nbd_add_meta_context - ask server to negotiate metadata context


 #include <libnbd.h>

 int nbd_add_meta_context (
       struct nbd_handle *h, const char *name


During connection libnbd can negotiate zero or more metadata contexts with the server. Metadata contexts are features (such as "base:allocation") which describe information returned by the nbd_block_status_64(3) command (for "base:allocation" this is whether blocks of data are allocated, zero or sparse).

This call adds one metadata context to the list to be negotiated. You can call it as many times as needed. The list is initially empty when the handle is created; you can check the contents of the list with nbd_get_nr_meta_contexts(3) and nbd_get_meta_context(3), or clear it with nbd_clear_meta_contexts(3).

The NBD protocol limits meta context names to 4096 bytes, but servers may not support the full length. The encoding of meta context names is always UTF-8.

Not all servers support all metadata contexts. To learn if a context was actually negotiated, call nbd_can_meta_context(3) after connecting.

The single parameter is the name of the metadata context, for example LIBNBD_CONTEXT_BASE_ALLOCATION. <libnbd.h> includes defined constants beginning with LIBNBD_CONTEXT_ for some well-known contexts, but you are free to pass in other contexts.

Other metadata contexts are server-specific, but include "qemu:dirty-bitmap:..." and "qemu:allocation-depth" for qemu-nbd (see qemu-nbd -B and -A options).


If the call is successful the function returns 0.


On error -1 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, name. For more information see "Non-NULL parameters" in libnbd(3).


nbd_add_meta_context can be called when the handle is in the following states:

 │ Handle created, before connecting   │ ✅ allowed              │
 │ Connecting                          │ ❌ error                │
 │ Connecting & handshaking (opt_mode) │ ✅ allowed              │
 │ Connected to the server             │ ❌ error                │
 │ Connection shut down                │ ❌ error                │
 │ Handle dead                         │ ❌ error                │


This function first appeared in libnbd 1.0.

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



nbd_block_status_64(3), nbd_can_meta_context(3), nbd_clear_meta_contexts(3), nbd_create(3), nbd_get_meta_context(3), nbd_get_nr_meta_contexts(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