nbd_opt_list_meta_context - Man Page

list available 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_list_meta_context (
       struct nbd_handle *h,
       nbd_context_callback context_callback


Request that the server list available meta contexts associated with the export previously specified by the most recent nbd_set_export_name(3) or nbd_connect_uri(3), and with a list of queries from prior calls to nbd_add_meta_context(3) (see nbd_opt_list_meta_context_queries(3) if you want to supply an explicit query list instead).  This can only be used if nbd_set_opt_mode(3) enabled option mode.

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. When the list is empty, a server will typically reply with all contexts that it supports; when the list is non-empty, the server will reply only with supported contexts that match the client's request.  Note that a reply by the server might be encoded to represent several feasible contexts within one string, rather than multiple strings per actual context name that would actually succeed during nbd_opt_go(3); so it is still necessary to use nbd_can_meta_context(3) after connecting to see which contexts are actually supported.

The context function is called once per server reply, with any user_data passed to this function, and with name supplied by the server.  Remember that it is not safe to call nbd_add_meta_context(3) from within the context of the callback function; rather, your code must copy any name needed for later use after this function completes.  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.  Likewise, the NBD protocol does not specify an upper bound for the number of replies that might be advertised, so client code should be aware that a server may send a lengthy list.

Return Value

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).

Handle State

The handle must be negotiating, otherwise this call will return an error.


This function first appeared in libnbd 1.6.

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


See Also

nbd_add_meta_context(3), nbd_aio_opt_list_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_list_meta_context_queries(3), nbd_opt_set_meta_context(3), nbd_set_export_name(3), nbd_set_opt_mode(3), libnbd(3).


Eric Blake

Richard W.M. Jones


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

Referenced By

libnbd(3), libnbd-release-notes-1.16(1), libnbd-release-notes-1.6(1), NBD(3), nbd_aio_opt_list_meta_context(3), nbd_opt_info(3), nbd_opt_list_meta_context_queries(3), nbd_opt_set_meta_context(3), nbd_set_opt_mode(3), nbd_set_request_meta_context(3).

2023-09-08 libnbd-1.17.5