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.\" ========================================================================
.\"
.IX Title "nbd_add_meta_context 3"
.TH nbd_add_meta_context 3 "2023-01-04" "libnbd-1.14.2" "LIBNBD"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
nbd_add_meta_context \- ask server to negotiate metadata context
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <libnbd.h>
\&
\& int nbd_add_meta_context (struct nbd_handle *h,
\&                           const char *name);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
During connection libnbd can negotiate zero or more metadata
contexts with the server.  Metadata contexts are features (such
as \f(CW"base:allocation"\fR) which describe information returned
by the \fBnbd_block_status\fR\|(3) command (for \f(CW"base:allocation"\fR
this is whether blocks of data are allocated, zero or sparse).
.PP
This call adds one metadata context to the list to be negotiated.
You can call it as many times as needed.  The list is initially
empty when the handle is created; you can check the contents of
the list with \fBnbd_get_nr_meta_contexts\fR\|(3) and
\&\fBnbd_get_meta_context\fR\|(3), or clear it with
\&\fBnbd_clear_meta_contexts\fR\|(3).
.PP
The \s-1NBD\s0 protocol limits meta context names to 4096 bytes, but
servers may not support the full length.  The encoding of meta
context names is always \s-1UTF\-8.\s0
.PP
Not all servers support all metadata contexts.  To learn if a context
was actually negotiated, call \fBnbd_can_meta_context\fR\|(3) after
connecting.
.PP
The single parameter is the name of the metadata context,
for example \f(CW\*(C`LIBNBD_CONTEXT_BASE_ALLOCATION\*(C'\fR.
\&\fB<libnbd.h>\fR includes defined constants beginning with
\&\f(CW\*(C`LIBNBD_CONTEXT_\*(C'\fR for some well-known contexts, but you are free
to pass in other contexts.
.PP
Other metadata contexts are server-specific, but include
\&\f(CW"qemu:dirty\-bitmap:..."\fR and \f(CW"qemu:allocation\-depth"\fR for
qemu-nbd (see qemu-nbd \fI\-B\fR and \fI\-A\fR options).
.SH "RETURN VALUE"
.IX Header "RETURN VALUE"
If the call is successful the function returns \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.
.PP
The following parameters must not be \s-1NULL:\s0 \f(CW\*(C`h\*(C'\fR, \f(CW\*(C`name\*(C'\fR.
For more information see \*(L"Non-NULL parameters\*(R" in \fBlibnbd\fR\|(3).
.SH "HANDLE STATE"
.IX Header "HANDLE STATE"
The handle must be
newly created, or
negotiating,
otherwise this call will return an error.
.SH "VERSION"
.IX Header "VERSION"
This function first appeared in libnbd 1.0.
.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_ADD_META_CONTEXT 1
.Ve
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbd_block_status\fR\|(3),
\&\fBnbd_can_meta_context\fR\|(3),
\&\fBnbd_clear_meta_contexts\fR\|(3),
\&\fBnbd_create\fR\|(3),
\&\fBnbd_get_meta_context\fR\|(3),
\&\fBnbd_get_nr_meta_contexts\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\-2021 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