comm(n) 4.2 comm "remote communication"
comm - A remote communications facility for Tcl (7.6, 8.0, and later)
package require Tcl 8.2
package require comm ?4.2?
The comm command provides an inter-interpreter remote
execution facility much like Tk's send(n), except that it uses
sockets rather than the X server for the communication path. As a
result, comm works with multiple interpreters, works on
Windows and Macintosh systems, and provides control over the remote
execution path.
These commands work just like send and winfo interps :
| |
::comm::comm send ?-async? id cmd ?arg arg ...?
::comm::comm interps
|
This is all that is really needed to know in order to use
comm
The package initializes ::comm::comm as the default chan.
comm names communication endpoints with an id unique
to each machine. Before sending commands, the id of another
interpreter is needed. Unlike Tk's send, comm doesn't
implicitly know the id's of all the interpreters on the system.
The following four methods make up the basic comm interface.
- ::comm::comm send ?-async? ?-command callback? id cmd ?arg arg ...?
-
This invokes the given command in the interpreter named by id. The
command waits for the result and remote errors are returned unless the
-async or -command option is given. If -async
is given, send returns immediately and there is no further notification of
result. If -command is used, callback specifies a command
to invoke when the result is received. These options are mutually
exclusive. The callback will receive arguments in the form
-option value, suitable for array set.
The options are: -id, the comm id of the interpreter that received
the command; -serial, a unique serial for each command sent to a
particular comm interpreter; -chan, the comm channel name;
-code, the result code of the command; -errorcode, the
errorcode, if any, of the command; -errorinfo, the errorinfo, if
any, of the command; and -result, the return value of the command.
If connection is lost before a reply is received, the callback will be
invoked with a connection lost message with -code equal to -1. When
-command is used, the command returns the unique serial for the
command.
- ::comm::comm self
-
Returns the id for this channel.
- ::comm::comm interps
-
Returns a list of all the remote id's to which this channel is
connected. comm learns a new remote id when a command
is first issued it, or when a remote id first issues a command
to this comm channel. ::comm::comm ids is an alias for this
method.
- ::comm::comm connect ?id?
-
Whereas ::comm::comm send will automatically connect to the
given id, this forces a connection to a remote id without
sending a command. After this, the remote id will appear in
::comm::comm interps.
The evaluation semantics of ::comm::comm send are intended to
match Tk's send exactly. This means that comm
evaluates arguments on the remote side.
If you find that ::comm::comm send doesn't work for a
particular command, try the same thing with Tk's send and see if the
result is different. If there is a problem, please report it. For
instance, there was had one report that this command produced an
error. Note that the equivalent send command also produces the
same error.
| |
% ::comm::comm send id llength {a b c}
wrong # args: should be "llength list"
% send name llength {a b c}
wrong # args: should be "llength list"
|
The eval hook (described below) can be used to change from
send's double eval semantics to single eval semantics.
More than one comm channel (or listener) can be created
in each Tcl interpreter. This allows flexibility to create full and
restricted channels. For instance, hook scripts are specific
to the channel they are defined against.
- ::comm::comm new chan ?name value ...?
-
This creates a new channel and Tcl command with the given channel
name. This new command controls the new channel and takes all the
same arguments as ::comm::comm. Any remaining arguments are
passed to the config method. The fully qualified channel
name is returned.
- ::comm::comm channels
-
This lists all the channels allocated in this Tcl interpreter.
The default configuration parameters for a new channel are:
| |
"-port 0 -local 1 -listen 0"
|
The default channel ::comm::comm is created with:
| |
"::comm::comm new ::comm::comm -port 0 -local 1 -listen 1"
|
The config method acts similar to fconfigure in that it
sets or queries configuration variables associated with a channel.
- ::comm::comm config
-
- ::comm::comm config name
-
- ::comm::comm config ?name value ...?
-
When given no arguments, config returns a list of all variables
and their value With one argument, config returns the value of
just that argument. With an even number of arguments, the given
variables are set to the given values.
These configuration variables can be changed (descriptions of them are
elsewhere in this manual page):
- -listen ?0|1?
-
- -local ?0|1?
-
- -port ?port?
-
These configuration variables are readonly:
- -chan chan
-
- -serial n
-
- -socket sockIn
-
When config changes the parameters of an existing channel, it
closes and reopens the listening socket. An automatically assigned
channel id will change when this happens. Recycling the socket
is done by invoking ::comm::comm abort, which causes all
active sends to terminate.
comm uses a TCP port for endpoint id. The
interps (or ids) method merely lists all the TCP ports
to which the channel is connected. By default, each channel's
id is randomly assigned by the operating system (but usually
starts at a low value around 1024 and increases each time a new socket
is opened). This behavior is accomplished by giving the
-port config option a value of 0. Alternately, a specific
TCP port number may be provided for a given channel. As a special
case, comm contains code to allocate a a high-numbered TCP port
(>10000) by using -port {}. Note that a channel won't be
created and initialized unless the specific port can be allocated.
As a special case, if the channel is configured with
-listen 0, then it will not create a listening socket and
will use an id of 0 for itself. Such a channel is only good
for outgoing connections (although once a connection is established,
it can carry send traffic in both directions).
By default, each channel is restricted to accepting connections from
the local system. This can be overridden by using the
-local 0 configuration option For such channels, the
id parameter takes the form { id host }.
WARNING: The host must always be specified in the same
form (e.g., as either a fully qualified domain name, plain hostname or
an IP address).
These methods give control over closing connections:
- ::comm::comm shutdown Iid
-
This closes the connection to id, aborting all outstanding
commands in progress. Note that nothing prevents the connection from
being immediately reopened by another incoming or outgoing command.
- ::comm::comm abort
-
This invokes shutdown on all open connections in this comm channel.
- ::comm::comm destroy
-
This aborts all connections and then destroys the this comm channel
itself, including closing the listening socket. Special code allows
the default ::comm::comm channel to be closed such that the
::comm::comm command it is not destroyed. Doing so closes the
listening socket, preventing both incoming and outgoing commands on
the channel. This sequence reinitializes the default channel:
| |
"::comm::comm destroy; ::comm::comm new ::comm::comm"
|
When a remote connection is lost (because the remote exited or called
shutdown), comm can invoke an application callback.
This can be used to cleanup or restart an ancillary process, for
instance. See the lost callback below.
This is a mechanism for setting hooks for particular events:
- ::comm::comm hook event ?+? ?script?
-
This uses a syntax similar to Tk's bind command. Prefixing
script with a + causes the new script to be appended.
Without this, a new script replaces any existing script. When
invoked without a script, no change is made. In all cases, the new
hook script is returned by the command.
When an event occurs, the script associated with it is
evaluated with the listed variables in scope and available. The
return code (not the return value) of the script is commonly
used decide how to further process after the hook.
Common variables include:
- chan
-
the name of the comm channel (and command)
- id
-
the id of the remote in question
- fid
-
the file id for the socket of the connection
These are the defined events:
- connecting
-
Variables:
chan id host port
This hook is invoked before making a connection to the remote named in
id. An error return (via error) will abort the connection
attempt with the error. Example:
| |
% ::comm::comm hook connecting {
if [lb]string match {*[lb]02468[rb]} $id[rb] {
error "Can't connect to even ids"
}
}
% ::comm::comm send 10000 puts ok
Connect to remote failed: Can't connect to even ids
%
|
- connected
-
Variables:
chan fid id host port
This hook is invoked immediately after making a remote connection to
id, allowing arbitrary authentication over the socket named by
fid. An error return (via error ) will close the
connection with the error. host and port are merely
extracted from the id; changing any of these will have no effect
on the connection, however. It is also possible to substitute and
replace fid.
- incoming
-
Variables:
chan fid addr remport
Hook invoked when receiving an incoming connection, allowing arbitrary
authentication over socket named by fid. An error return (via
error) will close the connection with the error. Note that the
peer is named by remport and addr but that the remote
id is still unknown. Example:
| |
::comm::comm hook incoming {
if [lb]string match 127.0.0.1 $addr[rb] {
error "I don't talk to myself"
}
}
|
- eval
-
Variables:
chan id cmd buffer
This hook is invoked after collecting a complete script from a remote
but before evaluating it. This allows complete control over
the processing of incoming commands. cmd contains either
send or async. buffer holds the script to
evaluate. At the time the hook is called, $chan remoteid is
identical in value to id.
By changing buffer, the hook can change the script to be
evaluated. The hook can short circuit evaluation and cause a value to
be immediately returned by using return result (or, from
within a procedure, return -code return result). An
error return (via error) will return an error result, as is if
the script caused the error. Any other return will evaluate the
script in buffer as normal. For compatibility with 3.2,
break and return -code break result is supported,
acting similarly to return {} and return -code return
result.
Examples:
-
augmenting a command
| |
% ::comm::comm send [lb]::comm::comm self[rb] pid
5013
% ::comm::comm hook eval {puts "going to execute $buffer"}
% ::comm::comm send [lb]::comm::comm self[rb] pid
going to execute pid
5013
|
-
short circuiting a command
| |
% ::comm::comm hook eval {puts "would have executed $buffer"; return 0}
% ::comm::comm send [lb]::comm::comm self[rb] pid
would have executed pid
0
|
-
Replacing double eval semantics
| |
% ::comm::comm send [lb]::comm::comm self[rb] llength {a b c}
wrong # args: should be "llength list"
% ::comm::comm hook eval {return [uplevel #0 $buffer]}
return [lb]uplevel #0 $buffer[rb]
% ::comm::comm send [lb]::comm::comm self[rb] llength {a b c}
3
|
-
Using a slave interpreter
| |
% interp create foo
% ::comm::comm hook eval {return [lb]foo eval $buffer[rb]}
% ::comm::comm send [lb]::comm::comm self[rb] set myvar 123
123
% set myvar
can't read "myvar": no such variable
% foo eval set myvar
123
|
-
Using a slave interpreter (double eval)
| |
% ::comm::comm hook eval {return [lb]eval foo eval $buffer[rb]}
|
-
Subverting the script to execute
| |
% ::comm::comm hook eval {
switch -- $buffer {
a {return A-OK} b {return B-OK} default {error "$buffer is a no-no"}
}
}
% ::comm::comm send [lb]::comm::comm self[rb] pid
pid is a no-no
% ::comm::comm send [lb]::comm::comm self[rb] a
A-OK
|
- reply
-
Variables:
chan id buffer ret return()
This hook is invoked after collecting a complete reply script from a
remote but before evaluating it. This allows complete
control over the processing of replies to sent commands. The reply
buffer is in one of the following forms
-
return result
-
return -code code result
-
return -code code -errorinfo info -errorcode ecode msg
For safety reasons, this is decomposed. The return result is in
ret, and the return switches are in the return array:
-
return(-code)
-
return(-errorinfo)
-
return(-errordcode)
Any of these may be the empty string. Modifying these four variables
can change the return value, whereas modifying buffer has no
effect.
- callback
-
Variables:
chan id buffer ret return()
Similar to reply, but used for callbacks.
- lost
-
Variables:
chan id reason
This hook is invoked when the connection to id is lost. Return
value (or thrown error) is ignored. reason is an explanatory
string indicating why the connection was lost. Example:
| |
::comm::comm hook lost {
global myvar
if {$myvar(id) == $id} {
myfunc
return
}
}
|
These interfaces may change or go away in subsequence releases.
- ::comm::comm remoteid
-
Returns the id of the sender of the last remote command
executed on this channel. If used by a proc being invoked remotely,
it must be called before any events are processed. Otherwise, another
command may get invoked and change the value.
- ::comm::comm_send
-
Invoking this procedure will substitute the Tk send and
winfo interps commands with these equivalents that use
::comm::comm.
| |
proc send {args} {
eval ::comm::comm send $args
}
rename winfo tk_winfo
proc winfo {cmd args} {
if ![lb]string match in* $cmd[rb] {return [lb]eval [lb]list tk_winfo $cmd[rb] $args[rb]}
return [lb]::comm::comm interps[rb]
}
|
Something here soon.
There is one outstanding difference between comm and
send. When blocking in a synchronous remote command, send
uses an internal C hook (Tk_RestrictEvents) to the event loop to look
ahead for send-related events and only process those without
processing any other events. In contrast, comm uses the
vwait command as a semaphore to indicate the return message has
arrived. The difference is that a synchronous send will block
the application and prevent all events (including window related ones)
from being processed, while a synchronous ::comm::comm send will block the
application but still allow other events will still get processed. In
particular, after idle handlers will fire immediately when
comm blocks.
What can be done about this? First, note that this behavior will come
from any code using vwait to block and wait for an event to
occur. At the cost of multiple channel support, comm could
be changed to do blocking I/O on the socket, giving send-like blocking
semantics. However, multiple channel support is a very useful feature
of comm that it is deemed too important to lose. The remaining
approaches involve a new loadable module written in C (which is
somewhat against the philosophy of comm) One way would be to
create a modified version of the vwait command that allow the
event flags passed to Tcl_DoOneEvent to be specified. For comm,
just the TCL_FILE_EVENTS would be processed. Another way would be to
implement a mechanism like Tk_RestrictEvents, but apply it to the Tcl
event loop (since comm doesn't require Tk). One of these
approaches will be available in a future comm release as an
optional component.
comm exports itself as a package. The package version number
is in the form major . minor, where the major version will
only change when a non-compatible change happens to the API or
protocol. Minor bug fixes and changes will only affect the minor
version. To load comm this command is usually used:
Note that requiring no version (or a specific version) can also be done.
The revision history of comm includes these releases:
- 4.2
-
Bugfixes, and most important, switched to utf-8 as default encoding
for full i18n without any problems.
- 4.1
-
Rewrite of internal code to remove old pseudo-object model. Addition
of send -command asynchronous callback option.
- 4.0
-
Per request by John LoVerso. Improved handling of error for async
invoked commands.
- 3.7
-
Moved into tcllib and placed in a proper namespace.
- 3.6
-
A bug in the looking up of the remoteid for a executed command could
be triggered when the connection was closed while several asynchronous
sends were queued to be executed.
- 3.5
-
Internal change to how reply messages from a send are handled.
Reply messages are now decoded into the value to pass to
return; a new return statement is then cons'd up to with this
value. Previously, the return code was passed in from the remote as a
command to evaluate. Since the wire protocol has not changed, this is
still the case. Instead, the reply handling code decodes the
reply message.
- 3.4
-
Added more source commentary, as well as documenting config variables
in this man page. Fixed bug were loss of connection would give error
about a variable named pending rather than the message about
the lost connection. comm ids is now an alias for
comm interps (previously, it an alias for comm chans).
Since the method invocation change of 3.0, break and other exceptional
conditions were not being returned correctly from comm send.
This has been fixed by removing the extra level of indirection into
the internal procedure commSend. Also added propagation of
the errorCode variable. This means that these commands return
exactly as they would with send:
| |
comm send id break
catch {comm send id break}
comm send id expr 1 / 0
|
Added a new hook for reply messages. Reworked method invocation to
avoid the use of comm:* procedures; this also cut the invocation time
down by 40%. Documented comm config (as this manual page
still listed the defunct comm init!)
- 3.3
-
Some minor bugs were corrected and the documentation was cleaned up.
Added some examples for hooks. The return semantics of the eval
hook were changed.
- 3.2
-
A new wire protocol, version 3, was added. This is backwards
compatible with version 2 but adds an exchange of supported protocol
versions to allow protocol negotiation in the future. Several bugs
with the hook implementation were fixed. A new section of the man
page on blocking semantics was added.
- 3.1
-
All the documented hooks were implemented. commLostHook was
removed. A bug in comm new was fixed.
- 3.0
-
This is a new version of comm with several major changes.
There is a new way of creating the methods available under the
comm command. The comm init method has been retired
and is replaced by comm configure which allows access to many
of the well-defined internal variables. This also generalizes the
options available to comm new. Finally, there is now a
protocol version exchanged when a connection is established. This
will allow for future on-wire protocol changes. Currently, the
protocol version is set to 2.
- 2.3
-
comm ids was renamed to comm channels. General
support for comm hook was fully implemented, but only the
lost hook exists, and it was changed to follow the general
hook API. commLostHook was unsupported (replaced by
comm hook lost) and commLost was removed.
- 2.2
-
The died hook was renamed lost, to be accessed by
commLostHook and an early implementation of
comm lost hook. As such, commDied is now
commLost.
- 2.1
-
Unsupported method comm remoteid was added.
- 2.0
-
comm has been rewritten from scratch (but is fully compatible
with Comm 1.0, without the requirement to use obTcl).
John LoVerso, John@LoVerso.Southborough.MA.US
http://www.opengroup.org/~loverso/tcl-tk/#comm
Please see the file comm.LICENSE that accompanied this source,
or
http://www.opengroup.org/www/dist_client/caubweb/COPYRIGHT.free.html.
This license for comm, new as of version 3.2, allows it to be
used for free, without any licensing fee or royalty.
-
If there is a failure initializing a channel created with
::comm::comm new, then the channel should be destroyed.
Currently, it is left in an inconsistent state.
-
There should be a way to force a channel to quiesce when changing the
configuration.
The following items can be implemented with the existing hooks and are
listed here as a reminder to provide a sample hook in a future
version.
-
Allow easier use of a slave interp for actual command execution
(especially when operating in "not local" mode).
-
Add host list (xhost-like) or "magic cookie" (xauth-like)
authentication to initial handshake.
The following are outstanding todo items.
-
Add an interp discovery and name->port mapping. This is likely to be
in a separate, optional nameserver. (See also the related work,
below.)
-
Fix the {id host} form so as not to be dependent upon
canonical hostnames. This requires fixes to Tcl to resolve hostnames!
This man page is bigger than the source file.
Tcl7.5 under Windows contains a bug that causes the interpreter to
hang when EOF is reached on non-blocking sockets. This can be
triggered with a command such as this:
Always make sure the channel is quiescent before closing/exiting or
use at least Tcl7.6 under Windows.
Tcl7.6 on the Mac contains several bugs. It is recommended you use
at least Tcl7.6p2.
Tcl8.0 on UNIX contains a socket bug that can crash Tcl. It is recommended
you use Tcl8.0p1 (or Tcl7.6p2).
Tcl-DP provides an RPC-based remote execution interface, but is a
compiled Tcl extension. See
http://www.cs.cornell.edu/Info/Projects/zeno/Projects/Tcl-DP.html.
Michael Doyle <miked@eolas.com> has code that implements the Tcl-DP
RPC interface using standard Tcl sockets, much like comm
Andreas Kupries <andreas_kupries@users.sourceforge.net> uses
comm and has built a simple nameserver as part of his Pool
library. See http://www.purl.org/net/akupries/soft/pool/index.htm.
send(n)
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