NAME

       nbdkit-tcl-plugin - nbdkit Tcl plugin

SYNOPSIS

        nbdkit tcl /path/to/plugin.tcl [arguments...]

DESCRIPTION

       "nbdkit-tcl-plugin" is an embedded Tcl interpreter for nbdkit(1), allowing you to write
       nbdkit plugins in Tcl.

   If you have been given an nbdkit Tcl plugin
       Assuming you have a Tcl script which is an nbdkit plugin, you run it like this:

        nbdkit tcl /path/to/plugin.tcl

       You may have to add further "key=value" arguments to the command line.  Read the Tcl
       script to see if it requires any.

WRITING A TCL NBDKIT PLUGIN

       For an example plugin written in Tcl, see:
       https://github.com/libguestfs/nbdkit/blob/master/plugins/tcl/example.tcl

       Broadly speaking, Tcl nbdkit plugins work like C ones, so you should read nbdkit-plugin(3)
       first.

       To write a Tcl nbdkit plugin, you create a Tcl file which contains at least the following
       required subroutines:

        proc plugin_open {readonly} {
            # see below
            return $h
        }
        proc get_size {h} {
            # see below
            return $size
        }
        proc pread {h count offset} {
            # see below
            return $buf
        }

       Note that the subroutines must have those literal names (like "plugin_open"), because the
       C part looks up and calls those functions directly.  You may want to include documentation
       and globals (eg. for storing global state).  Also any top-level statements are run when
       nbdkit starts up.

   Executable script
       If you want you can make the script executable and include a "shebang" at the top:

        #!/usr/sbin/nbdkit tcl

       See also "Shebang scripts" in nbdkit(1).

       These scripts can also be installed in the $plugindir.  See "WRITING PLUGINS IN OTHER
       PROGRAMMING LANGUAGES" in nbdkit-plugin(3).

   Exceptions
       Tcl plugin methods can indicate an error by calling "error".

   Binary data
       When writing your Tcl script, be careful to ensure that it is processing binary data (not
       Unicode).  If reading and writing from local disk files, you should use:

        fconfigure $fp -translation binary

       Note also that the value returned from "pread" should convertible to a byte array, and the
       buffer passed to "pwrite" is also a byte array.

       See also: https://wiki.tcl.tk/1180

   Tcl callbacks
       This just documents the arguments to the callbacks in Tcl, and any way that they differ
       from the C callbacks.  In all other respects they work the same way as the C callbacks, so
       you should go and read nbdkit-plugin(3).

       "dump_plugin"
           (Optional)

           There are no arguments or return value.

       "config"
           (Optional)

            proc config {key value} {
                # No return value.
            }

       "config_complete"
           (Optional)

           There are no arguments or return value.

       "plugin_open"
           (Required)

            proc plugin_open {readonly} {
                set handle ...
                return $handle
            }

           The "readonly" flag is a boolean.

           You can return any Tcl string or object as the handle.  It is passed back to
           subsequent calls.

       "plugin_close"
           (Optional)

            proc plugin_close {h} {
                # No return value
            }

           After "plugin_close" returns, the reference count of the handle is decremented in the
           C part, which usually means that the handle and its contents will be garbage
           collected.

       "get_size"
           (Required)

            proc get_size {h} {
                set size .. the size of the disk ..
                return $size
            }

           This returns the size of the disk.

       "can_write"
           (Optional)

            proc can_write {h} {
                return $bool
            }

           Return a boolean indicating whether the disk is writable.

       "can_flush"
           (Optional)

            proc can_flush {h} {
                return $bool
            }

           Return a boolean indicating whether flush can be performed.

       "is_rotational"
           (Optional)

            proc is_rotational {h} {
                return $bool
            }

           Return a boolean indicating whether the disk is rotational.

       "can_trim"
           (Optional)

            proc can_trim {h} {
                return $bool
            }

           Return a boolean indicating whether trim/discard can be performed.

       "pread"
           (Required)

            proc pread {h count offset} {
               # Construct a buffer of length $count bytes and return it.
               return $buf
            }

           The body of your "pread" function should construct a buffer of length (at least)
           $count bytes.  You should read $count bytes from the disk starting at $offset.

           NBD only supports whole reads, so your function should try to read the whole region
           (perhaps requiring a loop).  If the read fails or is partial, your function should
           call "error".

       "pwrite"
           (Optional)

            proc pwrite {h buf offset} {
               # No return value
            }

           The body of your "pwrite" function should write the $buf string to the disk.  You
           should write $count bytes to the disk starting at $offset.

           NBD only supports whole writes, so your function should try to write the whole region
           (perhaps requiring a loop).  If the write fails or is partial, your function should
           call "error".

       "plugin_flush"
           (Optional)

            proc plugin_flush {h} {
                # No return value
            }

           The body of your "plugin_flush" function should do a sync(2) or fdatasync(2) or
           equivalent on the backing store.

       "trim"
           (Optional)

            proc trim {h count offset} {
                # No return value
            }

           The body of your "trim" function should "punch a hole" in the backing store.

       "zero"
           (Optional)

            proc zero {h count offset may_trim} {
               # No return value
            }

           The body of your "zero" function should ensure that $count bytes of the disk, starting
           at $offset, will read back as zero.  If $may_trim is true, the operation may be
           optimized as a trim as long as subsequent reads see zeroes.

           NBD only supports whole writes, so your function should try to write the whole region
           (perhaps requiring a loop).  If the write fails or is partial, your function should
           call "error".

   Missing callbacks
       Missing: "load", "unload", "name", "version", "longname", "description", "config_help",
       "can_zero", "can_fua", "can_cache", "cache"
           These are not yet supported.

   Threads
       The thread model for Tcl callbacks currently cannot be set from Tcl.  It is hard-coded in
       the C part to "NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS".  This may change or be
       settable in future.

FILES

       $plugindir/nbdkit-tcl-plugin.so
           The plugin.

           Use "nbdkit --dump-config" to find the location of $plugindir.

VERSION

       "nbdkit-tcl-plugin" first appeared in nbdkit 1.4.

SEE ALSO

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

AUTHORS

       Richard W.M. Jones

COPYRIGHT

       Copyright (C) 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.

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