Provided by: nbdkit-plugin-dev_1.16.2-1ubuntu3_amd64 bug

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-rust-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 (but not during "nbdkit --dump-plugin").

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

       ".thread_model"
           In normal operation, ".thread_model" is called once after ".config_complete" has
           validated all configuration information, and before any connections are accepted.
           However, during "nbdkit --dump-plugin", it is called after any ".config" calls but
           without ".config_complete" (so a plugin which determines the results from a script
           must be prepared for a missing script).

       ".open"
           A new client has connected and finished the NBD handshake.  TLS negotiation (if
           required) has been completed successfully.

       ".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, they are called at most once and cached by nbdkit for
           that connection.

       ".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.

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

       "NBDKIT_CACHE_NONE"
           The server does not advertise caching support, and rejects any client-requested
           caching. Any ".cache" callback is ignored.

       "NBDKIT_CACHE_EMULATE"
           The nbdkit server advertises cache support to the client, where the client may request
           that the server cache a region of the export to potentially speed up future read
           and/or write operations on that region. The nbdkit server implements the caching by
           calling ".pread" and ignoring the results. This option exists to ease the
           implementation of a common form of caching; any ".cache" callback is ignored.

       "NBDKIT_CACHE_NATIVE"
           The nbdkit server advertises cache support to the client, where the client may request
           that the server cache a region of the export to potentially speed up future read
           and/or write operations on that region. The nbdkit server calls the ".cache" callback
           to perform the caching; if that callback is missing, the client's cache request
           succeeds without doing anything.

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 != 0" 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);

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.

   ".thread_model"
        int thread_model (void)

       This optional callback is called after all the configuration has been passed to the
       plugin.  It can be used to force a stricter thread model based on configuration, compared
       to "THREAD_MODEL".  See "THREADS" below for details.  Attempts to request a looser (more
       parallel) model are silently ignored.

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

   ".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 server was started with the -r flag on the
       command line which forces connections to be read-only.  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.

       This callback is called after the NBD handshake has completed, which includes TLS
       authentication (if required).

       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 zeroes is still advertised to the
       client (unless the nbdkit-nozero-filter(1) is also used), so returning false merely serves
       as a way to avoid complicating the ".zero" callback to have to fail with "ENOTSUP" or
       "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 for a normal zero request, nbdkit always
       tries ".zero" first if it is present, and gracefully falls back to ".pwrite" if ".zero"
       was absent or failed with "ENOTSUP" or "EOPNOTSUPP".

   ".can_fast_zero"
        int can_fast_zero (void *handle);

       This is called during the option negotiation phase to find out if the plugin wants to
       advertise support for fast zero requests.  If this support is not advertised, a client
       cannot attempt fast zero requests, and has no way to tell if writing zeroes offers any
       speedups compared to using ".pwrite" (other than compressed network traffic).  If support
       is advertised, then ".zero" will have "NBDKIT_FLAG_FAST_ZERO" set when the client has
       requested a fast zero, in which case the plugin must fail with "ENOTSUP" or "EOPNOTSUPP"
       up front if the request would not offer any benefits over ".pwrite".  Advertising support
       for fast zero requests does not require that writing zeroes be fast, only that the result
       (whether success or failure) is fast, so this should be advertised when feasible.

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

       This callback is not required.  If omitted, then nbdkit returns true if ".zero" is absent
       or ".can_zero" returns false (in those cases, nbdkit fails all fast zero requests, as its
       fallback to ".pwrite" is not inherently faster), otherwise false (since it cannot be
       determined in advance if the plugin's ".zero" will properly honor the semantics of
       "NBDKIT_FLAG_FAST_ZERO").

   ".can_extents"
        int can_extents (void *handle);

       This is called during the option negotiation phase to find out if the plugin supports
       detecting allocated (non-sparse) regions of the disk with the ".extents" callback.

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

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

   ".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 client; 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.

       Properly working clients should send the same export name for each of these connections.

       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.

   ".can_cache"
        int can_cache (void *handle);

       This is called during the option negotiation phase to find out if the plugin supports a
       cache operation. The nature of the caching is unspecified (including whether there are
       limits on how much can be cached at once, and whether writes to a cached region have
       write-through or write-back semantics), but the command exists to let clients issue a hint
       to the server that they will be accessing that region of the export.

       If this returns "NBDKIT_CACHE_NONE", cache support is not advertised to the client; if
       this returns "NBDKIT_CACHE_EMULATE", caching is emulated by the server calling ".pread"
       and ignoring the results; if this returns "NBDKIT_CACHE_NATIVE", then the ".cache"
       callback will be used.  If there is an error, ".can_cache" should call "nbdkit_error" with
       an error message and return "-1".

       This callback is not required.  If omitted, then we return "NBDKIT_CACHE_NONE" if the
       ".cache" callback is missing, or "NBDKIT_CACHE_NATIVE" if it is defined.

   ".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, "NBDKIT_FLAG_FUA" based on the
       result of ".can_fua", and "NBDKIT_FLAG_FAST_ZERO" based on the result of ".can_fast_zero".

       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 "NBDKIT_FLAG_FAST_ZERO" is requested, the plugin must decide up front if the
       implementation is likely to be faster than a corresponding ".pwrite"; if not, then it must
       immediately fail with "ENOTSUP" or "EOPNOTSUPP" (whether by "nbdkit_set_error" or "errno")
       and preferably without modifying the exported image.  It is acceptable to always fail a
       fast zero request (as a fast failure is better than attempting the write only to find out
       after the fact that it was not fast after all).  Note that on Linux, support for
       "ioctl(BLKZEROOUT)" is insufficient for determining whether a zero request to a block
       device will be fast (because the kernel will perform a slow fallback when needed).

       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".

       If this callback is omitted, or if it fails with "ENOTSUP" or "EOPNOTSUPP" (whether by
       "nbdkit_set_error" or "errno"), then ".pwrite" will be used as an automatic fallback
       except when the client requested a fast zero.

   ".extents"
        int extents (void *handle, uint32_t count, uint64_t offset,
                     uint32_t flags, struct nbdkit_extents *extents);

       During the data serving phase, this callback is used to detect allocated, sparse and
       zeroed regions of the disk.

       This function will not be called if ".can_extents" returned false.  nbdkit's default
       behaviour in this case is to treat the whole virtual disk as if it was allocated.  Also,
       this function will not be called by a client that does not request structured replies (the
       --no-sr option of nbdkit can be used to test behavior when ".extents" is unavailable to
       the client).

       The callback should detect and return the list of extents overlapping the range
       "[offset...offset+count-1]".  The "extents" parameter points to an opaque object which the
       callback should fill in by calling "nbdkit_add_extent".  See "Extents list" below.

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

       Extents list

       The plugin "extents" callback is passed an opaque pointer "struct nbdkit_extents
       *extents".  This structure represents a list of filesystem extents describing which areas
       of the disk are allocated, which are sparse (“holes”), and, if supported, which are
       zeroes.

       The "extents" callback should scan the disk starting at "offset" and call
       "nbdkit_add_extent" for each extent found.

       Extents overlapping the range "[offset...offset+count-1]" should be returned if possible.
       However nbdkit ignores extents < offset so the plugin may, if it is easier to implement,
       return all extent information for the whole disk.  The plugin may return extents beyond
       the end of the range.  It may also return extent information for less than the whole
       range, but it must return at least one extent overlapping "offset".

       The extents must be added in ascending order, and must be contiguous.

       The "flags" parameter of the ".extents" callback may contain the flag
       "NBDKIT_FLAG_REQ_ONE".  This means that the client is only requesting information about
       the extent overlapping "offset".  The plugin may ignore this flag, or as an optimization
       it may return just a single extent for "offset".

        int nbdkit_add_extent (struct nbdkit_extents *extents,
                               uint64_t offset, uint64_t length, uint32_t type);

       Add an extent covering "[offset...offset+length-1]" of one of the following four types:

       "type = 0"
           A normal, allocated data extent.

       "type = NBDKIT_EXTENT_HOLE|NBDKIT_EXTENT_ZERO"
           An unallocated extent, a.k.a. a “hole”, which reads back as zeroes.  This is the
           normal type of hole applicable to most disks.

       "type = NBDKIT_EXTENT_ZERO"
           An allocated extent which is known to contain only zeroes.

       "type = NBDKIT_EXTENT_HOLE"
           An unallocated extent (hole) which does not read back as zeroes.  Note this should
           only be used in specialized circumstances such as when writing a plugin for (or to
           emulate) certain SCSI drives which do not guarantee that trimmed blocks read back as
           zeroes.

       "nbdkit_add_extent" returns 0 on success or "-1" on failure.  On failure "nbdkit_error"
       and/or "nbdkit_set_error" has already been called.  "errno" will be set to a suitable
       value.

   ".cache"
        int cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags);

       During the data serving phase, this callback is used to give the plugin a hint that the
       client intends to make further accesses to the given region of the export.  The nature of
       caching is not specified further by the NBD specification (for example, a server may place
       limits on how much may be cached at once, and there is no way to control if writes to a
       cached area have write-through or write-back semantics).  In fact, the cache command can
       always fail and still be compliant, and success might not guarantee a performance gain.
       If this callback is omitted, then the results of ".can_cache" determine whether nbdkit
       will reject cache requests, treat them as instant success, or emulate caching by calling
       ".pread" over the same region and ignoring the results.

       This function will not be called if ".can_cache" did not return "NBDKIT_CACHE_NATIVE".
       The parameter "flags" exists in case of future NBD protocol extensions; at this time, it
       will be 0 on input. A plugin must fail this function if "flags" includes an unrecognized
       flag, as that may indicate a requirement that the plugin comply must with a specific
       caching semantic.

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

OTHER FIELDS

       The plugin struct also contains an integer field used as a boolean in C code, but unlikely
       to be exposed in other language bindings:

       ".errno_is_preserved"
           This defaults to 0; if non-zero, nbdkit can reliably use the value of "errno" when a
           callback reports failure, rather than the plugin having to call "nbdkit_set_error".

THREADS

       Each nbdkit plugin must declare its maximum thread safety model by defining the
       "THREAD_MODEL" macro.  (This macro is used by "NBDKIT_REGISTER_PLUGIN").  Additionally, a
       plugin may implement the ".thread_model" callback, called right after ".config_complete"
       to make a runtime decision on which thread model to use.  The nbdkit server chooses the
       most restrictive model between the plugin's "THREAD_MODEL", the ".thread_model" if
       present, any restrictions requested by filters, and any limitations imposed by the system
       (for example, a system without atomic "FD_CLOEXEC" will serialize all requests, so as to
       avoid nbdkit leaking a new file descriptor from one thread into a child process created by
       another thread).

       In "nbdkit --dump-plugin PLUGIN" output, the "max_thread_model" line matches the
       "THREAD_MODEL" macro, and the "thread_model" line matches what the system finally settled
       on after applying all restrictions.

       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).  The server may reorder replies, answering a later request before
           an earlier one.

           All the libraries you use must be thread-safe and reentrant, and any code that creates
           a file descriptor should atomically set "FD_CLOEXEC" if you do not want it
           accidentally leaked to another thread's child process.  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 COMMAND LINE PARAMETERS

   Parsing numbers
       There are several functions for parsing numbers.  These all deal correctly with overflow,
       out of range and parse errors, and you should use them instead of unsafe functions like
       sscanf(3), atoi(3) and similar.

        int nbdkit_parse_int (const char *what, const char *str, int *r);
        int nbdkit_parse_unsigned (const char *what,
                                   const char *str, unsigned *r);
        int nbdkit_parse_int8_t (const char *what,
                                 const char *str, int8_t *r);
        int nbdkit_parse_uint8_t (const char *what,
                                  const char *str, uint8_t *r);
        int nbdkit_parse_int16_t (const char *what,
                                  const char *str, int16_t *r);
        int nbdkit_parse_uint16_t (const char *what,
                                   const char *str, uint16_t *r);
        int nbdkit_parse_int32_t (const char *what,
                                  const char *str, int32_t *r);
        int nbdkit_parse_uint32_t (const char *what,
                                   const char *str, uint32_t *r);
        int nbdkit_parse_int64_t (const char *what,
                                  const char *str, int64_t *r);
        int nbdkit_parse_uint64_t (const char *what,
                                   const char *str, uint64_t *r);

       Parse string "str" into an integer of various types.  These functions parse a decimal,
       hexadecimal ("0x...") or octal ("0...") number.

       On success the functions return 0 and set *r to the parsed value (unless "*r == NULL" in
       which case the result is discarded).  On error, "nbdkit_error" is called and the functions
       return "-1".  On error *r is always unchanged.

       The "what" parameter is printed in error messages to provide context.  It should usually
       be a short descriptive string of what you are trying to parse, eg:

        if (nbdkit_parse_int ("random seed", argv[1], &seed) == -1)
          return -1;

       might print an error:

        random seed: could not parse number: "lalala"

   Parsing sizes
       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 booleans
       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

       or from a file descriptor inherited by nbdkit:

        nbdkit myplugin password=-99

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

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.

SLEEPING

       A plugin that needs to sleep may call sleep(2), nanosleep(2) and similar.  However that
       can cause nbdkit to delay excessively when shutting down (since it must wait for any
       plugin or filter which is sleeping).  To avoid this there is a special wrapper around
       nanosleep which plugins and filters should use instead.

   "nbdkit_nanosleep"
        int nbdkit_nanosleep (unsigned sec, unsigned nsec);

       The utility function "nbdkit_nanosleep" suspends the current thread, and returns 0 if it
       slept at least as many seconds and nanoseconds as requested, or -1 after calling
       "nbdkit_error" if there is no point in continuing the current command.  Attempts to sleep
       more than "INT_MAX" seconds are treated as an error.

EXPORT NAME

       If the client negotiated an NBD export name with nbdkit then plugins may read this from
       any connected callbacks.  Nbdkit's normal behaviour is to accept any export name passed by
       the client, log it in debug output, but otherwise ignore it.  By using
       "nbdkit_export_name" plugins may choose to filter by export name or serve different
       content.

   "nbdkit_export_name"
        const char *nbdkit_export_name (void);

       Return the optional NBD export name if one was negotiated with the current client (this
       uses thread-local magic so no parameter is required).  The returned string is only valid
       while the client is connected, so if you need to store it in the plugin you must copy it.

       The export name is a free-form text string, it is not necessarily a path or filename and
       it does not need to begin with a '/' character.  The NBD protocol describes the empty
       string ("") as a representing a "default export" or to be used in cases where the export
       name does not make sense.  The export name is untrusted client data, be cautious when
       parsing it.

       On error, "nbdkit_error" is called and the call returns "NULL".

PEER NAME

       It is possible to get the address of the client when you are running in any connected
       callback.

   "nbdkit_peer_name"
        int nbdkit_peer_name (struct sockaddr *addr, socklen_t *addrlen);

       Return the peer (client) address, if available.  The "addr" and "addrlen" parameters
       behave like getpeername(2).  In particular you must initialize "addrlen" with the size of
       the buffer pointed to by "addr", and if "addr" is not large enough then the address will
       be truncated.

       In some cases this is not available or the address returned will be meaningless (eg. if
       there is a proxy between the client and nbdkit).  This call uses thread-local magic so no
       parameter is required to specify the current connection.

       On success this returns 0.  On error, "nbdkit_error" is called and this call returns "-1".

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, Rust, 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-rust-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-nozero-filter(3), 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-info-plugin(1), nbdkit-iso-plugin(1), nbdkit-libvirt-plugin(1),
       nbdkit-linuxdisk-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-ssh-plugin(1),
       nbdkit-streaming-plugin(1), nbdkit-tar-plugin(1), nbdkit-vddk-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-rust-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.