nbd_get_protocol - return the NBD protocol variant


 #include <libnbd.h>

 const char * nbd_get_protocol (
                struct nbd_handle *h


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

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


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


On error NULL is returned.

Refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error.

The following parameters must not be NULL: h. For more information see "Non-NULL parameters" in libnbd(3).


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                │


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:



nbd_create(3), nbd_get_block_size(3), nbd_get_handshake_flags(3), nbd_get_structured_replies_negotiated(3), nbd_get_tls_negotiated(3), libnbd(3).


Eric Blake

Richard W.M. Jones


Copyright Red Hat


This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with this library; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA