For an example plugin written in Tcl, see:
https://gitlab.com/nbdkit/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.
Executablescript
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".
Binarydata
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
Tclcallbacks
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".
Missingcallbacks
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.
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