nbd_opt_set_meta_context - select specific meta contexts, using implicit query list


 #include <libnbd.h>

 typedef struct {
   int (*callback) (void *user_data, const char *name);
   void *user_data;
   void (*free) (void *user_data);
 } nbd_context_callback;

 int nbd_opt_set_meta_context (
       struct nbd_handle *h,
       nbd_context_callback context_callback


Request that the server supply all recognized meta contexts registered through prior calls to nbd_add_meta_context(3), in conjunction with the export previously specified by the most recent nbd_set_export_name(3) or nbd_connect_uri(3). This can only be used if nbd_set_opt_mode(3) enabled option mode. Normally, this function is redundant, as nbd_opt_go(3) automatically does the same task if structured replies or extended headers have already been negotiated. But manual control over meta context requests can be useful for fine-grained testing of how a server handles unusual negotiation sequences. Often, use of this function is coupled with nbd_set_request_meta_context(3) to bypass the automatic context request normally performed by nbd_opt_go(3).

The NBD protocol allows a client to decide how many queries to ask the server. Rather than taking that list of queries as a parameter to this function, libnbd reuses the current list of requested meta contexts as set by nbd_add_meta_context(3); you can use nbd_clear_meta_contexts(3) to set up a different list of queries (see nbd_opt_set_meta_context_queries(3) to pass an explicit list of contexts instead). Since this function is primarily designed for testing servers, libnbd does not prevent the use of this function on an empty list or when nbd_set_request_structured_replies(3) has disabled structured replies, in order to see how a server behaves.

The context function is called once per server reply, with any user_data passed to this function, and with name supplied by the server. Additionally, each server name will remain visible through nbd_can_meta_context(3) until the next attempt at nbd_set_export_name(3) or nbd_opt_set_meta_context(3), as well as nbd_opt_go(3) or nbd_opt_info(3) that trigger an automatic meta context request. Remember that it is not safe to call any nbd_* APIs from within the context of the callback function. At present, the return value of the callback is ignored, although a return of -1 should be avoided.

For convenience, when this function succeeds, it returns the number of replies returned by the server.

Not all servers understand this request, and even when it is understood, the server might intentionally send an empty list because it does not support the requested context, or may encounter a failure after delivering partial results. Thus, this function may succeed even when no contexts are reported, or may fail but have a non-empty list.


This call returns an integer ≥ 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. For more information see "Non-NULL parameters" in libnbd(3).


nbd_opt_set_meta_context can be called when the handle is in the following state:

 │ Handle created, before connecting   │ ❌ error                │
 │ 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 1.16.

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_opt_set_meta_context(3), nbd_can_meta_context(3), nbd_clear_meta_contexts(3), nbd_connect_uri(3), nbd_create(3), nbd_opt_go(3), nbd_opt_info(3), nbd_opt_list_meta_context(3), nbd_opt_set_meta_context(3), nbd_opt_set_meta_context_queries(3), nbd_set_export_name(3), nbd_set_opt_mode(3), nbd_set_request_meta_context(3), nbd_set_request_structured_replies(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