---
name: "nbd_get_protocol"
section: "3"
platform: "linux"
package: "libnbd"
version: "1.24.4-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/nbd_get_protocol"
---

# nbd\_get\_protocol(3)

return the NBD protocol variant

## NAME

nbd\_get\_protocol - return the NBD protocol variant

## SYNOPSIS

```
 #include <libnbd.h>
 const char * nbd_get_protocol (
                struct nbd_handle *h
              );
```

## DESCRIPTION

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.

## RETURN VALUE

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

## ERRORS

On error `"NULL"` is returned.

Refer to "ERROR HANDLING" in [**libnbd**(3)](/libnbd/3.md) 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)](/libnbd/3.md).

## 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                │
 └─────────────────────────────────────┴─────────────────────────┘
```

## 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
```

## SEE ALSO

[**nbd\_create**(3)](/nbd_create/3.md), [**nbd\_get\_block\_size**(3)](/nbd_get_block_size/3.md), [**nbd\_get\_handshake\_flags**(3)](/nbd_get_handshake_flags/3.md), [**nbd\_get\_structured\_replies\_negotiated**(3)](/nbd_get_structured_replies_negotiated/3.md), [**nbd\_get\_tls\_negotiated**(3)](/nbd_get_tls_negotiated/3.md), [**libnbd**(3)](/libnbd/3.md).

## AUTHORS

Eric Blake

Richard W.M. Jones

## COPYRIGHT

Copyright Red Hat

## LICENSE

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