1nbdkit-tcl-plugin(3) NBDKIT nbdkit-tcl-plugin(3)
2
6 nbdkit-tcl-plugin - nbdkit Tcl plugin
7
9 nbdkit tcl /path/to/plugin.tcl [arguments...]
10
12 "nbdkit-tcl-plugin" is an embedded Tcl interpreter for nbdkit(1),
13 allowing you to write nbdkit plugins in Tcl.
14 If you have been given an nbdkit Tcl plugin
16 Assuming you have a Tcl script which is an nbdkit plugin, you run it
17 like this:
18 nbdkit tcl /path/to/plugin.tcl
20 You may have to add further "key=value" arguments to the command line.
22 Read the Tcl script to see if it requires any.
23
25 For an example plugin written in Tcl, see:
26 https://github.com/libguestfs/nbdkit/blob/master/plugins/tcl/example.tcl
27 Broadly speaking, Tcl nbdkit plugins work like C ones, so you should
29 read nbdkit-plugin(3) first.
30 To write a Tcl nbdkit plugin, you create a Tcl file which contains at
32 least the following required subroutines:
33 proc plugin_open {readonly} {
35 # see below
36 return $h
37 }
38 proc get_size {h} {
39 # see below
40 return $size
41 }
42 proc pread {h count offset} {
43 # see below
44 return $buf
45 }
46 Note that the subroutines must have those literal names (like
48 "plugin_open"), because the C part looks up and calls those functions
49 directly. You may want to include documentation and globals (eg. for
50 storing global state). Also any top-level statements are run when
51 nbdkit starts up.
52 Executable script
54 If you want you can make the script executable and include a "shebang"
55 at the top:
56 #!/usr/sbin/nbdkit tcl
58 See also "Shebang scripts" in nbdkit(1).
60 These scripts can also be installed in the $plugindir. See "WRITING
62 PLUGINS IN OTHER PROGRAMMING LANGUAGES" in nbdkit-plugin(3).
63 Exceptions
65 Tcl plugin methods can indicate an error by calling "error".
66 Binary data
68 When writing your Tcl script, be careful to ensure that it is
69 processing binary data (not Unicode). If reading and writing from
70 local disk files, you should use:
71 fconfigure $fp -translation binary
73 Note also that the value returned from "pread" should convertible to a
75 byte array, and the buffer passed to "pwrite" is also a byte array.
76 See also: https://wiki.tcl.tk/1180
78 Tcl callbacks
80 This just documents the arguments to the callbacks in Tcl, and any way
81 that they differ from the C callbacks. In all other respects they work
82 the same way as the C callbacks, so you should go and read
83 nbdkit-plugin(3).
84 "dump_plugin"
86 (Optional)
87 There are no arguments or return value.
89 "config"
91 (Optional)
92 proc config {key value} {
94 # No return value.
95 }
96 "config_complete"
98 (Optional)
99 There are no arguments or return value.
101 "plugin_open"
103 (Required)
104 proc plugin_open {readonly} {
106 set handle ...
107 return $handle
108 }
109 The "readonly" flag is a boolean.
111 You can return any Tcl string or object as the handle. It is
113 passed back to subsequent calls.
114 "plugin_close"
116 (Optional)
117 proc plugin_close {h} {
119 # No return value
120 }
121 After "plugin_close" returns, the reference count of the handle is
123 decremented in the C part, which usually means that the handle and
124 its contents will be garbage collected.
125 "get_size"
127 (Required)
128 proc get_size {h} {
130 set size .. the size of the disk ..
131 return $size
132 }
133 This returns the size of the disk.
135 "can_write"
137 (Optional)
138 proc can_write {h} {
140 return $bool
141 }
142 Return a boolean indicating whether the disk is writable.
144 "can_flush"
146 (Optional)
147 proc can_flush {h} {
149 return $bool
150 }
151 Return a boolean indicating whether flush can be performed.
153 "is_rotational"
155 (Optional)
156 proc is_rotational {h} {
158 return $bool
159 }
160 Return a boolean indicating whether the disk is rotational.
162 "can_trim"
164 (Optional)
165 proc can_trim {h} {
167 return $bool
168 }
169 Return a boolean indicating whether trim/discard can be performed.
171 "pread"
173 (Required)
174 proc pread {h count offset} {
176 # Construct a buffer of length $count bytes and return it.
177 return $buf
178 }
179 The body of your "pread" function should construct a buffer of
181 length (at least) $count bytes. You should read $count bytes from
182 the disk starting at $offset.
183 NBD only supports whole reads, so your function should try to read
185 the whole region (perhaps requiring a loop). If the read fails or
186 is partial, your function should call "error".
187 "pwrite"
189 (Optional)
190 proc pwrite {h buf offset} {
192 # No return value
193 }
194 The body of your "pwrite" function should write the $buf string to
196 the disk. You should write $count bytes to the disk starting at
197 $offset.
198 NBD only supports whole writes, so your function should try to
200 write the whole region (perhaps requiring a loop). If the write
201 fails or is partial, your function should call "error".
202 "plugin_flush"
204 (Optional)
205 proc plugin_flush {h} {
207 # No return value
208 }
209 The body of your "plugin_flush" function should do a sync(2) or
211 fdatasync(2) or equivalent on the backing store.
212 "trim"
214 (Optional)
215 proc trim {h count offset} {
217 # No return value
218 }
219 The body of your "trim" function should "punch a hole" in the
221 backing store.
222 "zero"
224 (Optional)
225 proc zero {h count offset may_trim} {
227 # No return value
228 }
229 The body of your "zero" function should ensure that $count bytes of
231 the disk, starting at $offset, will read back as zero. If
232 $may_trim is true, the operation may be optimized as a trim as long
233 as subsequent reads see zeroes.
234 NBD only supports whole writes, so your function should try to
236 write the whole region (perhaps requiring a loop). If the write
237 fails or is partial, your function should call "error".
238 Missing callbacks
240 Missing: "load", "unload", "name", "version", "longname",
241 "description", "config_help", "can_zero", "can_fua"
242 These are not yet supported.
243 Threads
245 The thread model for Tcl callbacks currently cannot be set from Tcl.
246 It is hard-coded in the C part to
247 "NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS". This may change or be
248 settable in future.
249
251 nbdkit(1), nbdkit-plugin(3).
252
254 Richard W.M. Jones
255
257 Copyright (C) 2018 Red Hat Inc.
258
260 Redistribution and use in source and binary forms, with or without
261 modification, are permitted provided that the following conditions are
262 met:
263 · Redistributions of source code must retain the above copyright
265 notice, this list of conditions and the following disclaimer.
266 · Redistributions in binary form must reproduce the above copyright
268 notice, this list of conditions and the following disclaimer in the
269 documentation and/or other materials provided with the
270 distribution.
271 · Neither the name of Red Hat nor the names of its contributors may
273 be used to endorse or promote products derived from this software
274 without specific prior written permission.
275 THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
277 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
278 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
279 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
280 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
281 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
282 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
283 BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
284 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
285 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
286 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
287nbdkit-1.12.3 2019-05-22 nbdkit-tcl-plugin(3)