAVE_NBD_OPT_EXTENDED_HEADERS 1

=head1 SEE ALSO

L<nbd_aio_opt_extended_headers(3)>,
L<nbd_create(3)>,
L<nbd_get_extended_headers_negotiated(3)>,
L<nbd_opt_go(3)>,
L<nbd_set_opt_mode(3)>,
L<nbd_set_request_extended_headers(3)>,
L<nbd_set_request_structured_replies(3)>,
L<libnbd(3)>.

=head1 AUTHORS

Eric Blake

Richard W.M. Jones

=head1 COPYRIGHT

Copyright Red Hat
