nbd_can_meta_context - does the server support a specific meta context?


 #include <libnbd.h>

 int nbd_can_meta_context (
       struct nbd_handle *h, const char *metacontext


Returns true if the server supports the given meta context (see nbd_add_meta_context(3)). Returns false if the server does not. It is possible for this command to fail if meta contexts were requested but there is a missing or failed attempt at NBD_OPT_SET_META_CONTEXT during option negotiation.

If the server supports block status filtering (see nbd_can_block_status_payload(3), this function must return true for any filter name passed to nbd_block_status_filter(3).

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

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


This call returns a boolean value.


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, metacontext. For more information see "Non-NULL parameters" in libnbd(3).


nbd_can_meta_context 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 1.0.

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



nbd_add_meta_context(3), nbd_aio_block_status_64(3), nbd_block_status_64(3), nbd_block_status_filter(3), nbd_can_block_status_payload(3), nbd_create(3), nbd_opt_info(3), nbd_opt_set_meta_context(3), nbd_set_request_meta_context(3), "Flag calls" in libnbd(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