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.IX Title "nbd_opt_list_meta_context 3"
.TH nbd_opt_list_meta_context 3 2024-05-31 libnbd-1.20.0 LIBNBD
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.nh
.SH NAME
nbd_opt_list_meta_context \- list available meta contexts, using
implicit query list
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 1
\& #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
\&     );
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
Request that the server list available meta contexts associated with
the export previously specified by the most recent
\&\fBnbd_set_export_name\fR\|(3) or \fBnbd_connect_uri\fR\|(3), and with a
list of queries from prior calls to \fBnbd_add_meta_context\fR\|(3)
(see \fBnbd_opt_list_meta_context_queries\fR\|(3) if you want to supply
an explicit query list instead).  This can only be used if
\&\fBnbd_set_opt_mode\fR\|(3) enabled option mode.
.PP
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 \fBnbd_add_meta_context\fR\|(3); you can use
\&\fBnbd_clear_meta_contexts\fR\|(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 \fBnbd_opt_go\fR\|(3); so it is still necessary to use
\&\fBnbd_can_meta_context\fR\|(3) after connecting to see which contexts
are actually supported.
.PP
The \f(CW\*(C`context\*(C'\fR function is called once per server reply, with any
\&\f(CW\*(C`user_data\*(C'\fR passed to this function, and with \f(CW\*(C`name\*(C'\fR supplied by
the server.  Remember that it is not safe to call
\&\fBnbd_add_meta_context\fR\|(3) from within the context of the
callback function; rather, your code must copy any \f(CW\*(C`name\*(C'\fR 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.
.PP
For convenience, when this function succeeds, it returns the number
of replies returned by the server.
.PP
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.
.SH "RETURN VALUE"
.IX Header "RETURN VALUE"
This call returns an integer ≥ \f(CW0\fR.
.SH ERRORS
.IX Header "ERRORS"
On error \f(CW\-1\fR is returned.
.PP
Refer to "ERROR HANDLING" in \fBlibnbd\fR\|(3)
for how to get further details of the error.
.PP
The following parameters must not be NULL: \f(CW\*(C`h\*(C'\fR.
For more information see "Non-NULL parameters" in \fBlibnbd\fR\|(3).
.SH "HANDLE STATE"
.IX Header "HANDLE STATE"
nbd_opt_list_meta_context
can be called when the handle is in the following state:
.PP
.Vb 8
\& ┌─────────────────────────────────────┬─────────────────────────┐
\& │ Handle created, before connecting   │ ❌ error                │
\& │ Connecting                          │ ❌ error                │
\& │ Connecting & handshaking (opt_mode) │ ✅ allowed              │
\& │ Connected to the server             │ ❌ error                │
\& │ Connection shut down                │ ❌ error                │
\& │ Handle dead                         │ ❌ error                │
\& └─────────────────────────────────────┴─────────────────────────┘
.Ve
.SH VERSION
.IX Header "VERSION"
This function first appeared in libnbd 1.6.
.PP
If you need to test if this function is available at compile time
check if the following macro is defined:
.PP
.Vb 1
\& #define LIBNBD_HAVE_NBD_OPT_LIST_META_CONTEXT 1
.Ve
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbd_add_meta_context\fR\|(3),
\&\fBnbd_aio_opt_list_meta_context\fR\|(3),
\&\fBnbd_can_meta_context\fR\|(3),
\&\fBnbd_clear_meta_contexts\fR\|(3),
\&\fBnbd_connect_uri\fR\|(3),
\&\fBnbd_create\fR\|(3),
\&\fBnbd_opt_go\fR\|(3),
\&\fBnbd_opt_list_meta_context_queries\fR\|(3),
\&\fBnbd_opt_set_meta_context\fR\|(3),
\&\fBnbd_set_export_name\fR\|(3),
\&\fBnbd_set_opt_mode\fR\|(3),
\&\fBlibnbd\fR\|(3).
.SH AUTHORS
.IX Header "AUTHORS"
Eric Blake
.PP
Richard W.M. Jones
.SH COPYRIGHT
.IX Header "COPYRIGHT"
Copyright Red Hat
.SH LICENSE
.IX Header "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.
.PP
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.
.PP
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