=head1 NAME

nbd_get_protocol - return the NBD protocol variant

=head1 SYNOPSIS

 #include <libnbd.h>

 const char * nbd_get_protocol (
                struct nbd_handle *h
              );

=head1 DESCRIPTION

Return the NBD protocol variant in use on the connection.
At the moment this returns one of the strings C<"oldstyle">,
C<"newstyle"> or C<"newstyle-fixed">.  Other strings might
be returned in the future.
Most modern NBD servers use C<"newstyle-fixed">.


This call does not block, because it returns data that is saved in
the handle from the NBD protocol handshake.

=head1 RETURN VALUE

This call returns a statically allocated string, valid for the
lifetime of the process or until libnbd is unloaded by
L<dlclose(3)>.  You B<must not> try to free the string.

=head1 ERRORS

On error C<NULL> 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_get_protocol
can be called when the handle is in the following states:

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

=head1 VERSION

This function first appeared in libnbd 1.2.

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

 #define LIBNBD_HAVE_NBD_GET_PROTOCOL 1

=head1 SEE ALSO

L<nbd_create(3)>,
L<nbd_get_block_size(3)>,
L<nbd_get_handshake_flags(3)>,
L<nbd_get_structured_replies_negotiated(3)>,
L<nbd_get_tls_negotiated(3)>,
L<libnbd(3)>.

=head1 AUTHORS

Eric Blake

Richard W.M. Jones

=head1 COPYRIGHT

Copyright Red Hat
