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nbdkit-plugin(3) NBDKIT nbdkit-plugin(3)

NAME

nbdkit-plugin - how to write nbdkit plugins

SYNOPSIS

 #define NBDKIT_API_VERSION 2
 
 #include <nbdkit-plugin.h>
 
 #define THREAD_MODEL NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS
 
 static void *
 myplugin_open (void)
 {
   /* create a handle ... */
   return handle;
 }
 
 static struct nbdkit_plugin plugin = {
   .name              = "myplugin",
   .open              = myplugin_open,
   .get_size          = myplugin_get_size,
   .pread             = myplugin_pread,
   .pwrite            = myplugin_pwrite,
   /* etc */
 };
 
 NBDKIT_REGISTER_PLUGIN(plugin)

When this has been compiled to a shared library, do:

 nbdkit [--args ...] ./myplugin.so [key=value ...]

When debugging, use the -fv options:

 nbdkit -fv ./myplugin.so [key=value ...]

DESCRIPTION

An nbdkit plugin is a new source device which can be served using the Network Block Device (NBD) protocol. This manual page describes how to create an nbdkit plugin in C.

To see example plugins: https://github.com/libguestfs/nbdkit/tree/master/plugins

Plugins written in C have an ABI guarantee: a plugin compiled against an older version of nbdkit will still work correctly when loaded with a newer nbdkit. We also try (but cannot guarantee) to support plugins compiled against a newer version of nbdkit when loaded with an older nbdkit, although the plugin may have reduced functionality if it depends on features only provided by newer nbdkit.

For plugins written in C, we also provide an API guarantee: a plugin written against an older header will still compile unmodified with a newer nbdkit.

The API guarantee does not always apply to plugins written in other (non-C) languages which may have to adapt to changes when recompiled against a newer nbdkit.

To write plugins in other languages, see: nbdkit-lua-plugin(3), nbdkit-ocaml-plugin(3), nbdkit-perl-plugin(3), nbdkit-python-plugin(3), nbdkit-ruby-plugin(3), nbdkit-sh-plugin(3), nbdkit-tcl-plugin(3).

"#define NBDKIT_API_VERSION 2"

Plugins must choose which API version they want to use, by defining NBDKIT_API_VERSION to a positive integer prior to including "nbdkit-plugin.h" (or any other nbdkit header). The default version is 1 for backwards-compatibility with nbdkit v1.1.26 and earlier; however, it is recommended that new plugins be written to the maximum version (currently 2) as it enables more features and better interaction with nbdkit filters. Therefore, the rest of this document only covers the version 2 interface. A newer nbdkit will always support plugins written in C which were compiled against any prior API version.

"#include <nbdkit-plugin.h>"

All plugins should start by including this header file, after optionally choosing an API version.

"#define THREAD_MODEL"

All plugins must define a thread model. See "THREADS" below for details. It is generally safe to use: 

 #define THREAD_MODEL NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS

"struct nbdkit_plugin"

All plugins must define and register one "struct nbdkit_plugin", which contains the name of the plugin and pointers to callback functions. 

 static struct nbdkit_plugin plugin = {
   .name              = "myplugin",
   .longname          = "My Plugin",
   .description       = "This is my great plugin for nbdkit",
   .open              = myplugin_open,
   .get_size          = myplugin_get_size,
   .pread             = myplugin_pread,
   .pwrite            = myplugin_pwrite,
   /* etc */
 };
 
 NBDKIT_REGISTER_PLUGIN(plugin)

The ".name" field is the name of the plugin.

The callbacks are described below (see "CALLBACKS"). Only ".name", ".open", ".get_size" and ".pread" are required. All other callbacks can be omitted. However almost all plugins should have a ".close" callback. Most real-world plugins will also want to declare some of the other callbacks.

The nbdkit server calls the callbacks in the following order over the lifetime of the plugin:

".load"
is called once just after the plugin is loaded into memory.
".config" and ".config_complete"
".config" is called zero or more times during command line parsing. ".config_complete" is called once after all configuration information has been passed to the plugin.

Both are called after loading the plugin but before any connections are accepted.

".open"
A new client has connected.
".can_write", ".get_size" and other option negotiation callbacks
These are called during option negotiation with the client, but before any data is served. These callbacks may return different values across different ".open" calls, but within a single connection, must always return the same value; other code in nbdkit may cache the per-connection value returned rather than using the callback a second time.
".pread", ".pwrite" and other data serving callbacks
After option negotiation has finished, these may be called to serve data. Depending on the thread model chosen, they might be called in parallel from multiple threads. The data serving callbacks include a flags argument; the results of the negotiation callbacks influence whether particular flags will ever be passed to a data callback.
".close"
The client has disconnected.
".open" ... ".close"
The sequence ".open" ... ".close" can be called repeatedly over the lifetime of the plugin, and can be called in parallel (depending on the thread model).
".unload"
is called once just before the plugin is unloaded from memory.

FLAGS

The following flags are defined by nbdkit, and used in various data serving callbacks as follows:
"NBDKIT_FLAG_MAY_TRIM"
This flag is used by the ".zero" callback; there is no way to disable this flag, although a plugin that does not support trims as a way to write zeroes may ignore the flag without violating expected semantics.
"NBDKIT_FLAG_FUA"
This flag represents Forced Unit Access semantics. It is used by the ".pwrite", ".zero", and ".trim" callbacks to indicate that the plugin must not return a result until the action has landed in persistent storage. This flag will not be sent to the plugin unless ".can_fua" is provided and returns "NBDKIT_FUA_NATIVE".

The following defines are valid as successful return values for ".can_fua":

"NBDKIT_FUA_NONE"
Forced Unit Access is not supported; the client must manually request a flush after writes have completed. The "NBDKIT_FLAG_FUA" flag will not be passed to the plugin's write callbacks.
"NBDKIT_FUA_EMULATE"
The client may request Forced Unit Access, but it is implemented by emulation, where nbdkit calls ".flush" after a write operation; this is semantically correct, but may hurt performance as it tends to flush more data than just what the client requested. The "NBDKIT_FLAG_FUA" flag will not be passed to the plugin's write callbacks.
"NBDKIT_FUA_NATIVE"
The client may request Forced Unit Access, which results in the "NBDKIT_FLAG_FUA" flag being passed to the plugin's write callbacks (".pwrite", ".trim", and ".zero"). When the flag is set, these callbacks must not return success until the client's request has landed in persistent storage.

ERROR HANDLING

If there is an error in the plugin, the plugin should call "nbdkit_error" to report an error message; additionally, if the callback is involved in serving data, the plugin should call "nbdkit_set_error" to influence the error code that will be sent to the client. These two functions can be called in either order. Then, the callback should return the appropriate error indication, eg. "NULL" or "-1".

If the call to "nbdkit_set_error" is omitted while serving data, then the global variable "errno" may be used. For plugins which have ".errno_is_preserved == 1" the core code will use "errno". In plugins written in non-C languages, we usually cannot trust that "errno" will not be overwritten when returning from that language to C. In that case, either the plugin must call "nbdkit_set_error" or hard-coded "EIO" is used.

"nbdkit_error" has the following prototype and works like printf(3):

 void nbdkit_error (const char *fs, ...);
 void nbdkit_verror (const char *fs, va_list args);

For convenience, "nbdkit_error" preserves the value of "errno", and also supports the glibc extension of a single %m in a format string expanding to "strerror(errno)", even on platforms that don't support that natively.

"nbdkit_set_error" can be called at any time, but only has an impact during callbacks for serving data, and only when the callback returns an indication of failure. It has the following prototype:

 void nbdkit_set_error (int err);

FILENAMES AND PATHS

The server usually (not always) changes directory to "/" before it starts serving connections. This means that any relative paths passed during configuration will not work when the server is running (example: "nbdkit plugin.so disk.img").

To avoid problems, prepend relative paths with the current directory before storing them in the handle. Or open files and store the file descriptor.

"nbdkit_absolute_path"

 char *nbdkit_absolute_path (const char *filename);

The utility function "nbdkit_absolute_path" converts any path to an absolute path: if it is relative, then all this function does is prepend the current working directory to the path, with no extra checks.

Note that this function works only when used in the ".config", and ".config_complete" callbacks.

If conversion was not possible, this calls "nbdkit_error" and returns "NULL". Note that this function does not check that the file exists.

The returned string must be freed by the caller.

"nbdkit_realpath"

 char *nbdkit_realpath (const char *filename);

The utility function "nbdkit_realpath" converts any path to an absolute path, resolving symlinks. Under the hood it uses the "realpath" function, and thus it fails if the path does not exist, or it is not possible to access to any of the components of the path.

Note that this function works only when used in the ".config", and ".config_complete" callbacks.

If the path resolution was not possible, this calls "nbdkit_error" and returns "NULL".

The returned string must be freed by the caller.

umask

All plugins will see a umask(2) of 0022.

CALLBACKS

".name"

 const char *name;

This field (a string) is required, and must contain only ASCII alphanumeric characters and be unique amongst all plugins.

".version"

 const char *version;

Plugins may optionally set a version string which is displayed in help and debugging output.

".longname"

 const char *longname;

An optional free text name of the plugin. This field is used in error messages.

".description"

 const char *description;

An optional multi-line description of the plugin.

".load"

 void load (void);

This is called once just after the plugin is loaded into memory. You can use this to perform any global initialization needed by the plugin.

".unload"

 void unload (void);

This may be called once just before the plugin is unloaded from memory. Note that it's not guaranteed that ".unload" will always be called (eg. the server might be killed or segfault), so you should try to make the plugin as robust as possible by not requiring cleanup. See also "SHUTDOWN" below.

".dump_plugin"

 void dump_plugin (void);

This optional callback is called when the "nbdkit plugin --dump-plugin" command is used. It should print any additional informative "key=value" fields to stdout as needed. Prefixing the keys with the name of the plugin will avoid conflicts.

".config"

 int config (const char *key, const char *value);

On the nbdkit command line, after the plugin filename, come an optional list of "key=value" arguments. These are passed to the plugin through this callback when the plugin is first loaded and before any connections are accepted.

This callback may be called zero or more times.

Both "key" and "value" parameters will be non-NULL. The strings are owned by nbdkit but will remain valid for the lifetime of the plugin, so the plugin does not need to copy them.

The key will be a non-empty string beginning with an ASCII alphabetic character ("A-Z" "a-z"). The rest of the key must contain only ASCII alphanumeric plus period, underscore or dash characters ("A-Z" "a-z" "0-9" "." "_" "-"). The value may be an arbitrary string, including an empty string.

The names of "key"s accepted by plugins is up to the plugin, but you should probably look at other plugins and follow the same conventions.

If the value is a relative path, then note that the server changes directory when it starts up. See "FILENAMES AND PATHS" above.

If the ".config" callback is not provided by the plugin, and the user tries to specify any "key=value" arguments, then nbdkit will exit with an error.

If there is an error, ".config" should call "nbdkit_error" with an error message and return "-1".

".magic_config_key"

 const char *magic_config_key;

This optional string can be used to set a "magic" key used when parsing plugin parameters. It affects how "bare parameters" (those which do not contain an "=" character) are parsed on the command line.

If "magic_config_key != NULL" then any bare parameters are passed to the ".config" method as: "config (magic_config_key, argv[i]);".

If "magic_config_key" is not set then we behave as in nbdkit < 1.7: If the first parameter on the command line is bare then it is passed to the ".config" method as: "config ("script", value);". Any other bare parameters give errors.

".config_complete"

 int config_complete (void);

This optional callback is called after all the configuration has been passed to the plugin. It is a good place to do checks, for example that the user has passed the required parameters to the plugin.

If there is an error, ".config_complete" should call "nbdkit_error" with an error message and return "-1".

".config_help"

 const char *config_help;

This optional multi-line help message should summarize any "key=value" parameters that it takes. It does not need to repeat what already appears in ".description".

If the plugin doesn't take any config parameters you should probably omit this.

".open"

 void *open (int readonly);

This is called when a new client connects to the nbdkit server. The callback should allocate a handle and return it. This handle is passed back to other callbacks and could be freed in the ".close" callback.

Note that the handle is completely opaque to nbdkit, but it must not be NULL. If you don't need to use a handle, return "NBDKIT_HANDLE_NOT_NEEDED" which is a static non-NULL pointer.

The "readonly" flag informs the plugin that the user requested a read-only connection using the -r flag on the command line. Note that the plugin may additionally force the connection to be readonly (even if this flag is false) by returning false from the ".can_write" callback. So if your plugin can only serve read-only, you can ignore this parameter.

If there is an error, ".open" should call "nbdkit_error" with an error message and return "NULL".

".close"

 void close (void *handle);

This is called when the client closes the connection. It should clean up any per-connection resources.

Note there is no way in the NBD protocol to communicate close errors back to the client, for example if your plugin calls close(2) and you are checking for errors (as you should do). Therefore the best you can do is to log the error on the server. Well-behaved NBD clients should try to flush the connection before it is closed and check for errors, but obviously this is outside the scope of nbdkit.

".get_size"

 int64_t get_size (void *handle);

This is called during the option negotiation phase of the protocol to get the size (in bytes) of the block device being exported.

The returned size must be ≥ 0. If there is an error, ".get_size" should call "nbdkit_error" with an error message and return "-1".

".can_write"

 int can_write (void *handle);

This is called during the option negotiation phase to find out if the handle supports writes.

If there is an error, ".can_write" should call "nbdkit_error" with an error message and return "-1".

This callback is not required. If omitted, then we return true iff a ".pwrite" callback has been defined.

".can_flush"

 int can_flush (void *handle);

This is called during the option negotiation phase to find out if the handle supports the flush-to-disk operation.

If there is an error, ".can_flush" should call "nbdkit_error" with an error message and return "-1".

This callback is not required. If omitted, then we return true iff a ".flush" callback has been defined.

".is_rotational"

 int is_rotational (void *handle);

This is called during the option negotiation phase to find out if the backing disk is a rotational medium (like a traditional hard disk) or not (like an SSD). If true, this may cause the client to reorder requests to make them more efficient for a slow rotating disk.

If there is an error, ".is_rotational" should call "nbdkit_error" with an error message and return "-1".

This callback is not required. If omitted, then we return false.

".can_trim"

 int can_trim (void *handle);

This is called during the option negotiation phase to find out if the plugin supports the trim/discard operation for punching holes in the backing storage.

If there is an error, ".can_trim" should call "nbdkit_error" with an error message and return "-1".

This callback is not required. If omitted, then we return true iff a ".trim" callback has been defined.

".can_zero"

 int can_zero (void *handle);

This is called during the option negotiation phase to find out if the plugin wants the ".zero" callback to be utilized. Support for writing zeros is still advertised to the client (unless the nbdkit filter nozero is also used), so returning false merely serves as a way to avoid complicating the ".zero" callback to have to fail with "EOPNOTSUPP" on the connections where it will never be more efficient than using ".pwrite" up front.

If there is an error, ".can_zero" should call "nbdkit_error" with an error message and return "-1".

This callback is not required. If omitted, then nbdkit always tries ".zero" first if it is present, and gracefully falls back to ".pwrite" if ".zero" was absent or failed with "EOPNOTSUPP".

".can_fua"

 int can_fua (void *handle);

This is called during the option negotiation phase to find out if the plugin supports the Forced Unit Access (FUA) flag on write, zero, and trim requests. If this returns "NBDKIT_FUA_NONE", FUA support is not advertised to the guest; if this returns "NBDKIT_FUA_EMULATE", the ".flush" callback must work (even if ".can_flush" returns false), and FUA support is emulated by calling ".flush" after any write operation; if this returns "NBDKIT_FUA_NATIVE", then the ".pwrite", ".zero", and ".trim" callbacks (if implemented) must handle the flag "NBDKIT_FLAG_FUA", by not returning until that action has landed in persistent storage.

If there is an error, ".can_fua" should call "nbdkit_error" with an error message and return "-1".

This callback is not required unless a plugin wants to specifically handle FUA requests. If omitted, nbdkit checks whether ".flush" exists, and behaves as if this function returns "NBDKIT_FUA_NONE" or "NBDKIT_FUA_EMULATE" as appropriate.

".can_multi_conn"

 int can_multi_conn (void *handle);

This is called during the option negotiation phase to find out if the plugin is prepared to handle multiple connections from a single client. If the plugin sets this to true then a client may try to open multiple connections to the nbdkit server and spread requests across all connections to maximize parallelism. If the plugin sets it to false (which is the default) then well-behaved clients should only open a single connection, although we cannot control what clients do in practice.

Specifically it means that either the plugin does not cache requests at all. Or if it does cache them then the effects of a ".flush" request or setting "NBDKIT_FLAG_FUA" on a request must be visible across all connections to the plugin before the plugin replies to that request.

If you use Linux nbd-client(8) option -C num with num > 1 then Linux checks this flag and will refuse to connect if ".can_multi_conn" is false.

If there is an error, ".can_multi_conn" should call "nbdkit_error" with an error message and return "-1".

This callback is not required. If omitted, then we return false.

".pread"

 int pread (void *handle, void *buf, uint32_t count, uint64_t offset,
            uint32_t flags);

During the data serving phase, nbdkit calls this callback to read data from the backing store. "count" bytes starting at "offset" in the backing store should be read and copied into "buf". nbdkit takes care of all bounds- and sanity-checking, so the plugin does not need to worry about that.

The parameter "flags" exists in case of future NBD protocol extensions; at this time, it will be 0 on input.

The callback must read the whole "count" bytes if it can. The NBD protocol doesn't allow partial reads (instead, these would be errors). If the whole "count" bytes was read, the callback should return 0 to indicate there was no error.

If there is an error (including a short read which couldn't be recovered from), ".pread" should call "nbdkit_error" with an error message, and "nbdkit_set_error" to record an appropriate error (unless "errno" is sufficient), then return "-1".

".pwrite"

 int pwrite (void *handle, const void *buf, uint32_t count, uint64_t offset,
             uint32_t flags);

During the data serving phase, nbdkit calls this callback to write data to the backing store. "count" bytes starting at "offset" in the backing store should be written using the data in "buf". nbdkit takes care of all bounds- and sanity-checking, so the plugin does not need to worry about that.

This function will not be called if ".can_write" returned false. The parameter "flags" may include "NBDKIT_FLAG_FUA" on input based on the result of ".can_fua".

The callback must write the whole "count" bytes if it can. The NBD protocol doesn't allow partial writes (instead, these would be errors). If the whole "count" bytes was written successfully, the callback should return 0 to indicate there was no error.

If there is an error (including a short write which couldn't be recovered from), ".pwrite" should call "nbdkit_error" with an error message, and "nbdkit_set_error" to record an appropriate error (unless "errno" is sufficient), then return "-1".

".flush"

 int flush (void *handle, uint32_t flags);

During the data serving phase, this callback is used to fdatasync(2) the backing store, ie. to ensure it has been completely written to a permanent medium. If that is not possible then you can omit this callback.

This function will not be called directly by the client if ".can_flush" returned false; however, it may still be called by nbdkit if ".can_fua" returned "NBDKIT_FUA_EMULATE". The parameter "flags" exists in case of future NBD protocol extensions; at this time, it will be 0 on input.

If there is an error, ".flush" should call "nbdkit_error" with an error message, and "nbdkit_set_error" to record an appropriate error (unless "errno" is sufficient), then return "-1".

".trim"

 int trim (void *handle, uint32_t count, uint64_t offset, uint32_t flags);

During the data serving phase, this callback is used to "punch holes" in the backing store. If that is not possible then you can omit this callback.

This function will not be called if ".can_trim" returned false. The parameter "flags" may include "NBDKIT_FLAG_FUA" on input based on the result of ".can_fua".

If there is an error, ".trim" should call "nbdkit_error" with an error message, and "nbdkit_set_error" to record an appropriate error (unless "errno" is sufficient), then return "-1".

".zero"

 int zero (void *handle, uint32_t count, uint64_t offset, uint32_t flags);

During the data serving phase, this callback is used to write "count" bytes of zeroes at "offset" in the backing store.

This function will not be called if ".can_zero" returned false. On input, the parameter "flags" may include "NBDKIT_FLAG_MAY_TRIM" unconditionally, and "NBDKIT_FLAG_FUA" based on the result of ".can_fua".

If "NBDKIT_FLAG_MAY_TRIM" is requested, the operation can punch a hole instead of writing actual zero bytes, but only if subsequent reads from the hole read as zeroes. If this callback is omitted, or if it fails with "EOPNOTSUPP" (whether by "nbdkit_set_error" or "errno"), then ".pwrite" will be used instead.

The callback must write the whole "count" bytes if it can. The NBD protocol doesn't allow partial writes (instead, these would be errors). If the whole "count" bytes was written successfully, the callback should return 0 to indicate there was no error.

If there is an error, ".zero" should call "nbdkit_error" with an error message, and "nbdkit_set_error" to record an appropriate error (unless "errno" is sufficient), then return "-1".

THREADS

Each nbdkit plugin must declare its thread safety model by defining the "THREAD_MODEL" macro. (This macro is used by "NBDKIT_REGISTER_PLUGIN").

The possible settings for "THREAD_MODEL" are defined below.

"#define THREAD_MODEL NBDKIT_THREAD_MODEL_SERIALIZE_CONNECTIONS"
Only a single handle can be open at any time, and all requests happen from one thread.

Note this means only one client can connect to the server at any time. If a second client tries to connect it will block waiting for the first client to close the connection.

"#define THREAD_MODEL NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS"
This is a safe default for most plugins.

Multiple handles can be open at the same time, but requests are serialized so that for the plugin as a whole only one open/read/write/close (etc) request will be in progress at any time.

This is a useful setting if the library you are using is not thread-safe. However performance may not be good.

"#define THREAD_MODEL NBDKIT_THREAD_MODEL_SERIALIZE_REQUESTS"
Multiple handles can be open and multiple data requests can happen in parallel. However only one request will happen per handle at a time (but requests on different handles might happen concurrently).
"#define THREAD_MODEL NBDKIT_THREAD_MODEL_PARALLEL"
Multiple handles can be open and multiple data requests can happen in parallel (even on the same handle).

All the libraries you use must be thread-safe and reentrant. You may also need to provide mutexes for fields in your connection handle.

If none of the above thread models are suitable, then use "NBDKIT_THREAD_MODEL_PARALLEL" and implement your own locking using "pthread_mutex_t" etc.

SHUTDOWN

When nbdkit receives certain signals it will shut down (see "SIGNALS" in nbdkit(1)). The server will wait for any currently running plugin callbacks to finish and also call the ".unload" callback before unloading the plugin.

Note that it's not guaranteed this can always happen (eg. the server might be killed by "SIGKILL" or segfault).

PARSING SIZE PARAMETERS

Use the "nbdkit_parse_size" utility function to parse human-readable size strings such as "100M" into the size in bytes. 

 int64_t nbdkit_parse_size (const char *str);

"str" can be a string in a number of common formats. The function returns the size in bytes. If there was an error, it returns "-1".

PARSING BOOLEAN PARAMETERS

Use the "nbdkit_parse_bool" utility function to parse human-readable strings such as "on" into a boolean value. 

 int nbdkit_parse_bool (const char *str);

"str" can be a string containing a case-insensitive form of various common toggle values. The function returns 0 or 1 if the parse was successful. If there was an error, it returns "-1".

READING PASSWORDS

The "nbdkit_read_password" utility function can be used to read passwords from config parameters: 

 int nbdkit_read_password (const char *value, char **password);

For example:

 char *password = NULL;
 
 static int
 myplugin_config (const char *key, const char *value)
 {
   ..
   if (strcmp (key, "password") == 0) {
     free (password);
     if (nbdkit_read_password (value, &password) == -1)
       return -1;
   }
   ..
 }

The "password" result string is allocated by malloc, and so you may need to free it.

This function recognizes several password formats. A password may be used directly on the command line, eg:

 nbdkit myplugin password=mostsecret

But more securely this function can also read a password interactively:

 nbdkit myplugin password=-

or from a file:

 nbdkit myplugin password=+/tmp/secret

(If the password begins with a "-" or "+" character then it must be passed in a file).

DEBUGGING

Run the server with -f and -v options so it doesn't fork and you can see debugging information: 

 nbdkit -fv ./myplugin.so [key=value [key=value [...]]]

To print debugging information from within the plugin, call "nbdkit_debug", which has the following prototype and works like printf(3):

 void nbdkit_debug (const char *fs, ...);
 void nbdkit_vdebug (const char *fs, va_list args);

For convenience, "nbdkit_debug" preserves the value of "errno", and also supports the glibc extension of a single %m in a format string expanding to "strerror(errno)", even on platforms that don't support that natively. Note that "nbdkit_debug" only prints things when the server is in verbose mode (-v option).

Debug Flags

The -v option switches general debugging on or off, and this debugging should be used for messages which are useful for all users of your plugin.

In cases where you want to enable specific extra debugging to track down bugs in plugins or filters — mainly for use by the plugin/filter developers themselves — you can define Debug Flags. These are global ints called "myplugin_debug_*":

 int myplugin_debug_foo;
 int myplugin_debug_bar;
 ...
 if (myplugin_debug_foo) {
   nbdkit_debug ("lots of extra debugging about foo: ...");
 }

Debug Flags can be controlled on the command line using the -D (or --debug) option:

 nbdkit -f -v -D myplugin.foo=1 -D myplugin.bar=2 myplugin [...]

Note "myplugin" is the name passed to ".name" in the "struct nbdkit_plugin".

You should only use this feature for debug settings. For general settings use ordinary plugin parameters. Debug Flags can only be C ints. They are not supported by non-C language plugins.

INSTALLING THE PLUGIN

The plugin is a "*.so" file and possibly a manual page. You can of course install the plugin "*.so" file wherever you want, and users will be able to use it by running: 

 nbdkit /path/to/plugin.so [args]

However if the shared library has a name of the form "nbdkit-name-plugin.so" and if the library is installed in the $plugindir directory, then users can be run it by only typing:

 nbdkit name [args]

The location of the $plugindir directory is set when nbdkit is compiled and can be found by doing:

 nbdkit --dump-config

If using the pkg-config/pkgconf system then you can also find the plugin directory at compile time by doing:

 pkgconf nbdkit --variable=plugindir

PKG-CONFIG/PKGCONF

nbdkit provides a pkg-config/pkgconf file called "nbdkit.pc" which should be installed on the correct path when the nbdkit plugin development environment is installed. You can use this in autoconf configure.ac scripts to test for the development environment: 

 PKG_CHECK_MODULES([NBDKIT], [nbdkit >= 1.2.3])

The above will fail unless nbdkit ≥ 1.2.3 and the header file is installed, and will set "NBDKIT_CFLAGS" and "NBDKIT_LIBS" appropriately for compiling plugins.

You can also run pkg-config/pkgconf directly, for example:

 if ! pkgconf nbdkit --exists; then
   echo "you must install the nbdkit plugin development environment"
   exit 1
 fi

You can also substitute the plugindir variable by doing:

 PKG_CHECK_VAR([NBDKIT_PLUGINDIR], [nbdkit], [plugindir])

which defines "$(NBDKIT_PLUGINDIR)" in automake-generated Makefiles.

WRITING PLUGINS IN OTHER PROGRAMMING LANGUAGES

You can also write nbdkit plugins in Lua, OCaml, Perl, Python, Ruby, shell script or Tcl. Other programming languages may be offered in future.

For more information see: nbdkit-lua-plugin(3), nbdkit-ocaml-plugin(3), nbdkit-perl-plugin(3), nbdkit-python-plugin(3), nbdkit-ruby-plugin(3), nbdkit-sh-plugin(3), nbdkit-tcl-plugin(3).

Plugins written in scripting languages may also be installed in $plugindir. These must be called "nbdkit-name-plugin" without any extension. They must be executable, and they must use the shebang header (see "Shebang scripts" in nbdkit(1)). For example a plugin written in Perl called "foo.pl" might be installed like this:

 $ head -1 foo.pl
 #!/usr/sbin/nbdkit perl

 $ sudo install -m 0755 foo.pl $plugindir/nbdkit-foo-plugin

and then users will be able to run it like this:

 $ nbdkit foo [args ...]

SEE ALSO

nbdkit(1), nbdkit-filter(3).

Standard plugins provided by nbdkit:

nbdkit-curl-plugin(1), nbdkit-data-plugin(1), nbdkit-example1-plugin(1), nbdkit-example2-plugin(1), nbdkit-example3-plugin(1), nbdkit-example4-plugin(1), nbdkit-ext2-plugin(1), nbdkit-file-plugin(1), nbdkit-floppy-plugin(1), nbdkit-full-plugin(1), nbdkit-guestfs-plugin(1), nbdkit-gzip-plugin(1), nbdkit-iso-plugin(1), nbdkit-libvirt-plugin(1), nbdkit-memory-plugin(1), nbdkit-nbd-plugin(1), nbdkit-null-plugin(1), nbdkit-partitioning-plugin(1), nbdkit-pattern-plugin(1), nbdkit-random-plugin(1), nbdkit-split-plugin(1), nbdkit-streaming-plugin(1), nbdkit-tar-plugin(1), nbdkit-vddk-plugin(1), nbdkit-xz-plugin(1), nbdkit-zero-plugin(1) ; nbdkit-lua-plugin(3), nbdkit-ocaml-plugin(3), nbdkit-perl-plugin(3), nbdkit-python-plugin(3), nbdkit-ruby-plugin(3), nbdkit-sh-plugin(3), nbdkit-tcl-plugin(3) .

AUTHORS

Eric Blake

Richard W.M. Jones

Pino Toscano

COPYRIGHT

Copyright (C) 2013-2018 Red Hat Inc.

LICENSE

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
  • Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
  • 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.
  • 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.

THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND 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 (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

2019-01-26 nbdkit-1.10.3