NAME

       nbd_opt_list_meta_context - request the server to list available meta contexts

SYNOPSIS

        #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);

DESCRIPTION

       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).  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.

ERRORS

       On error "-1" is returned.

       Refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error.

HANDLE STATE

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

VERSION

       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:

        #define LIBNBD_HAVE_NBD_OPT_LIST_META_CONTEXT 1

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_set_export_name(3), nbd_set_opt_mode(3), libnbd(3).

AUTHORS

       Eric Blake

       Richard W.M. Jones

COPYRIGHT

       Copyright (C) 2019-2021 Red Hat Inc.

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