.\" Automatically generated by Podwrapper::Man 1.24.1 (Pod::Simple 3.40)
.\"
.\" Standard preamble:
.\" ========================================================================
.de Sp \" Vertical space (when we can't use .PP)
.if t .sp .5v
.if n .sp
..
.de Vb \" Begin verbatim text
.ft CW
.nf
.ne \\$1
..
.de Ve \" End verbatim text
.ft R
.fi
..
.\" Set up some character translations and predefined strings.  \*(-- will
.\" give an unbreakable dash, \*(PI will give pi, \*(L" will give a left
.\" double quote, and \*(R" will give a right double quote.  \*(C+ will
.\" give a nicer C++.  Capital omega is used to do unbreakable dashes and
.\" therefore won't be available.  \*(C` and \*(C' expand to `' in nroff,
.\" nothing in troff, for use with C<>.
.tr \(*W-
.ds C+ C\v'-.1v'\h'-1p'\s-2+\h'-1p'+\s0\v'.1v'\h'-1p'
.ie n \{\
.    ds -- \(*W-
.    ds PI pi
.    if (\n(.H=4u)&(1m=24u) .ds -- \(*W\h'-12u'\(*W\h'-12u'-\" diablo 10 pitch
.    if (\n(.H=4u)&(1m=20u) .ds -- \(*W\h'-12u'\(*W\h'-8u'-\"  diablo 12 pitch
.    ds L" ""
.    ds R" ""
.    ds C` ""
.    ds C' ""
'br\}
.el\{\
.    ds -- \|\(em\|
.    ds PI \(*p
.    ds L" ``
.    ds R" ''
.    ds C`
.    ds C'
'br\}
.\"
.\" Escape single quotes in literal strings from groff's Unicode transform.
.ie \n(.g .ds Aq \(aq
.el       .ds Aq '
.\"
.\" If the F register is >0, we'll generate index entries on stderr for
.\" titles (.TH), headers (.SH), subsections (.SS), items (.Ip), and index
.\" entries marked with X<> in POD.  Of course, you'll have to process the
.\" output yourself in some meaningful fashion.
.\"
.\" Avoid warning from groff about undefined register 'F'.
.de IX
..
.nr rF 0
.if \n(.g .if rF .nr rF 1
.if (\n(rF:(\n(.g==0)) \{\
.    if \nF \{\
.        de IX
.        tm Index:\\$1\t\\n%\t"\\$2"
..
.        if !\nF==2 \{\
.            nr % 0
.            nr F 2
.        \}
.    \}
.\}
.rr rF
.\" ========================================================================
.\"
.IX Title "nbdkit-tcl-plugin 3"
.TH nbdkit-tcl-plugin 3 "2021-01-20" "nbdkit-1.24.1" "NBDKIT"
.\" 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"
nbdkit\-tcl\-plugin \- nbdkit Tcl plugin
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& nbdkit tcl /path/to/plugin.tcl [arguments...]
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\f(CW\*(C`nbdkit\-tcl\-plugin\*(C'\fR is an embedded Tcl interpreter for
\&\fBnbdkit\fR\|(1), allowing you to write nbdkit plugins in Tcl.
.SS "If you have been given an nbdkit Tcl plugin"
.IX Subsection "If you have been given an nbdkit Tcl plugin"
Assuming you have a Tcl script which is an nbdkit plugin, you run it
like this:
.PP
.Vb 1
\& nbdkit tcl /path/to/plugin.tcl
.Ve
.PP
You may have to add further \f(CW\*(C`key=value\*(C'\fR arguments to the command
line.  Read the Tcl script to see if it requires any.
.SH "WRITING A TCL NBDKIT PLUGIN"
.IX Header "WRITING A TCL NBDKIT PLUGIN"
For an example plugin written in Tcl, see:
https://github.com/libguestfs/nbdkit/blob/master/plugins/tcl/example.tcl
.PP
Broadly speaking, Tcl nbdkit plugins work like C ones, so you should
read \fBnbdkit\-plugin\fR\|(3) first.
.PP
To write a Tcl nbdkit plugin, you create a Tcl file which contains
at least the following required subroutines:
.PP
.Vb 12
\& proc plugin_open {readonly} {
\&     # see below
\&     return $h
\& }
\& proc get_size {h} {
\&     # see below
\&     return $size
\& }
\& proc pread {h count offset} {
\&     # see below
\&     return $buf
\& }
.Ve
.PP
Note that the subroutines must have those literal names (like
\&\f(CW\*(C`plugin_open\*(C'\fR), because the C part looks up and calls those functions
directly.  You may want to include documentation and globals (eg. for
storing global state).  Also any top-level statements are run when
nbdkit starts up.
.SS "Executable script"
.IX Subsection "Executable script"
If you want you can make the script executable and include a \*(L"shebang\*(R"
at the top:
.PP
.Vb 1
\& #!/usr/sbin/nbdkit tcl
.Ve
.PP
See also \*(L"Shebang scripts\*(R" in \fBnbdkit\fR\|(1).
.PP
These scripts can also be installed in the \f(CW$plugindir\fR.  See
\&\*(L"\s-1WRITING PLUGINS IN OTHER PROGRAMMING LANGUAGES\*(R"\s0 in \fBnbdkit\-plugin\fR\|(3).
.SS "Exceptions"
.IX Subsection "Exceptions"
Tcl plugin methods can indicate an error by calling \f(CW\*(C`error\*(C'\fR.
.SS "Binary data"
.IX Subsection "Binary data"
When writing your Tcl script, be careful to ensure that it is
processing binary data (not Unicode).  If reading and writing from
local disk files, you should use:
.PP
.Vb 1
\& fconfigure $fp \-translation binary
.Ve
.PP
Note also that the value returned from \f(CW\*(C`pread\*(C'\fR should convertible to
a byte array, and the buffer passed to \f(CW\*(C`pwrite\*(C'\fR is also a byte array.
.PP
See also: https://wiki.tcl.tk/1180
.SS "Tcl callbacks"
.IX Subsection "Tcl callbacks"
This just documents the arguments to the callbacks in Tcl, and any
way that they differ from the C callbacks.  In all other respects they
work the same way as the C callbacks, so you should go and read
\&\fBnbdkit\-plugin\fR\|(3).
.ie n .IP """dump_plugin""" 4
.el .IP "\f(CWdump_plugin\fR" 4
.IX Item "dump_plugin"
(Optional)
.Sp
There are no arguments or return value.
.ie n .IP """config""" 4
.el .IP "\f(CWconfig\fR" 4
.IX Item "config"
(Optional)
.Sp
.Vb 3
\& proc config {key value} {
\&     # No return value.
\& }
.Ve
.ie n .IP """config_complete""" 4
.el .IP "\f(CWconfig_complete\fR" 4
.IX Item "config_complete"
(Optional)
.Sp
There are no arguments or return value.
.ie n .IP """plugin_open""" 4
.el .IP "\f(CWplugin_open\fR" 4
.IX Item "plugin_open"
(Required)
.Sp
.Vb 4
\& proc plugin_open {readonly} {
\&     set handle ...
\&     return $handle
\& }
.Ve
.Sp
The \f(CW\*(C`readonly\*(C'\fR flag is a boolean.
.Sp
You can return any Tcl string or object as the handle.  It is passed
back to subsequent calls.
.ie n .IP """plugin_close""" 4
.el .IP "\f(CWplugin_close\fR" 4
.IX Item "plugin_close"
(Optional)
.Sp
.Vb 3
\& proc plugin_close {h} {
\&     # No return value
\& }
.Ve
.Sp
After \f(CW\*(C`plugin_close\*(C'\fR returns, the reference count of the handle is
decremented in the C part, which usually means that the handle and its
contents will be garbage collected.
.ie n .IP """get_size""" 4
.el .IP "\f(CWget_size\fR" 4
.IX Item "get_size"
(Required)
.Sp
.Vb 4
\& proc get_size {h} {
\&     set size .. the size of the disk ..
\&     return $size
\& }
.Ve
.Sp
This returns the size of the disk.
.ie n .IP """can_write""" 4
.el .IP "\f(CWcan_write\fR" 4
.IX Item "can_write"
(Optional)
.Sp
.Vb 3
\& proc can_write {h} {
\&     return $bool
\& }
.Ve
.Sp
Return a boolean indicating whether the disk is writable.
.ie n .IP """can_flush""" 4
.el .IP "\f(CWcan_flush\fR" 4
.IX Item "can_flush"
(Optional)
.Sp
.Vb 3
\& proc can_flush {h} {
\&     return $bool
\& }
.Ve
.Sp
Return a boolean indicating whether flush can be performed.
.ie n .IP """is_rotational""" 4
.el .IP "\f(CWis_rotational\fR" 4
.IX Item "is_rotational"
(Optional)
.Sp
.Vb 3
\& proc is_rotational {h} {
\&     return $bool
\& }
.Ve
.Sp
Return a boolean indicating whether the disk is rotational.
.ie n .IP """can_trim""" 4
.el .IP "\f(CWcan_trim\fR" 4
.IX Item "can_trim"
(Optional)
.Sp
.Vb 3
\& proc can_trim {h} {
\&     return $bool
\& }
.Ve
.Sp
Return a boolean indicating whether trim/discard can be performed.
.ie n .IP """pread""" 4
.el .IP "\f(CWpread\fR" 4
.IX Item "pread"
(Required)
.Sp
.Vb 4
\& proc pread {h count offset} {
\&    # Construct a buffer of length $count bytes and return it.
\&    return $buf
\& }
.Ve
.Sp
The body of your \f(CW\*(C`pread\*(C'\fR function should construct a buffer of length
(at least) \f(CW$count\fR bytes.  You should read \f(CW$count\fR bytes from the
disk starting at \f(CW$offset\fR.
.Sp
\&\s-1NBD\s0 only supports whole reads, so your function should try to read the
whole region (perhaps requiring a loop).  If the read fails or is
partial, your function should call \f(CW\*(C`error\*(C'\fR.
.ie n .IP """pwrite""" 4
.el .IP "\f(CWpwrite\fR" 4
.IX Item "pwrite"
(Optional)
.Sp
.Vb 3
\& proc pwrite {h buf offset} {
\&    # No return value
\& }
.Ve
.Sp
The body of your \f(CW\*(C`pwrite\*(C'\fR function should write the \f(CW$buf\fR string to
the disk.  You should write \f(CW$count\fR bytes to the disk starting at
\&\f(CW$offset\fR.
.Sp
\&\s-1NBD\s0 only supports whole writes, so your function should try to write
the whole region (perhaps requiring a loop).  If the write fails or is
partial, your function should call \f(CW\*(C`error\*(C'\fR.
.ie n .IP """plugin_flush""" 4
.el .IP "\f(CWplugin_flush\fR" 4
.IX Item "plugin_flush"
(Optional)
.Sp
.Vb 3
\& proc plugin_flush {h} {
\&     # No return value
\& }
.Ve
.Sp
The body of your \f(CW\*(C`plugin_flush\*(C'\fR function should do a \fBsync\fR\|(2) or
\&\fBfdatasync\fR\|(2) or equivalent on the backing store.
.ie n .IP """trim""" 4
.el .IP "\f(CWtrim\fR" 4
.IX Item "trim"
(Optional)
.Sp
.Vb 3
\& proc trim {h count offset} {
\&     # No return value
\& }
.Ve
.Sp
The body of your \f(CW\*(C`trim\*(C'\fR function should \*(L"punch a hole\*(R" in the backing
store.
.ie n .IP """zero""" 4
.el .IP "\f(CWzero\fR" 4
.IX Item "zero"
(Optional)
.Sp
.Vb 3
\& proc zero {h count offset may_trim} {
\&    # No return value
\& }
.Ve
.Sp
The body of your \f(CW\*(C`zero\*(C'\fR function should ensure that \f(CW$count\fR bytes
of the disk, starting at \f(CW$offset\fR, will read back as zero.  If
\&\f(CW$may_trim\fR is true, the operation may be optimized as a trim as long
as subsequent reads see zeroes.
.Sp
\&\s-1NBD\s0 only supports whole writes, so your function should try to write
the whole region (perhaps requiring a loop).  If the write fails or is
partial, your function should call \f(CW\*(C`error\*(C'\fR.
.SS "Missing callbacks"
.IX Subsection "Missing callbacks"
.ie n .IP "Missing: ""load"", ""unload"", ""name"", ""version"", ""longname"", ""description"", ""config_help"", ""can_zero"", ""can_fua"", ""can_cache"", ""cache""" 4
.el .IP "Missing: \f(CWload\fR, \f(CWunload\fR, \f(CWname\fR, \f(CWversion\fR, \f(CWlongname\fR, \f(CWdescription\fR, \f(CWconfig_help\fR, \f(CWcan_zero\fR, \f(CWcan_fua\fR, \f(CWcan_cache\fR, \f(CWcache\fR" 4
.IX Item "Missing: load, unload, name, version, longname, description, config_help, can_zero, can_fua, can_cache, cache"
These are not yet supported.
.SS "Threads"
.IX Subsection "Threads"
The thread model for Tcl callbacks currently cannot be set from Tcl.
It is hard-coded in the C part to
\&\f(CW\*(C`NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS\*(C'\fR.  This may change or be
settable in future.
.SH "FILES"
.IX Header "FILES"
.IP "\fI\f(CI$plugindir\fI/nbdkit\-tcl\-plugin.so\fR" 4
.IX Item "$plugindir/nbdkit-tcl-plugin.so"
The plugin.
.Sp
Use \f(CW\*(C`nbdkit \-\-dump\-config\*(C'\fR to find the location of \f(CW$plugindir\fR.
.SH "VERSION"
.IX Header "VERSION"
\&\f(CW\*(C`nbdkit\-tcl\-plugin\*(C'\fR first appeared in nbdkit 1.4.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbdkit\fR\|(1),
\&\fBnbdkit\-plugin\fR\|(3).
.SH "AUTHORS"
.IX Header "AUTHORS"
Richard W.M. Jones
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright (C) 2018 Red Hat Inc.
.SH "LICENSE"
.IX Header "LICENSE"
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:
.IP "\(bu" 4
Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
.IP "\(bu" 4
Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
.IP "\(bu" 4
Neither the name of Red Hat nor the names of its contributors may be
used to endorse or promote products derived from this software without
specific prior written permission.
.PP
\&\s-1THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS\s0 ''\s-1AS IS\s0'' \s-1AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES\s0 (\s-1INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES\s0; \s-1LOSS OF
USE, DATA, OR PROFITS\s0; \s-1OR BUSINESS INTERRUPTION\s0) \s-1HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
OR TORT\s0 (\s-1INCLUDING NEGLIGENCE OR OTHERWISE\s0) \s-1ARISING IN ANY WAY OUT
OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.\s0