1nbdkit-plugin(3) NBDKIT nbdkit-plugin(3)
2
6 nbdkit-plugin - how to write nbdkit plugins
7
9 #define NBDKIT_API_VERSION 2
10 #include <nbdkit-plugin.h>
11 #define THREAD_MODEL NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS
13 static void *
15 myplugin_open (void)
16 {
17 /* create a handle ... */
18 return handle;
19 }
20 static struct nbdkit_plugin plugin = {
22 .name = "myplugin",
23 .open = myplugin_open,
24 .get_size = myplugin_get_size,
25 .pread = myplugin_pread,
26 .pwrite = myplugin_pwrite,
27 /* etc */
28 };
29 NBDKIT_REGISTER_PLUGIN(plugin)
30 Compile the plugin as a shared library:
32 gcc -fPIC -shared myplugin.c -o myplugin.so
34 and load it into nbdkit:
36 nbdkit [--args ...] ./myplugin.so [key=value ...]
38 When debugging, use the -fv options:
40 nbdkit -fv ./myplugin.so [key=value ...]
42
44 An nbdkit plugin is a new source device which can be served using the
45 Network Block Device (NBD) protocol. This manual page describes how to
46 create an nbdkit plugin in C.
47 To see example plugins:
49 https://gitlab.com/nbdkit/nbdkit/tree/master/plugins
50 To write plugins in other languages, see: nbdkit-cc-plugin(3),
52 nbdkit-golang-plugin(3), nbdkit-lua-plugin(3), nbdkit-ocaml-plugin(3),
53 nbdkit-perl-plugin(3), nbdkit-python-plugin(3), nbdkit-ruby-plugin(3),
54 nbdkit-rust-plugin(3), nbdkit-sh-plugin(3), nbdkit-tcl-plugin(3) .
55 API and ABI guarantee for C plugins
57 Plugins written in C have an ABI guarantee: a plugin compiled against
58 an older version of nbdkit will still work correctly when loaded with a
59 newer nbdkit. We also try (but cannot guarantee) to support plugins
60 compiled against a newer version of nbdkit when loaded with an older
61 nbdkit, although the plugin may have reduced functionality if it
62 depends on features only provided by newer nbdkit.
63 For plugins written in C, we also provide an API guarantee: a plugin
65 written against an older header will still compile unmodified with a
66 newer nbdkit.
67 The API guarantee does not always apply to plugins written in other
69 (non-C) languages which may have to adapt to changes when recompiled
70 against a newer nbdkit.
71
73 "#define NBDKIT_API_VERSION 2"
74 Plugins must choose which API version they want to use, by defining
75 NBDKIT_API_VERSION before including "<nbdkit-plugin.h>" (or any other
76 nbdkit header).
77 If omitted, the default version is 1 for backwards-compatibility with
79 nbdkit v1.1.26 and earlier; however, it is recommended that new plugins
80 be written to the maximum version (currently 2) as it enables more
81 features and better interaction with nbdkit filters.
82 The rest of this document only covers the version 2 interface. A newer
84 nbdkit will always support plugins written in C which use any prior API
85 version.
86 "#include <nbdkit-plugin.h>"
88 All plugins should start by including this header file (after
89 optionally choosing an API version).
90 "#define THREAD_MODEL ..."
92 All plugins must define a thread model. See "Threads" below for
93 details. It is generally safe to use:
94 #define THREAD_MODEL NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS
96 "struct nbdkit_plugin"
98 All plugins must define and register one "struct nbdkit_plugin", which
99 contains the name of the plugin and pointers to callback functions, and
100 use the "NBDKIT_REGISTER_PLUGIN(plugin)" macro:
101 static struct nbdkit_plugin plugin = {
103 .name = "myplugin",
104 .longname = "My Plugin",
105 .description = "This is my great plugin for nbdkit",
106 .open = myplugin_open,
107 .get_size = myplugin_get_size,
108 .pread = myplugin_pread,
109 .pwrite = myplugin_pwrite,
110 /* etc */
111 };
112 NBDKIT_REGISTER_PLUGIN(plugin)
113 The ".name" field is the name of the plugin.
115 The callbacks are described below (see "CALLBACKS"). Only ".name",
117 ".open", ".get_size" and ".pread" are required. All other callbacks
118 can be omitted, although typical plugins need to use more.
119 Callback lifecycle
121 Callbacks are called in the following order over the lifecycle of the
122 plugin:
123 ┌──────────────────┐
125 │ load │
126 └─────────┬────────┘
127 │ configuration phase starts ─┐
128 ┌─────────┴────────┐ ┆
129 │ config │ config is called once per ┆
130 └─────────┬────────┘↺ key=value on the command line ┆
131 ┌─────────┴────────┐ ┆
132 │ config_complete │ ┆
133 └─────────┬────────┘ ┆
134 ┌─────────┴────────┐ ┆
135 │ thread_model │ ┆
136 └─────────┬────────┘ configuration phase ends ─┘
137 ┌─────────┴────────┐
138 │ get_ready │
139 └─────────┬────────┘
140 │ nbdkit forks into the background
141 ┌─────────┴────────┐
142 │ after_fork │
143 └─────────┬────────┘
144 │ nbdkit starts serving clients
145 │
146 ┌──────────┴─────────────┬─ ─ ─ ─ ─ ─ ─ ─ ─
147 ┌──────┴─────┐ client #1 │
148 │ preconnect │ │
149 └──────┬─────┘ │
150 ┌──────┴─────┐ │
151 │list_exports│ │
152 └──────┬─────┘ │
153 ┌──────┴─────┐ │
154 │ open │ │
155 └──────┬─────┘ │
156 ┌──────┴─────┐ NBD option │
157 │ can_write │ negotiation │
158 └──────┬─────┘ │
159 ┌──────┴─────┐ ┌──────┴─────┐ client #2
160 │ get_size │ │ preconnect │
161 └──────┬─────┘ └──────┬─────┘
162 ┌──────┴─────┐ data
163 │ pread │ serving
164 └──────┬─────┘↺ ...
165 ┌──────┴─────┐
166 │ pwrite │
167 └──────┬─────┘↺ ┌──────┴─────┐
168 ┌──────┴─────┐ │ close │
169 │ close │ └────────────┘
170 └────────────┘
171 │ before nbdkit exits
173 │
174 ┌─────────┴────────┐
175 │ cleanup │
176 └─────────┬────────┘
177 ┌─────────┴────────┐
178 │ unload │
179 └──────────────────┘
180 ".load"
182 is called once just after the plugin is loaded into memory.
183 ".config" and ".config_complete"
185 ".config" is called zero or more times during command line parsing.
186 ".config_complete" is called once after all configuration
187 information has been passed to the plugin (but not during "nbdkit
188 --dump-plugin").
189 Both are called after loading the plugin but before any connections
191 are accepted.
192 ".thread_model"
194 In normal operation, ".thread_model" is called once after
195 ".config_complete" has validated all configuration information, and
196 before any connections are accepted. However, during "nbdkit
197 --dump-plugin", it is called after any ".config" calls but without
198 ".config_complete" (so a plugin which determines the results from a
199 script must be prepared for a missing script).
200 ".get_ready"
202 In normal operation, ".get_ready" is called before the server
203 starts serving. It is called before the server forks or changes
204 directory. It is normally the last chance to do any global
205 preparation that is needed to serve connections.
206 Plugins should not create background threads here. Use
208 ".after_fork" instead.
209 ".after_fork"
211 In normal operation, ".after_fork" is called after the server has
212 forked into the background and changed UID and directory. If a
213 plugin needs to create background threads (or uses an external
214 library that creates threads) it should do so here, because
215 background threads are invalidated by fork.
216 Because the server may have forked into the background, error
218 messages and failures from ".after_fork" cannot be seen by the user
219 unless they look through syslog. An error in ".after_fork" can
220 appear to the user as if nbdkit “just died”. So in almost all
221 cases it is better to use ".get_ready" instead of this callback, or
222 to do as much preparation work as possible in ".get_ready" and only
223 start background threads here.
224 The server doesn't always fork (eg. if the -f flag is used), but
226 even so this callback will be called. If you want to find out if
227 the server forked between ".get_ready" and ".after_fork" use
228 getpid(2).
229 ".preconnect"
231 Called when a TCP connection has been made to the server. This
232 happens early, before NBD or TLS negotiation.
233 ".list_exports"
235 Early in option negotiation the client may try to list the exports
236 served by the plugin, and plugins can optionally implement this
237 callback to answer the client. See "EXPORT NAME" below.
238 ".default_export"
240 During option negotiation, if the client requests the default
241 export name (""), this optional callback provides a canonical name
242 to use in its place prior to calling ".open".
243 ".open"
245 A new client has connected and finished the NBD handshake. TLS
246 negotiation (if required) has been completed successfully.
247 ".can_write", ".get_size" and other option negotiation callbacks
249 These are called during option negotiation with the client, but
250 before any data is served. These callbacks may return different
251 values across different ".open" calls, but within a single
252 connection, they are called at most once and cached by nbdkit for
253 that connection.
254 ".pread", ".pwrite" and other data serving callbacks
256 After option negotiation has finished, these may be called to serve
257 data. Depending on the thread model chosen, they might be called
258 in parallel from multiple threads. The data serving callbacks
259 include a flags argument; the results of the negotiation callbacks
260 influence whether particular flags will ever be passed to a data
261 callback.
262 ".close"
264 The client has disconnected.
265 ".preconnect", ".open" ... ".close"
267 The sequence ".preconnect", ".open" ... ".close" can be called
268 repeatedly over the lifetime of the plugin, and can be called in
269 parallel (depending on the thread model).
270 ".cleanup"
272 is called once after all connections have been closed, prior to
273 unloading the plugin from memory. This is only called if
274 ".after_fork" succeeded earlier (even in cases where nbdkit did not
275 fork but is running in the foreground), which makes it a good place
276 to gracefully end any background threads.
277 ".unload"
279 is called once just before the plugin is unloaded from memory.
280 This is called even when nbdkit did not need to use ".after_fork"
281 (such as when using --dump-plugin to display documentation about
282 the plugin).
283 Flags
285 The following flags are defined by nbdkit, and used in various data
286 serving callbacks as follows:
287 "NBDKIT_FLAG_MAY_TRIM"
289 This flag is used by the ".zero" callback; there is no way to
290 disable this flag, although a plugin that does not support trims as
291 a way to write zeroes may ignore the flag without violating
292 expected semantics.
293 "NBDKIT_FLAG_FUA"
295 This flag represents Forced Unit Access semantics. It is used by
296 the ".pwrite", ".zero", and ".trim" callbacks to indicate that the
297 plugin must not return a result until the action has landed in
298 persistent storage. This flag will not be sent to the plugin
299 unless ".can_fua" is provided and returns "NBDKIT_FUA_NATIVE".
300 The following defines are valid as successful return values for
302 ".can_fua":
303 "NBDKIT_FUA_NONE"
305 Forced Unit Access is not supported; the client must manually
306 request a flush after writes have completed. The "NBDKIT_FLAG_FUA"
307 flag will not be passed to the plugin's write callbacks.
308 "NBDKIT_FUA_EMULATE"
310 The client may request Forced Unit Access, but it is implemented by
311 emulation, where nbdkit calls ".flush" after a write operation;
312 this is semantically correct, but may hurt performance as it tends
313 to flush more data than just what the client requested. The
314 "NBDKIT_FLAG_FUA" flag will not be passed to the plugin's write
315 callbacks.
316 "NBDKIT_FUA_NATIVE"
318 The client may request Forced Unit Access, which results in the
319 "NBDKIT_FLAG_FUA" flag being passed to the plugin's write callbacks
320 (".pwrite", ".trim", and ".zero"). When the flag is set, these
321 callbacks must not return success until the client's request has
322 landed in persistent storage.
323 The following defines are valid as successful return values for
325 ".can_cache":
326 "NBDKIT_CACHE_NONE"
328 The server does not advertise caching support, and rejects any
329 client-requested caching. Any ".cache" callback is ignored.
330 "NBDKIT_CACHE_EMULATE"
332 The nbdkit server advertises cache support to the client, where the
333 client may request that the server cache a region of the export to
334 potentially speed up future read and/or write operations on that
335 region. The nbdkit server implements the caching by calling
336 ".pread" and ignoring the results. This option exists to ease the
337 implementation of a common form of caching; any ".cache" callback
338 is ignored.
339 "NBDKIT_CACHE_NATIVE"
341 The nbdkit server advertises cache support to the client, where the
342 client may request that the server cache a region of the export to
343 potentially speed up future read and/or write operations on that
344 region. The nbdkit server calls the ".cache" callback to perform
345 the caching; if that callback is missing, the client's cache
346 request succeeds without doing anything.
347 Threads
349 Each nbdkit plugin must declare its maximum thread safety model by
350 defining the "THREAD_MODEL" macro. (This macro is used by
351 "NBDKIT_REGISTER_PLUGIN"). Additionally, a plugin may implement the
352 ".thread_model" callback, called right after ".config_complete" to make
353 a runtime decision on which thread model to use. The nbdkit server
354 chooses the most restrictive model between the plugin's "THREAD_MODEL",
355 the ".thread_model" if present, any restrictions requested by filters,
356 and any limitations imposed by the operating system.
357 In "nbdkit --dump-plugin PLUGIN" output, the "max_thread_model" line
359 matches the "THREAD_MODEL" macro, and the "thread_model" line matches
360 what the system finally settled on after applying all restrictions.
361 The possible settings for "THREAD_MODEL" are defined below.
363 "#define THREAD_MODEL NBDKIT_THREAD_MODEL_SERIALIZE_CONNECTIONS"
365 Only a single handle can be open at any time, and all requests
366 happen from one thread.
367 Note this means only one client can connect to the server at any
369 time. If a second client tries to connect it will block waiting
370 for the first client to close the connection.
371 "#define THREAD_MODEL NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS"
373 This is a safe default for most plugins.
374 Multiple handles can be open at the same time, but requests are
376 serialized so that for the plugin as a whole only one
377 open/read/write/close (etc) request will be in progress at any
378 time.
379 This is a useful setting if the library you are using is not
381 thread-safe. However performance may not be good.
382 "#define THREAD_MODEL NBDKIT_THREAD_MODEL_SERIALIZE_REQUESTS"
384 Multiple handles can be open and multiple data requests can happen
385 in parallel. However only one request will happen per handle at a
386 time (but requests on different handles might happen concurrently).
387 "#define THREAD_MODEL NBDKIT_THREAD_MODEL_PARALLEL"
389 Multiple handles can be open and multiple data requests can happen
390 in parallel (even on the same handle). The server may reorder
391 replies, answering a later request before an earlier one.
392 All the libraries you use must be thread-safe and reentrant, and
394 any code that creates a file descriptor should atomically set
395 "FD_CLOEXEC" if you do not want it accidentally leaked to another
396 thread's child process. You may also need to provide mutexes for
397 fields in your connection handle.
398 If none of the above thread models are suitable, use
400 "NBDKIT_THREAD_MODEL_PARALLEL" and implement your own locking using
401 "pthread_mutex_t" etc.
402 Error handling
404 If there is an error in the plugin, the plugin should call
405 "nbdkit_error" to report an error message; additionally, if the
406 callback is involved in serving data, the plugin should call
407 "nbdkit_set_error" to influence the error code that will be sent to the
408 client. These two functions can be called in either order. Then, the
409 callback should return the appropriate error indication, eg. "NULL" or
410 "-1".
411 If the call to "nbdkit_set_error" is omitted while serving data, then
413 the global variable "errno" may be used. For plugins which have
414 ".errno_is_preserved != 0" the core code will use "errno". In plugins
415 written in non-C languages, we usually cannot trust that "errno" will
416 not be overwritten when returning from that language to C. In that
417 case, either the plugin must call "nbdkit_set_error" or hard-coded
418 "EIO" is used.
419 "nbdkit_error" has the following prototype and works like printf(3):
421 void nbdkit_error (const char *fs, ...);
423 void nbdkit_verror (const char *fs, va_list args);
424 For convenience, "nbdkit_error" preserves the value of "errno", and
426 also supports the glibc extension of a single %m in a format string
427 expanding to "strerror(errno)", even on platforms that don't support
428 that natively.
429 "nbdkit_set_error" can be called at any time, but only has an impact
431 during callbacks for serving data, and only when the callback returns
432 an indication of failure. It has the following prototype:
433 void nbdkit_set_error (int err);
435
437 ".name"
438 const char *name;
439 This field (a string) is required, and must contain only ASCII
441 alphanumeric characters or non-leading dashes, and be unique amongst
442 all plugins.
443 ".version"
445 const char *version;
446 Plugins may optionally set a version string which is displayed in help
448 and debugging output. (See also "VERSION" below)
449 ".longname"
451 const char *longname;
452 An optional free text name of the plugin. This field is used in error
454 messages.
455 ".description"
457 const char *description;
458 An optional multi-line description of the plugin.
460 ".load"
462 void load (void);
463 This is called once just after the plugin is loaded into memory. You
465 can use this to perform any global initialization needed by the plugin.
466 ".unload"
468 void unload (void);
469 This may be called once just before the plugin is unloaded from memory.
471 Note that it's not guaranteed that ".unload" will always be called (eg.
472 the server might be killed or segfault), so you should try to make the
473 plugin as robust as possible by not requiring cleanup. See also
474 "SHUTDOWN" below.
475 ".dump_plugin"
477 void dump_plugin (void);
478 This optional callback is called when the "nbdkit plugin --dump-plugin"
480 command is used. It should print any additional informative
481 "key=value" fields to stdout as needed. Prefixing the keys with the
482 name of the plugin will avoid conflicts.
483 ".config"
485 int config (const char *key, const char *value);
486 On the nbdkit command line, after the plugin filename, come an optional
488 list of "key=value" arguments. These are passed to the plugin through
489 this callback when the plugin is first loaded and before any
490 connections are accepted.
491 This callback may be called zero or more times.
493 Both "key" and "value" parameters will be non-NULL. The strings are
495 owned by nbdkit but will remain valid for the lifetime of the plugin,
496 so the plugin does not need to copy them.
497 The key will be a non-empty string beginning with an ASCII alphabetic
499 character ("A-Z" "a-z"). The rest of the key must contain only ASCII
500 alphanumeric plus period, underscore or dash characters ("A-Z" "a-z"
501 "0-9" "." "_" "-"). The value may be an arbitrary string, including an
502 empty string.
503 The names of "key"s accepted by plugins is up to the plugin, but you
505 should probably look at other plugins and follow the same conventions.
506 If the value is a relative path, then note that the server changes
508 directory when it starts up. See "FILENAMES AND PATHS" above.
509 If "nbdkit_stdio_safe" returns 1, the value of the configuration
511 parameter may be used to trigger reading additional data through stdin
512 (such as a password or inline script).
513 If the ".config" callback is not provided by the plugin, and the user
515 tries to specify any "key=value" arguments, then nbdkit will exit with
516 an error.
517 If there is an error, ".config" should call "nbdkit_error" with an
519 error message and return "-1".
520 ".magic_config_key"
522 const char *magic_config_key;
523 This optional string can be used to set a "magic" key used when parsing
525 plugin parameters. It affects how "bare parameters" (those which do
526 not contain an "=" character) are parsed on the command line.
527 If "magic_config_key != NULL" then any bare parameters are passed to
529 the ".config" method as: "config (magic_config_key, argv[i]);".
530 If "magic_config_key" is not set then we behave as in nbdkit < 1.7: If
532 the first parameter on the command line is bare then it is passed to
533 the ".config" method as: "config ("script", value);". Any other bare
534 parameters give errors.
535 ".config_complete"
537 int config_complete (void);
538 This optional callback is called after all the configuration has been
540 passed to the plugin. It is a good place to do checks, for example
541 that the user has passed the required parameters to the plugin.
542 If there is an error, ".config_complete" should call "nbdkit_error"
544 with an error message and return "-1".
545 ".config_help"
547 const char *config_help;
548 This optional multi-line help message should summarize any "key=value"
550 parameters that it takes. It does not need to repeat what already
551 appears in ".description".
552 If the plugin doesn't take any config parameters you should probably
554 omit this.
555 ".thread_model"
557 int thread_model (void)
558 This optional callback is called after all the configuration has been
560 passed to the plugin. It can be used to force a stricter thread model
561 based on configuration, compared to "THREAD_MODEL". See "Threads"
562 above for details. Attempts to request a looser (more parallel) model
563 are silently ignored.
564 If there is an error, ".thread_model" should call "nbdkit_error" with
566 an error message and return "-1".
567 ".get_ready"
569 int get_ready (void);
570 This optional callback is called before the server starts serving. It
572 is called before the server forks or changes directory. It is
573 ordinarily the last chance to do any global preparation that is needed
574 to serve connections.
575 If there is an error, ".get_ready" should call "nbdkit_error" with an
577 error message and return "-1".
578 ".after_fork"
580 int after_fork (void);
581 This optional callback is called before the server starts serving. It
583 is called after the server forks and changes directory. If a plugin
584 needs to create background threads (or uses an external library that
585 creates threads) it should do so here, because background threads are
586 killed by fork. However you should try to do as little as possible
587 here because error reporting is difficult. See "Callback lifecycle"
588 above.
589 If there is an error, ".after_fork" should call "nbdkit_error" with an
591 error message and return "-1".
592 ".cleanup"
594 void cleanup (void);
595 This optional callback is called after the server has closed all
597 connections and is preparing to unload. It is only reached in the same
598 cases that the ".after_fork" callback was used, making it a good place
599 to clean up any background threads. However, it is not guaranteed that
600 this callback will be reached, so you should try to make the plugin as
601 robust as possible by not requiring cleanup. See also "SHUTDOWN"
602 below.
603 ".preconnect"
605 int preconnect (int readonly);
606 This optional callback is called when a TCP connection has been made to
608 the server. This happens early, before NBD or TLS negotiation. If TLS
609 authentication is required to access the server, then it has not been
610 negotiated at this point.
611 For security reasons (to avoid denial of service attacks) this callback
613 should be written to be as fast and take as few resources as possible.
614 If you use this callback, only use it to do basic access control, such
615 as checking "nbdkit_peer_name", "nbdkit_peer_pid", "nbdkit_peer_uid",
616 "nbdkit_peer_gid" against a list of permitted source addresses (see
617 "PEER NAME" and nbdkit-ip-filter(1)). It may be better to do access
618 control outside the server, for example using TCP wrappers or a
619 firewall.
620 The "readonly" flag informs the plugin that the server was started with
622 the -r flag on the command line.
623 Returning 0 will allow the connection to continue. If there is an
625 error or you want to deny the connection, call "nbdkit_error" with an
626 error message and return "-1".
627 ".list_exports"
629 int list_exports (int readonly, int is_tls,
630 struct nbdkit_exports *exports);
631 This optional callback is called if the client tries to list the
633 exports served by the plugin (using "NBD_OPT_LIST"). If the plugin
634 does not supply this callback then the result of ".default_export" is
635 advertised as the lone export. The NBD protocol defines "" as the
636 default export, so this is suitable for plugins which ignore the export
637 name and always serve the same content. See also "EXPORT NAME" below.
638 The "readonly" flag informs the plugin that the server was started with
640 the -r flag on the command line, which is the same value passed to
641 ".preconnect" and ".open". However, the NBD protocol does not yet have
642 a way to let the client advertise an intent to be read-only even when
643 the server allows writes, so this parameter may not be as useful as it
644 appears.
645 The "is_tls" flag informs the plugin whether this listing was requested
647 after the client has completed TLS negotiation. When running the
648 server in a mode that permits but does not require TLS, be careful that
649 any exports listed when "is_tls" is "false" do not leak unintended
650 information.
651 The "exports" parameter is an opaque object for collecting the list of
653 exports. Call "nbdkit_add_export" as needed to add specific exports to
654 the list.
655 int nbdkit_add_export (struct nbdkit_export *exports,
657 const char *name, const char *description);
658 The "name" must be a non-NULL, UTF-8 string between 0 and 4096 bytes in
660 length. Export names must be unique. "description" is an optional
661 description of the export which some clients can display but which is
662 otherwise unused (if you don't want a description, you can pass this
663 parameter as "NULL"). The string(s) are copied into the exports list
664 so you may free them immediately after calling this function.
665 "nbdkit_add_export" returns 0 on success or "-1" on failure; on failure
666 "nbdkit_error" has already been called, with "errno" set to a suitable
667 value.
668 There are also situations where a plugin may wish to duplicate the
670 nbdkit default behavior of supplying an export list containing only the
671 result of ".default_export" when ".list_exports" is missing; this is
672 most common in a language binding where it is not known at compile time
673 whether the language script will be providing an implementation for
674 ".list_exports", and is done by calling "nbdkit_use_default_export".
675 int nbdkit_use_default_export (struct nbdkit_export *exports);
677 "nbdkit_use_default_export" returns 0 on success or "-1" on failure; on
679 failure "nbdkit_error" has already been called, with "errno" set to a
680 suitable value.
681 The plugin may also leave the export list empty, by not calling either
683 helper. Once the plugin is happy with the list contents, returning 0
684 will send the list of exports back to the client. If there is an
685 error, ".list_exports" should call "nbdkit_error" with an error message
686 and return "-1".
687 ".default_export"
689 const char *default_export (int readonly, int is_tls);
690 This optional callback is called if the client tries to connect to the
692 default export "", where the plugin provides a UTF-8 string between 0
693 and 4096 bytes. If the plugin does not supply this callback, the
694 connection continues with the empty name; if the plugin returns a valid
695 string, nbdkit behaves as if the client had passed that string instead
696 of an empty name, and returns that name to clients that support it (see
697 the "NBD_INFO_NAME" response to "NBD_OPT_GO"). Similarly, if the
698 plugin does not supply a ".list_exports" callback, the result of this
699 callback determines what export name to advertise to a client
700 requesting an export list.
701 The "readonly" flag informs the plugin that the server was started with
703 the -r flag on the command line, which is the same value passed to
704 ".preconnect" and ".open". However, the NBD protocol does not yet have
705 a way to let the client advertise an intent to be read-only even when
706 the server allows writes, so this parameter may not be as useful as it
707 appears.
708 The "is_tls" flag informs the plugin whether the canonical name for the
710 default export is being requested after the client has completed TLS
711 negotiation. When running the server in a mode that permits but does
712 not require TLS, be careful that a default export name does not leak
713 unintended information.
714 If the plugin returns "NULL" or an invalid string (such as longer than
716 4096 bytes), the client is not permitted to connect to the default
717 export. However, this is not an error in the protocol, so it is not
718 necessary to call "nbdkit_error".
719 ".open"
721 void *open (int readonly);
722 This is called when a new client connects to the nbdkit server. The
724 callback should allocate a handle and return it. This handle is passed
725 back to other callbacks and could be freed in the ".close" callback.
726 Note that the handle is completely opaque to nbdkit, but it must not be
728 NULL. If you don't need to use a handle, return
729 "NBDKIT_HANDLE_NOT_NEEDED" which is a static non-NULL pointer.
730 The "readonly" flag informs the plugin that the server was started with
732 the -r flag on the command line which forces connections to be read-
733 only. Note that the plugin may additionally force the connection to be
734 readonly (even if this flag is false) by returning false from the
735 ".can_write" callback. So if your plugin can only serve read-only, you
736 can ignore this parameter.
737 If the plugin wants to differentiate the content it serves based on
739 client input, then this is the spot to use "nbdkit_export_name()" to
740 determine which export the client requested. See also "EXPORT NAME"
741 below.
742 This callback is called after the NBD handshake has completed; if the
744 server requires TLS authentication, then that has occurred as well.
745 But if the server is set up to have optional TLS authentication, you
746 may check "nbdkit_is_tls" to learn whether the client has completed TLS
747 authentication. When running the server in a mode that permits but
748 does not require TLS, be careful that you do not allow unauthenticated
749 clients to cause a denial of service against authentication.
750 If there is an error, ".open" should call "nbdkit_error" with an error
752 message and return "NULL".
753 ".close"
755 void close (void *handle);
756 This is called when the client closes the connection. It should clean
758 up any per-connection resources.
759 Note there is no way in the NBD protocol to communicate close errors
761 back to the client, for example if your plugin calls close(2) and you
762 are checking for errors (as you should do). Therefore the best you can
763 do is to log the error on the server. Well-behaved NBD clients should
764 try to flush the connection before it is closed and check for errors,
765 but obviously this is outside the scope of nbdkit.
766 ".get_size"
768 int64_t get_size (void *handle);
769 This is called during the option negotiation phase of the protocol to
771 get the size (in bytes) of the block device being exported.
772 The returned size must be ≥ 0. If there is an error, ".get_size"
774 should call "nbdkit_error" with an error message and return "-1".
775 ".export_description"
777 const char *export_description (void *handle);
778 This is called during the option negotiation phase only if the client
780 specifically requested an export description (see the
781 "NBD_INFO_DESCRIPTION" response to "NBD_OPT_GO"). Any description
782 provided must be human-readable UTF-8, no longer than 4096 bytes.
783 Ideally, this description should match any description set during
784 ".list_exports", but that is not enforced.
785 If the plugin returns "NULL" or an invalid string (such as longer than
787 4096 bytes), or if this callback is omitted, no description is offered
788 to the client. As this is not an error in the protocol, it is not
789 necessary to call "nbdkit_error". If the callback will not be
790 returning a compile-time constant string, you may find
791 "nbdkit_strdup_intern" helpful for returning a value that avoids a
792 memory leak.
793 ".can_write"
795 int can_write (void *handle);
796 This is called during the option negotiation phase to find out if the
798 handle supports writes.
799 If there is an error, ".can_write" should call "nbdkit_error" with an
801 error message and return "-1".
802 This callback is not required. If omitted, then we return true iff a
804 ".pwrite" callback has been defined.
805 ".can_flush"
807 int can_flush (void *handle);
808 This is called during the option negotiation phase to find out if the
810 handle supports the flush-to-disk operation.
811 If there is an error, ".can_flush" should call "nbdkit_error" with an
813 error message and return "-1".
814 This callback is not required. If omitted, then we return true iff a
816 ".flush" callback has been defined.
817 ".is_rotational"
819 int is_rotational (void *handle);
820 This is called during the option negotiation phase to find out if the
822 backing disk is a rotational medium (like a traditional hard disk) or
823 not (like an SSD). If true, this may cause the client to reorder
824 requests to make them more efficient for a slow rotating disk.
825 If there is an error, ".is_rotational" should call "nbdkit_error" with
827 an error message and return "-1".
828 This callback is not required. If omitted, then we return false.
830 ".can_trim"
832 int can_trim (void *handle);
833 This is called during the option negotiation phase to find out if the
835 plugin supports the trim/discard operation for punching holes in the
836 backing storage.
837 If there is an error, ".can_trim" should call "nbdkit_error" with an
839 error message and return "-1".
840 This callback is not required. If omitted, then we return true iff a
842 ".trim" callback has been defined.
843 ".can_zero"
845 int can_zero (void *handle);
846 This is called during the option negotiation phase to find out if the
848 plugin wants the ".zero" callback to be utilized. Support for writing
849 zeroes is still advertised to the client (unless the
850 nbdkit-nozero-filter(1) is also used), so returning false merely serves
851 as a way to avoid complicating the ".zero" callback to have to fail
852 with "ENOTSUP" or "EOPNOTSUPP" on the connections where it will never
853 be more efficient than using ".pwrite" up front.
854 If there is an error, ".can_zero" should call "nbdkit_error" with an
856 error message and return "-1".
857 This callback is not required. If omitted, then for a normal zero
859 request, nbdkit always tries ".zero" first if it is present, and
860 gracefully falls back to ".pwrite" if ".zero" was absent or failed with
861 "ENOTSUP" or "EOPNOTSUPP".
862 ".can_fast_zero"
864 int can_fast_zero (void *handle);
865 This is called during the option negotiation phase to find out if the
867 plugin wants to advertise support for fast zero requests. If this
868 support is not advertised, a client cannot attempt fast zero requests,
869 and has no way to tell if writing zeroes offers any speedups compared
870 to using ".pwrite" (other than compressed network traffic). If support
871 is advertised, then ".zero" will have "NBDKIT_FLAG_FAST_ZERO" set when
872 the client has requested a fast zero, in which case the plugin must
873 fail with "ENOTSUP" or "EOPNOTSUPP" up front if the request would not
874 offer any benefits over ".pwrite". Advertising support for fast zero
875 requests does not require that writing zeroes be fast, only that the
876 result (whether success or failure) is fast, so this should be
877 advertised when feasible.
878 If there is an error, ".can_fast_zero" should call "nbdkit_error" with
880 an error message and return "-1".
881 This callback is not required. If omitted, then nbdkit returns true if
883 ".zero" is absent or ".can_zero" returns false (in those cases, nbdkit
884 fails all fast zero requests, as its fallback to ".pwrite" is not
885 inherently faster), otherwise false (since it cannot be determined in
886 advance if the plugin's ".zero" will properly honor the semantics of
887 "NBDKIT_FLAG_FAST_ZERO").
888 ".can_extents"
890 int can_extents (void *handle);
891 This is called during the option negotiation phase to find out if the
893 plugin supports detecting allocated (non-sparse) regions of the disk
894 with the ".extents" callback.
895 If there is an error, ".can_extents" should call "nbdkit_error" with an
897 error message and return "-1".
898 This callback is not required. If omitted, then we return true iff a
900 ".extents" callback has been defined.
901 ".can_fua"
903 int can_fua (void *handle);
904 This is called during the option negotiation phase to find out if the
906 plugin supports the Forced Unit Access (FUA) flag on write, zero, and
907 trim requests. If this returns "NBDKIT_FUA_NONE", FUA support is not
908 advertised to the client; if this returns "NBDKIT_FUA_EMULATE", the
909 ".flush" callback must work (even if ".can_flush" returns false), and
910 FUA support is emulated by calling ".flush" after any write operation;
911 if this returns "NBDKIT_FUA_NATIVE", then the ".pwrite", ".zero", and
912 ".trim" callbacks (if implemented) must handle the flag
913 "NBDKIT_FLAG_FUA", by not returning until that action has landed in
914 persistent storage.
915 If there is an error, ".can_fua" should call "nbdkit_error" with an
917 error message and return "-1".
918 This callback is not required unless a plugin wants to specifically
920 handle FUA requests. If omitted, nbdkit checks whether ".flush"
921 exists, and behaves as if this function returns "NBDKIT_FUA_NONE" or
922 "NBDKIT_FUA_EMULATE" as appropriate.
923 ".can_multi_conn"
925 int can_multi_conn (void *handle);
926 This is called during the option negotiation phase to find out if the
928 plugin is prepared to handle multiple connections from a single client.
929 If the plugin sets this to true then a client may try to open multiple
930 connections to the nbdkit server and spread requests across all
931 connections to maximize parallelism. If the plugin sets it to false
932 (which is the default) then well-behaved clients should only open a
933 single connection, although we cannot control what clients do in
934 practice.
935 Specifically it means that either the plugin does not cache requests at
937 all. Or if it does cache them then the effects of a ".flush" request
938 or setting "NBDKIT_FLAG_FUA" on a request must be visible across all
939 connections to the plugin before the plugin replies to that request.
940 Properly working clients should send the same export name for each of
942 these connections.
943 If you use Linux nbd-client(8) option -C num with num > 1 then Linux
945 checks this flag and will refuse to connect if ".can_multi_conn" is
946 false.
947 If there is an error, ".can_multi_conn" should call "nbdkit_error" with
949 an error message and return "-1".
950 This callback is not required. If omitted, then we return false.
952 ".can_cache"
954 int can_cache (void *handle);
955 This is called during the option negotiation phase to find out if the
957 plugin supports a cache or prefetch operation.
958 This can return:
960 "NBDKIT_CACHE_NONE"
962 Cache support is not advertised to the client.
963 "NBDKIT_CACHE_EMULATE"
965 Caching is emulated by the server calling ".pread" and discarding
966 the result.
967 "NBDKIT_CACHE_NATIVE"
969 The ".cache" callback will be called.
970 If there is an error, ".can_cache" should call "nbdkit_error" with an
972 error message and return "-1".
973 This callback is not required. If omitted, then we return
975 "NBDKIT_CACHE_NONE" if the ".cache" callback is missing, or
976 "NBDKIT_CACHE_NATIVE" if it is defined.
977 ".pread"
979 int pread (void *handle, void *buf, uint32_t count, uint64_t offset,
980 uint32_t flags);
981 During the data serving phase, nbdkit calls this callback to read data
983 from the backing store. "count" bytes starting at "offset" in the
984 backing store should be read and copied into "buf". nbdkit takes care
985 of all bounds- and sanity-checking, so the plugin does not need to
986 worry about that.
987 The parameter "flags" exists in case of future NBD protocol extensions;
989 at this time, it will be 0 on input.
990 The callback must read the whole "count" bytes if it can. The NBD
992 protocol doesn't allow partial reads (instead, these would be errors).
993 If the whole "count" bytes was read, the callback should return 0 to
994 indicate there was no error.
995 If there is an error (including a short read which couldn't be
997 recovered from), ".pread" should call "nbdkit_error" with an error
998 message, and "nbdkit_set_error" to record an appropriate error (unless
999 "errno" is sufficient), then return "-1".
1000 ".pwrite"
1002 int pwrite (void *handle, const void *buf, uint32_t count, uint64_t offset,
1003 uint32_t flags);
1004 During the data serving phase, nbdkit calls this callback to write data
1006 to the backing store. "count" bytes starting at "offset" in the
1007 backing store should be written using the data in "buf". nbdkit takes
1008 care of all bounds- and sanity-checking, so the plugin does not need to
1009 worry about that.
1010 This function will not be called if ".can_write" returned false. The
1012 parameter "flags" may include "NBDKIT_FLAG_FUA" on input based on the
1013 result of ".can_fua".
1014 The callback must write the whole "count" bytes if it can. The NBD
1016 protocol doesn't allow partial writes (instead, these would be errors).
1017 If the whole "count" bytes was written successfully, the callback
1018 should return 0 to indicate there was no error.
1019 If there is an error (including a short write which couldn't be
1021 recovered from), ".pwrite" should call "nbdkit_error" with an error
1022 message, and "nbdkit_set_error" to record an appropriate error (unless
1023 "errno" is sufficient), then return "-1".
1024 ".flush"
1026 int flush (void *handle, uint32_t flags);
1027 During the data serving phase, this callback is used to fdatasync(2)
1029 the backing store, ie. to ensure it has been completely written to a
1030 permanent medium. If that is not possible then you can omit this
1031 callback.
1032 This function will not be called directly by the client if ".can_flush"
1034 returned false; however, it may still be called by nbdkit if ".can_fua"
1035 returned "NBDKIT_FUA_EMULATE". The parameter "flags" exists in case of
1036 future NBD protocol extensions; at this time, it will be 0 on input.
1037 If there is an error, ".flush" should call "nbdkit_error" with an error
1039 message, and "nbdkit_set_error" to record an appropriate error (unless
1040 "errno" is sufficient), then return "-1".
1041 ".trim"
1043 int trim (void *handle, uint32_t count, uint64_t offset, uint32_t flags);
1044 During the data serving phase, this callback is used to "punch holes"
1046 in the backing store. If that is not possible then you can omit this
1047 callback.
1048 This function will not be called if ".can_trim" returned false. The
1050 parameter "flags" may include "NBDKIT_FLAG_FUA" on input based on the
1051 result of ".can_fua".
1052 If there is an error, ".trim" should call "nbdkit_error" with an error
1054 message, and "nbdkit_set_error" to record an appropriate error (unless
1055 "errno" is sufficient), then return "-1".
1056 ".zero"
1058 int zero (void *handle, uint32_t count, uint64_t offset, uint32_t flags);
1059 During the data serving phase, this callback is used to write "count"
1061 bytes of zeroes at "offset" in the backing store.
1062 This function will not be called if ".can_zero" returned false. On
1064 input, the parameter "flags" may include "NBDKIT_FLAG_MAY_TRIM"
1065 unconditionally, "NBDKIT_FLAG_FUA" based on the result of ".can_fua",
1066 and "NBDKIT_FLAG_FAST_ZERO" based on the result of ".can_fast_zero".
1067 If "NBDKIT_FLAG_MAY_TRIM" is requested, the operation can punch a hole
1069 instead of writing actual zero bytes, but only if subsequent reads from
1070 the hole read as zeroes.
1071 If "NBDKIT_FLAG_FAST_ZERO" is requested, the plugin must decide up
1073 front if the implementation is likely to be faster than a corresponding
1074 ".pwrite"; if not, then it must immediately fail with "ENOTSUP" or
1075 "EOPNOTSUPP" (whether by "nbdkit_set_error" or "errno") and preferably
1076 without modifying the exported image. It is acceptable to always fail
1077 a fast zero request (as a fast failure is better than attempting the
1078 write only to find out after the fact that it was not fast after all).
1079 Note that on Linux, support for "ioctl(BLKZEROOUT)" is insufficient for
1080 determining whether a zero request to a block device will be fast
1081 (because the kernel will perform a slow fallback when needed).
1082 The callback must write the whole "count" bytes if it can. The NBD
1084 protocol doesn't allow partial writes (instead, these would be errors).
1085 If the whole "count" bytes was written successfully, the callback
1086 should return 0 to indicate there was no error.
1087 If there is an error, ".zero" should call "nbdkit_error" with an error
1089 message, and "nbdkit_set_error" to record an appropriate error (unless
1090 "errno" is sufficient), then return "-1".
1091 If this callback is omitted, or if it fails with "ENOTSUP" or
1093 "EOPNOTSUPP" (whether by "nbdkit_set_error" or "errno"), then ".pwrite"
1094 will be used as an automatic fallback except when the client requested
1095 a fast zero.
1096 ".extents"
1098 int extents (void *handle, uint32_t count, uint64_t offset,
1099 uint32_t flags, struct nbdkit_extents *extents);
1100 During the data serving phase, this callback is used to detect
1102 allocated, sparse and zeroed regions of the disk.
1103 This function will not be called if ".can_extents" returned false.
1105 nbdkit's default behaviour in this case is to treat the whole virtual
1106 disk as if it was allocated. Also, this function will not be called by
1107 a client that does not request structured replies (the --no-sr option
1108 of nbdkit can be used to test behavior when ".extents" is unavailable
1109 to the client).
1110 The callback should detect and return the list of extents overlapping
1112 the range "[offset...offset+count-1]". The "extents" parameter points
1113 to an opaque object which the callback should fill in by calling
1114 "nbdkit_add_extent". See "Extents list" below.
1115 If there is an error, ".extents" should call "nbdkit_error" with an
1117 error message, and "nbdkit_set_error" to record an appropriate error
1118 (unless "errno" is sufficient), then return "-1".
1119 Extents list
1121 The plugin "extents" callback is passed an opaque pointer "struct
1123 nbdkit_extents *extents". This structure represents a list of
1124 filesystem extents describing which areas of the disk are allocated,
1125 which are sparse (“holes”), and, if supported, which are zeroes.
1126 The "extents" callback should scan the disk starting at "offset" and
1128 call "nbdkit_add_extent" for each extent found.
1129 Extents overlapping the range "[offset...offset+count-1]" should be
1131 returned if possible. However nbdkit ignores extents < offset so the
1132 plugin may, if it is easier to implement, return all extent information
1133 for the whole disk. The plugin may return extents beyond the end of
1134 the range. It may also return extent information for less than the
1135 whole range, but it must return at least one extent overlapping
1136 "offset".
1137 The extents must be added in ascending order, and must be contiguous.
1139 The "flags" parameter of the ".extents" callback may contain the flag
1141 "NBDKIT_FLAG_REQ_ONE". This means that the client is only requesting
1142 information about the extent overlapping "offset". The plugin may
1143 ignore this flag, or as an optimization it may return just a single
1144 extent for "offset".
1145 int nbdkit_add_extent (struct nbdkit_extents *extents,
1147 uint64_t offset, uint64_t length, uint32_t type);
1148 Add an extent covering "[offset...offset+length-1]" of one of the
1150 following four types:
1151 "type = 0"
1153 A normal, allocated data extent.
1154 "type = NBDKIT_EXTENT_HOLE|NBDKIT_EXTENT_ZERO"
1156 An unallocated extent, a.k.a. a “hole”, which reads back as zeroes.
1157 This is the normal type of hole applicable to most disks.
1158 "type = NBDKIT_EXTENT_ZERO"
1160 An allocated extent which is known to contain only zeroes.
1161 "type = NBDKIT_EXTENT_HOLE"
1163 An unallocated extent (hole) which does not read back as zeroes.
1164 Note this should only be used in specialized circumstances such as
1165 when writing a plugin for (or to emulate) certain SCSI drives which
1166 do not guarantee that trimmed blocks read back as zeroes.
1167 "nbdkit_add_extent" returns 0 on success or "-1" on failure. On
1169 failure "nbdkit_error" and/or "nbdkit_set_error" has already been
1170 called. "errno" will be set to a suitable value.
1171 ".cache"
1173 int cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags);
1174 During the data serving phase, this callback is used to give the plugin
1176 a hint that the client intends to make further accesses to the given
1177 region of the export.
1178 The nature of caching/prefetching is not specified further by the NBD
1180 specification. For example, a server may place limits on how much may
1181 be cached at once, and there is no way to control if writes to a cached
1182 area have write-through or write-back semantics. In fact, the cache
1183 command can always fail and still be compliant, and success might not
1184 guarantee a performance gain.
1185 If this callback is omitted, then the results of ".can_cache" determine
1187 whether nbdkit will reject cache requests, treat them as instant
1188 success, or emulate caching by calling ".pread" over the same region
1189 and ignoring the results.
1190 This function will not be called if ".can_cache" did not return
1192 "NBDKIT_CACHE_NATIVE".
1193 The "flags" parameter exists in case of future NBD protocol extensions;
1195 at this time, it will be 0 on input. A plugin must fail this function
1196 if "flags" includes an unrecognized flag, as that may indicate a
1197 requirement that the plugin must comply with to provide a specific
1198 caching semantic.
1199 If there is an error, ".cache" should call "nbdkit_error" with an error
1201 message, and "nbdkit_set_error" to record an appropriate error (unless
1202 "errno" is sufficient), then return "-1".
1203 ".errno_is_preserved"
1205 This field defaults to 0; if non-zero, nbdkit can reliably use the
1206 value of "errno" when a callback reports failure, rather than the
1207 plugin having to call "nbdkit_set_error".
1208
1210 When nbdkit receives certain signals it will shut down (see "SIGNALS"
1211 in nbdkit(1)). The server will wait for any currently running plugin
1212 callbacks to finish, call ".close" on those connections, then call the
1213 ".cleanup" and ".unload" callbacks before unloading the plugin.
1214 Note that it's not guaranteed this can always happen (eg. the server
1216 might be killed by "SIGKILL" or segfault).
1217 Requesting asynchronous shutdown
1219 Plugins and filters can call exit(3) in the configuration phase (before
1220 and including ".get_ready", but not in connected callbacks).
1221 Once nbdkit has started serving connections, plugins and filters should
1223 not call exit(3). However they may instruct nbdkit to shut down by
1224 calling "nbdkit_shutdown":
1225 void nbdkit_shutdown (void);
1227 This function requests an asynchronous shutdown and returns (note that
1229 it does not exit the process immediately). It ensures that the plugin
1230 and all filters are unloaded cleanly which may take some time. Further
1231 callbacks from nbdkit into the plugin or filter may occur after you
1232 have called this.
1233
1235 Parsing numbers
1236 There are several functions for parsing numbers. These all deal
1237 correctly with overflow, out of range and parse errors, and you should
1238 use them instead of unsafe functions like sscanf(3), atoi(3) and
1239 similar.
1240 int nbdkit_parse_int (const char *what, const char *str, int *r);
1242 int nbdkit_parse_unsigned (const char *what,
1243 const char *str, unsigned *r);
1244 int nbdkit_parse_int8_t (const char *what,
1245 const char *str, int8_t *r);
1246 int nbdkit_parse_uint8_t (const char *what,
1247 const char *str, uint8_t *r);
1248 int nbdkit_parse_int16_t (const char *what,
1249 const char *str, int16_t *r);
1250 int nbdkit_parse_uint16_t (const char *what,
1251 const char *str, uint16_t *r);
1252 int nbdkit_parse_int32_t (const char *what,
1253 const char *str, int32_t *r);
1254 int nbdkit_parse_uint32_t (const char *what,
1255 const char *str, uint32_t *r);
1256 int nbdkit_parse_int64_t (const char *what,
1257 const char *str, int64_t *r);
1258 int nbdkit_parse_uint64_t (const char *what,
1259 const char *str, uint64_t *r);
1260 Parse string "str" into an integer of various types. These functions
1262 parse a decimal, hexadecimal ("0x...") or octal ("0...") number.
1263 On success the functions return 0 and set *r to the parsed value
1265 (unless "*r == NULL" in which case the result is discarded). On error,
1266 "nbdkit_error" is called and the functions return "-1". On error *r is
1267 always unchanged.
1268 The "what" parameter is printed in error messages to provide context.
1270 It should usually be a short descriptive string of what you are trying
1271 to parse, eg:
1272 if (nbdkit_parse_int ("random seed", argv[1], &seed) == -1)
1274 return -1;
1275 might print an error:
1277 random seed: could not parse number: "lalala"
1279 Parsing sizes
1281 Use the "nbdkit_parse_size" utility function to parse human-readable
1282 size strings such as "100M" into the size in bytes.
1283 int64_t nbdkit_parse_size (const char *str);
1285 "str" can be a string in a number of common formats. The function
1287 returns the size in bytes. If there was an error, it returns "-1".
1288 Parsing booleans
1290 Use the "nbdkit_parse_bool" utility function to parse human-readable
1291 strings such as "on" into a boolean value.
1292 int nbdkit_parse_bool (const char *str);
1294 "str" can be a string containing a case-insensitive form of various
1296 common toggle values. The function returns 0 or 1 if the parse was
1297 successful. If there was an error, it returns "-1".
1298 Reading passwords
1300 The "nbdkit_read_password" utility function can be used to read
1301 passwords from config parameters:
1302 int nbdkit_read_password (const char *value, char **password);
1304 For example:
1306 char *password = NULL;
1308 static int
1310 myplugin_config (const char *key, const char *value)
1311 {
1312 ..
1313 if (strcmp (key, "password") == 0) {
1314 free (password);
1315 if (nbdkit_read_password (value, &password) == -1)
1316 return -1;
1317 }
1318 ..
1319 }
1320 The "password" result string is allocated by malloc, and so you may
1322 need to free it.
1323 This function recognizes several password formats. A password may be
1325 used directly on the command line, eg:
1326 nbdkit myplugin password=mostsecret
1328 But more securely this function can also read a password interactively:
1330 nbdkit myplugin password=-
1332 or from a file:
1334 nbdkit myplugin password=+/tmp/secret
1336 or from a file descriptor inherited by nbdkit:
1338 nbdkit myplugin password=-99
1340 Notes on reading passwords
1342 If the password begins with a "-" or "+" character then it must be
1344 passed in a file.
1345 "password=-" can only be used when stdin is a terminal.
1347 "password=-FD" cannot be used with stdin, stdout or stderr (ie. "-0",
1349 "-1" or "-2"). The reason is that after reading the password the file
1350 descriptor is closed, which causes bad stuff to happen.
1351 Safely interacting with stdin and stdout
1353 int nbdkit_stdio_safe (void);
1354 The "nbdkit_stdio_safe" utility function returns 1 if it is safe to
1356 interact with stdin and stdout during the configuration phase, and 0
1357 otherwise. This is because when the nbdkit -s option is used the
1358 plugin must not directly interact with stdin, because that would
1359 interfere with the client.
1360 The result of this function only matters in callbacks up to
1362 ".config_complete". Once nbdkit reaches ".get_ready", the plugin
1363 should assume that nbdkit may have closed the original stdin and stdout
1364 in order to become a daemon.
1365 nbdkit-sh-plugin(3) uses this function to determine whether it is safe
1367 to support "script=-" to read a script from stdin. Also constructs
1368 like "password=-" (see "Reading passwords" above) are disabled when
1369 reading from stdio is not safe.
1370
1372 The server usually (not always) changes directory to "/" before it
1373 starts serving connections. This means that any relative paths passed
1374 during configuration will not work when the server is running (example:
1375 "nbdkit plugin.so disk.img").
1376 To avoid problems, prepend relative paths with the current directory
1378 before storing them in the handle. Or open files and store the file
1379 descriptor.
1380 "nbdkit_absolute_path"
1382 char *nbdkit_absolute_path (const char *filename);
1383 The utility function "nbdkit_absolute_path" converts any path to an
1385 absolute path: if it is relative, then all this function does is
1386 prepend the current working directory to the path, with no extra
1387 checks.
1388 Note that this function works only when used in the ".config",
1390 ".config_complete" and ".get_ready" callbacks.
1391 If conversion was not possible, this calls "nbdkit_error" and returns
1393 "NULL". Note that this function does not check that the file exists.
1394 The returned string must be freed by the caller.
1396 "nbdkit_realpath"
1398 char *nbdkit_realpath (const char *filename);
1399 The utility function "nbdkit_realpath" converts any path to an absolute
1401 path, resolving symlinks. Under the hood it uses the "realpath"
1402 function, and thus it fails if the path does not exist, or it is not
1403 possible to access to any of the components of the path.
1404 Note that this function works only when used in the ".config",
1406 ".config_complete" and ".get_ready" callbacks.
1407 If the path resolution was not possible, this calls "nbdkit_error" and
1409 returns "NULL".
1410 The returned string must be freed by the caller.
1412 umask
1414 All plugins will see a umask(2) of 0022.
1415
1417 A plugin that needs to sleep may call sleep(2), nanosleep(2) and
1418 similar. However that can cause nbdkit to delay excessively when
1419 shutting down (since it must wait for any plugin or filter which is
1420 sleeping). To avoid this there is a special wrapper around nanosleep
1421 which plugins and filters should use instead.
1422 "nbdkit_nanosleep"
1424 int nbdkit_nanosleep (unsigned sec, unsigned nsec);
1425 The utility function "nbdkit_nanosleep" suspends the current thread,
1427 and returns 0 if it slept at least as many seconds and nanoseconds as
1428 requested, or -1 after calling "nbdkit_error" if there is no point in
1429 continuing the current command. Attempts to sleep more than "INT_MAX"
1430 seconds are treated as an error.
1431
1433 If the client negotiated an NBD export name with nbdkit then plugins
1434 may read this from any connected callbacks. Nbdkit's normal behaviour
1435 is to accept any export name passed by the client, log it in debug
1436 output, but otherwise ignore it. By using "nbdkit_export_name" plugins
1437 may choose to filter by export name or serve different content.
1438 "nbdkit_export_name"
1440 const char *nbdkit_export_name (void);
1441 Return the optional NBD export name if one was negotiated with the
1443 current client (this uses thread-local magic so no parameter is
1444 required). The returned string is valid at least through the ".close"
1445 of the current connection, but if you need to store it in the plugin
1446 for use by more than one client you must copy it.
1447 The export name is a free-form text string, it is not necessarily a
1449 path or filename and it does not need to begin with a '/' character.
1450 The NBD protocol describes the empty string ("") as a representing a
1451 "default export" or to be used in cases where the export name does not
1452 make sense. The export name is untrusted client data, be cautious when
1453 parsing it.
1454 On error, "nbdkit_error" is called and the call returns "NULL".
1456
1458 Some callbacks are specified to return "const char *", even when a
1459 plugin may not have a suitable compile-time constant to return.
1460 Returning dynamically-allocated memory for such a callback would induce
1461 a memory leak or otherwise complicate the plugin to perform additional
1462 bookkeeping. For these cases, nbdkit provides several convenience
1463 functions for creating a copy of a string for better lifetime
1464 management.
1465 "nbdkit_strdup_intern"
1467 "nbdkit_strndup_intern"
1468 const char *nbdkit_strdup_intern (const char *str);
1469 const char *nbdkit_strndup_intern (const char *str, size_t n);
1470 Returns a copy of "str", possibly limited to a maximum of "n" bytes, so
1472 that the caller may reclaim str and use the copy in its place. If the
1473 copy is created outside the scope of a connection (such as during
1474 ".load" or ".config"), the lifetime of the copy will last at least
1475 through ".unload". If the copy is created after a client has triggered
1476 a connection (such as during ".preconnect" or ".open"), the lifetime
1477 will last at least through ".close", but the copy is not safe to share
1478 with other connections.
1479 On error, "nbdkit_error" is called and the call returns "NULL".
1481 "nbdkit_printf_intern"
1483 "nbdkit_vprintf_intern"
1484 const char *nbdkit_printf_intern (const char *fmt, ...);
1485 const char *nbdkit_vprintf_intern (const char *fmt, va_list ap);
1486 Return a string created from a format template, with a lifetime longer
1488 than the current connection. Shorthand for passing "fmt" to
1489 asprintf(3) on a temporary string, then passing that result to
1490 "nbdkit_strdup_intern".
1491 On error, "nbdkit_error" is called and the call returns "NULL".
1493
1495 A server may use "nbdkit_is_tls" to limit which export names work until
1496 after a client has completed TLS authentication. See nbdkit-tls(1).
1497 It is also possible to use nbdkit-tls-fallback-filter(1) to
1498 automatically ensure that the plugin is only used with authentication.
1499 "nbdkit_is_tls"
1501 int nbdkit_is_tls (void);
1502 Return true if the client has completed TLS authentication, or false if
1504 the connection is still plaintext.
1505 On error (such as calling this function outside of the context of
1507 ".open"), "nbdkit_error" is called and the call returns "-1".
1508
1510 It is possible to get the source address of the client when you are
1511 running in any connected callback.
1512 "nbdkit_peer_name"
1514 int nbdkit_peer_name (struct sockaddr *addr, socklen_t *addrlen);
1515 Return the peer (client) address, if available. The "addr" and
1517 "addrlen" parameters behave like getpeername(2). In particular you
1518 must initialize "addrlen" with the size of the buffer pointed to by
1519 "addr", and if "addr" is not large enough then the address will be
1520 truncated.
1521 In some cases this is not available or the address returned will be
1523 meaningless (eg. if there is a proxy between the client and nbdkit).
1524 This call uses thread-local magic so no parameter is required to
1525 specify the current connection.
1526 On success this returns 0. On error, "nbdkit_error" is called and this
1528 call returns "-1".
1529 "nbdkit_peer_pid"
1531 (nbdkit ≥ 1.24, Linux only)
1532 int64_t nbdkit_peer_pid (void);
1534 Return the peer process ID. This is only available when the client
1536 connected over a Unix domain socket.
1537 On success this returns the peer process ID. On error, "nbdkit_error"
1539 is called and this call returns "-1".
1540 "nbdkit_peer_uid"
1542 (nbdkit ≥ 1.24)
1543 int64_t nbdkit_peer_uid (void);
1545 Return the peer user ID. This is only available when the client
1547 connected over a Unix domain socket.
1548 On success this returns the user ID. On error, "nbdkit_error" is
1550 called and this call returns "-1".
1551 "nbdkit_peer_gid"
1553 (nbdkit ≥ 1.24)
1554 int64_t nbdkit_peer_gid (void);
1556 Return the peer group ID. This is only available when the client
1558 connected over a Unix domain socket.
1559 On success this returns the user ID. On error, "nbdkit_error" is
1561 called and this call returns "-1".
1562
1564 Compile-time version of nbdkit
1565 The macros "NBDKIT_VERSION_MAJOR", "NBDKIT_VERSION_MINOR" and
1566 "NBDKIT_VERSION_MICRO" expand to integers containing the version of the
1567 nbdkit headers that you are compiling against.
1568 "NBDKIT_VERSION_MAJOR" is always 1. "NBDKIT_VERSION_MINOR" is even for
1570 stable releases of nbdkit and odd for development releases.
1571 The macro "NBDKIT_VERSION_STRING" expands to the same version as a
1573 string.
1574 Run-time version of nbdkit
1576 When the plugin is loaded into nbdkit, it may not be the same version
1577 that it was compiled against. nbdkit guarantees backwards
1578 compatibility of the API and ABI, so provided that nbdkit is the same
1579 version or newer, the plugin will still work. There is no way to get
1580 the version of nbdkit from the plugin.
1581 Version of the plugin
1583 The plugin itself can use any versioning scheme you want, and put any
1584 string into the ".version" field of the plugin struct (or leave the
1585 field NULL).
1586 API version
1588 See "WRITING AN NBDKIT PLUGIN" above.
1589
1591 Run the server with -f and -v options so it doesn't fork and you can
1592 see debugging information:
1593 nbdkit -fv ./myplugin.so [key=value [key=value [...]]]
1595 To print debugging information from within the plugin, call
1597 "nbdkit_debug", which has the following prototype and works like
1598 printf(3):
1599 void nbdkit_debug (const char *fs, ...);
1601 void nbdkit_vdebug (const char *fs, va_list args);
1602 For convenience, "nbdkit_debug" preserves the value of "errno", and
1604 also supports the glibc extension of a single %m in a format string
1605 expanding to "strerror(errno)", even on platforms that don't support
1606 that natively. Note that "nbdkit_debug" only prints things when the
1607 server is in verbose mode (-v option).
1608 Debug Flags
1610 The -v option switches general debugging on or off, and this debugging
1611 should be used for messages which are useful for all users of your
1612 plugin.
1613 In cases where you want to enable specific extra debugging to track
1615 down bugs in plugins or filters — mainly for use by the plugin/filter
1616 developers themselves — you can define Debug Flags. These are global
1617 ints called "myplugin_debug_*":
1618 int myplugin_debug_foo;
1620 int myplugin_debug_bar;
1621 ...
1622 if (myplugin_debug_foo) {
1623 nbdkit_debug ("lots of extra debugging about foo: ...");
1624 }
1625 Debug Flags can be controlled on the command line using the -D (or
1627 --debug) option:
1628 nbdkit -f -v -D myplugin.foo=1 -D myplugin.bar=2 myplugin [...]
1630 Note "myplugin" is the name passed to ".name" in the "struct
1632 nbdkit_plugin".
1633 You should only use this feature for debug settings. For general
1635 settings use ordinary plugin parameters. Debug Flags can only be C
1636 ints. They are not supported by non-C language plugins.
1637 For convenience '.' characters are replaced with '_' characters in the
1639 variable name, so both of these parameters:
1640 -D myplugin.foo_bar=1
1642 -D myplugin.foo.bar=1
1643 correspond to the plugin variable "myplugin_debug_foo_bar".
1645
1647 Plugins should be compiled as shared libraries. There are various ways
1648 to achieve this, but most Linux compilers support a -shared option to
1649 create the shared library directly, for example:
1650 gcc -fPIC -shared myplugin.c -o myplugin.so
1652 Note that the shared library will have undefined symbols for functions
1654 that you call like "nbdkit_parse_int" or "nbdkit_error". These will be
1655 resolved by the server binary when nbdkit dlopens the plugin.
1656 PKG-CONFIG/PKGCONF
1658 nbdkit provides a pkg-config/pkgconf file called "nbdkit.pc" which
1659 should be installed on the correct path when the nbdkit plugin
1660 development environment is installed. You can use this in autoconf
1661 configure.ac scripts to test for the development environment:
1662 PKG_CHECK_MODULES([NBDKIT], [nbdkit >= 1.2.3])
1664 The above will fail unless nbdkit ≥ 1.2.3 and the header file is
1666 installed, and will set "NBDKIT_CFLAGS" and "NBDKIT_LIBS" appropriately
1667 for compiling plugins.
1668 You can also run pkg-config/pkgconf directly, for example:
1670 if ! pkg-config nbdkit --exists; then
1672 echo "you must install the nbdkit plugin development environment"
1673 exit 1
1674 fi
1675 You can also substitute the plugindir variable by doing:
1677 PKG_CHECK_VAR([NBDKIT_PLUGINDIR], [nbdkit], [plugindir])
1679 which defines "$(NBDKIT_PLUGINDIR)" in automake-generated Makefiles.
1681 If nbdkit development headers are installed in a non-standard location
1683 then you may need to compile plugins using:
1684 gcc -fPIC -shared myplugin.c -o myplugin.so \
1686 `pkg-config nbdkit --cflags --libs`
1687
1689 The plugin is a "*.so" file and possibly a manual page. You can of
1690 course install the plugin "*.so" file wherever you want, and users will
1691 be able to use it by running:
1692 nbdkit /path/to/plugin.so [args]
1694 However if the shared library has a name of the form
1696 "nbdkit-name-plugin.so" and if the library is installed in the
1697 $plugindir directory, then users can be run it by only typing:
1698 nbdkit name [args]
1700 The location of the $plugindir directory is set when nbdkit is compiled
1702 and can be found by doing:
1703 nbdkit --dump-config
1705 If using the pkg-config/pkgconf system then you can also find the
1707 plugin directory at compile time by doing:
1708 pkg-config nbdkit --variable=plugindir
1710
1712 You can also write nbdkit plugins in Go, Lua, OCaml, Perl, Python,
1713 Ruby, Rust, shell script or Tcl. Other programming languages may be
1714 offered in future.
1715 For more information see: nbdkit-cc-plugin(3), nbdkit-golang-plugin(3),
1717 nbdkit-lua-plugin(3), nbdkit-ocaml-plugin(3), nbdkit-perl-plugin(3),
1718 nbdkit-python-plugin(3), nbdkit-ruby-plugin(3), nbdkit-rust-plugin(3),
1719 nbdkit-sh-plugin(3), nbdkit-tcl-plugin(3) .
1720 Plugins written in scripting languages may also be installed in
1722 $plugindir. These must be called "nbdkit-name-plugin" without any
1723 extension. They must be executable, and they must use the shebang
1724 header (see "Shebang scripts" in nbdkit(1)). For example a plugin
1725 written in Perl called "foo.pl" might be installed like this:
1726 $ head -1 foo.pl
1728 #!/usr/sbin/nbdkit perl
1729 $ sudo install -m 0755 foo.pl $plugindir/nbdkit-foo-plugin
1731 and then users will be able to run it like this:
1733 $ nbdkit foo [args ...]
1735
1737 nbdkit(1), nbdkit-nozero-filter(1), nbdkit-tls-fallback-filter(1),
1738 nbdkit-filter(3).
1739 Standard plugins provided by nbdkit:
1741 nbdkit-cdi-plugin(1), nbdkit-curl-plugin(1), nbdkit-data-plugin(1),
1743 nbdkit-eval-plugin(1), nbdkit-example1-plugin(1),
1744 nbdkit-example2-plugin(1), nbdkit-example3-plugin(1),
1745 nbdkit-example4-plugin(1), nbdkit-file-plugin(1),
1746 nbdkit-floppy-plugin(1), nbdkit-full-plugin(1),
1747 nbdkit-guestfs-plugin(1), nbdkit-info-plugin(1), nbdkit-iso-plugin(1),
1748 nbdkit-libvirt-plugin(1), nbdkit-linuxdisk-plugin(1),
1749 nbdkit-memory-plugin(1), nbdkit-nbd-plugin(1), nbdkit-null-plugin(1),
1750 nbdkit-ondemand-plugin(1), nbdkit-partitioning-plugin(1),
1751 nbdkit-pattern-plugin(1), nbdkit-random-plugin(1), nbdkit-S3-plugin(1),
1752 nbdkit-sparse-random-plugin(1), nbdkit-split-plugin(1),
1753 nbdkit-ssh-plugin(1), nbdkit-streaming-plugin(1),
1754 nbdkit-tmpdisk-plugin(1), nbdkit-torrent-plugin(1),
1755 nbdkit-vddk-plugin(1), nbdkit-zero-plugin(1) ; nbdkit-cc-plugin(3),
1756 nbdkit-golang-plugin(3), nbdkit-lua-plugin(3), nbdkit-ocaml-plugin(3),
1757 nbdkit-perl-plugin(3), nbdkit-python-plugin(3), nbdkit-ruby-plugin(3),
1758 nbdkit-rust-plugin(3), nbdkit-sh-plugin(3), nbdkit-tcl-plugin(3) .
1759
1761 Eric Blake
1762 Richard W.M. Jones
1764 Pino Toscano
1766
1768 Copyright (C) 2013-2020 Red Hat Inc.
1769
1771 Redistribution and use in source and binary forms, with or without
1772 modification, are permitted provided that the following conditions are
1773 met:
1774 • Redistributions of source code must retain the above copyright
1776 notice, this list of conditions and the following disclaimer.
1777 • Redistributions in binary form must reproduce the above copyright
1779 notice, this list of conditions and the following disclaimer in the
1780 documentation and/or other materials provided with the
1781 distribution.
1782 • Neither the name of Red Hat nor the names of its contributors may
1784 be used to endorse or promote products derived from this software
1785 without specific prior written permission.
1786 THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
1788 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
1789 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
1790 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
1791 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
1792 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
1793 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
1794 BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
1795 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
1796 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
1797 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1798nbdkit-1.28.2 2021-11-09 nbdkit-plugin(3)