comm.tcl - A remote communications facility for Tcl (7.6, 8.0, and later)


package require Comm 3

chan send ?-async? id cmd ?arg arg ...?

chan interps

chan ids

chan self

chan connect ?id?

chan config
chan config name
chan config ?name value ...?

-listen ?0|1? -local ?0|1? -port ?port?

chan new chan ?name value ...?

chan channels

chan shutdown id

chan abort

chan destroy

chan remoteid

chan hook event ?+??script?

The package initializes comm as the default chan.


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 send ?-async? id cmd ?arg arg ...?
comm interps
This is all that is really needed to know in order to use comm.


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.

comm send ?-async? 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 option is given.
comm self
Returns the id for this channel.
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 ids is an alias for this method.
comm connect ?id?
Whereas 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 interps.

These four methods make up the basic comm interface.


The evaluation semantics of 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 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 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 interpeter. This allows flexibility to create full and restricted channels. For instance, hook scripts are specific to the channel they are defined against.

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. Any remaining arguments are passed to the config method.
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 is created with:
comm new 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 config
comm config name
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 sockn

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 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 overriden 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 shutdown id
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 abort
This invokes shutdown on all open connections in this comm channel.
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 channel to be closed such that the 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 destroy; comm new 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 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:

the name of the comm channel (and command)
the id of the remote in question
the file id for the socket of the connection
These are the defined events:
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 hook connecting {
if [string match {*[02468]} $id] {
error "Can't connect to even ids"
% comm send 10000 puts ok
Connect to remote failed: Can't connect to even ids
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 .
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 hook incoming {
if [string match $addr] {
error "I don't talk to myself"
Variables: chan id cmd buffer
This hook is invoked after collecting a complete script from a remote but before evalutating 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.


1. augmenting a command
% comm send [comm self] pid
% comm hook eval {puts "going to execute $buffer"}
% comm send [comm self] pid
going to execute pid
2. short circuting a command
% comm hook eval {puts "would have executed $buffer"; return 0}
% comm send [comm self] pid
would have executed pid
3. Replacing double eval semantics
% comm send [comm self] llength {a b c}
wrong # args: should be "llength list"
% comm hook eval {return [uplevel #0 $buffer]}
return [uplevel #0 $buffer]
% comm send [comm self] llength {a b c}
4. Using a slave interpreter
% interp create foo
% comm hook eval {return [foo eval $buffer]}
% comm send [comm self] set myvar 123
% set myvar
can't read "myvar": no such variable
% foo eval set myvar
5. Using a slave interpreter (double eval)
% comm hook eval {return [eval foo eval $buffer]}
6. Subverting the script to execute
% comm hook eval {
switch -- $buffer {
a {return A-OK} b {return B-OK} default {error "$buffer is a no-no"}
% comm send [comm self] pid
pid is a no-no
% comm send [comm self] a
Variables: chan id buffer ret return()
This hook is invoked after collecting a complete reply script from a remote but before evalutating 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.
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 hook lost {
global myvar
if {$myvar(id) == $id} {


These interfaces may change or go away in subsequence releases.

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.
Invoking this procedure will substitute the Tk send and winfo interps commands with these equivalents that use comm.

proc send {args} {
eval comm send $args
rename winfo tk_winfo
proc winfo {cmd args} {
if ![string match in* $cmd] {return [eval [list tk_winfo $cmd] $args]}
return [comm interps]


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 synchornous send will block the application and prevent all events (including window related ones) from being processed, while a synchronous comm 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, givng 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:

package require Comm 3
Note that requiring no version (or a specific version) can also be done.

The revision history of comm includes these releases:

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.
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.
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 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 propogation 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!)
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.
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.
All the documented hooks were implemented. commLostHook was removed. A bug in comm new was fixed.
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.
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.
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.
Unsupported method comm remoteid was added.
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


Copyright (C) 1995-1998 The Open Group. All Rights Reserved. Please see the file comm.LICENSE that accompanied this source, or

This license for comm, new as of version 3.2, allows it to be used for free, without any licensing fee or royalty.


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.

The following are outstanding todo items.

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:

comm send $other exit
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

Michael Doyle <> has code that implements the Tcl-DP RPC interface using standard Tcl sockets, much like comm.

Andreas Kupries <> uses comm and has built a simple nameserver as part of his Pool library. See

Markup created by unroff 1.0,    May 30, 1998.