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.\" ========================================================================
.\"
.IX Title "nbd_opt_list_meta_context 3"
.TH nbd_opt_list_meta_context 3 "2021-02-09" "libnbd-1.6.1" "LIBNBD"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.SH "NAME"
nbd_opt_list_meta_context \- request the server to list available meta 
contexts
.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).  This can only be
used if \fBnbd_set_opt_mode\fR\|(3) enabled option mode.
.PP
The \s-1NBD\s0 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 \s-1NBD\s0 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\*(C`\-1\*(C'\fR is returned.
.PP
Refer to \*(L"\s-1ERROR HANDLING\*(R"\s0 in \fBlibnbd\fR\|(3)
for how to get further details of the error.
.SH "HANDLE STATE"
.IX Header "HANDLE STATE"
The handle must be
negotiating,
otherwise this call will return an error.
.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_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 (C) 2019\-2020 Red Hat Inc.
.SH "LICENSE"
.IX Header "LICENSE"
This library is free software; you can redistribute it and/or
modify it under the terms of the \s-1GNU\s0 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 \s-1WITHOUT ANY WARRANTY\s0; without even the implied warranty of
\&\s-1MERCHANTABILITY\s0 or \s-1FITNESS FOR A PARTICULAR PURPOSE.\s0  See the \s-1GNU\s0
Lesser General Public License for more details.
.PP
You should have received a copy of the \s-1GNU\s0 Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, \s-1MA 02110\-1301 USA\s0