Provided by: tcllib_1.21+dfsg-1_all 
NAME
comm - A remote communication facility for Tcl (8.5 and later)
SYNOPSIS
package require Tcl 8.5
package require comm ?4.7?
::comm::comm send ?-async? ?-command callback? id cmd ?arg arg ...?
::comm::comm self
::comm::comm interps
::comm::comm connect ?id?
::comm::comm new chan ?name value ...?
::comm::comm channels
::comm::comm config
::comm::comm config name
::comm::comm config ?name value ...?
::comm::comm shutdown id
::comm::comm abort
::comm::comm destroy
::comm::comm hook event ?+? ?script?
::comm::comm remoteid
::comm::comm_send
::comm::comm return_async
$future return ?-code code? ?value?
$future configure ?-command ?cmdprefix??
$future cget -command
________________________________________________________________________________________________________________
DESCRIPTION
The comm command provides an inter-interpreter remote execution facility much like Tk's send(3tk), 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
COMMANDS
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.
EVAL SEMANTICS
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.
MULTIPLE CHANNELS
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 -silent 0"
The default channel ::comm::comm is created with:
"::comm::comm new ::comm::comm -port 0 -local 1 -listen 1 -silent 0"
CHANNEL CONFIGURATION
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?
-silent ?0|1?
-socketcmd ?commandname?
-interp ?interpreter?
-events ?eventlist?
These configuration variables are read only:
-chan chan
-serial n
-socket sockIn
When config changes the parameters of an existing channel (with the exception of -interp and -events), 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.
ID/PORT ASSIGNMENTS
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). As another
special case, if the channel is configured with -silent 0, then the listening side will ignore connection
attempts where the protocol negotiation phase failed, instead of throwing an error.
EXECUTION ENVIRONMENT
A communication channel in its default configuration will use the current interpreter for the execution
of all received scripts, and of the event scripts associated with the various hooks.
This insecure setup can be changed by the user via the two options -interp, and -events.
When -interp is set all received scripts are executed in the slave interpreter specified as the value of
the option. This interpreter is expected to exist before configuration. I.e. it is the responsibility of
the user to create it. However afterward the communication channel takes ownership of this interpreter,
and will destroy it when the communication channel is destroyed. Note that reconfiguration of the
communication channel to either a different interpreter or the empty string will release the ownership
without destroying the previously configured interpreter. The empty string has a special meaning, it
restores the default behaviour of executing received scripts in the current interpreter.
Also of note is that replies and callbacks (a special form of reply) are not considered as received
scripts. They are trusted, part of the internal machinery of comm, and therefore always executed in the
current interpreter.
Even if an interpreter has been configured as the execution environment for received scripts the event
scripts associated with the various hooks will by default still be executed in the current interpreter.
To change this use the option -events to declare a list of the events whose scripts should be executed in
the declared interpreter as well. The contents of this option are ignored if the communication channel is
configured to execute received scripts in the current interpreter.
REMOTE INTERPRETERS
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).
CLOSING CONNECTIONS
These methods give control over closing connections:
::comm::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::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.
CALLBACKS
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
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 {[string match {*[02468]} $id]} {
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, and 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, and 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 {[string match 127.0.0.1 $addr]} {
error "I don't talk to myself"
}
}
eval Variables: chan, id, cmd, and 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:
[1] augmenting a command
% ::comm::comm send [::comm::comm self] pid
5013
% ::comm::comm hook eval {puts "going to execute $buffer"}
% ::comm::comm send [::comm::comm self] pid
going to execute pid
5013
[2] short circuiting a command
% ::comm::comm hook eval {puts "would have executed $buffer"; return 0}
% ::comm::comm send [::comm::comm self] pid
would have executed pid
0
[3] Replacing double eval semantics
% ::comm::comm send [::comm::comm self] llength {a b c}
wrong # args: should be "llength list"
% ::comm::comm hook eval {return [uplevel #0 $buffer]}
return [uplevel #0 $buffer]
% ::comm::comm send [::comm::comm self] llength {a b c}
3
[4] Using a slave interpreter
% interp create foo
% ::comm::comm hook eval {return [foo eval $buffer]}
% ::comm::comm send [::comm::comm self] set myvar 123
123
% set myvar
can't read "myvar": no such variable
% foo eval set myvar
123
[5] Using a slave interpreter (double eval)
% ::comm::comm hook eval {return [eval foo eval $buffer]}
[6] 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 [::comm::comm self] pid
pid is a no-no
% ::comm::comm send [::comm::comm self] a
A-OK
reply Variables: chan, id, buffer, ret, and 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(-errorcode)
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, and return().
Similar to reply, but used for callbacks.
lost Variables: chan, id, and 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
}
}
UNSUPPORTED
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 {![string match in* $cmd]} {
return [eval [list tk_winfo $cmd] $args]
}
return [::comm::comm interps]
}
SECURITY
Starting with version 4.6 of the package an option -socketcmd is supported, allowing the user of a comm
channel to specify which command to use when opening a socket. Anything which is API-compatible with the
builtin ::socket (the default) can be used.
The envisioned main use is the specification of the tls::socket command, see package tls, to secure the
communication.
# Load and initialize tls
package require tls
tls::init -cafile /path/to/ca/cert -keyfile ...
# Create secured comm channel
::comm::comm new SECURE -socketcmd tls::socket -listen 1
...
The sections Execution Environment and Callbacks are also relevant to the security of the system,
providing means to restrict the execution to a specific environment, perform additional authentication,
and the like.
BLOCKING SEMANTICS
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 to 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.
ASYNCHRONOUS RESULT GENERATION
By default the result returned by a remotely invoked command is the result sent back to the invoker. This
means that the result is generated synchronously, and the server handling the call is blocked for the
duration of the command.
While this is tolerable as long as only short-running commands are invoked on the server long-running
commands, like database queries make this a problem. One command can prevent the processing requests of
all other clients for an arbitrary period of time.
Before version 4.5 of comm the only solution was to rewrite the server command to use the Tcl builtin
command vwait, or one of its relatives like tkwait, to open a new event loop which processes requests
while the long-running operation is executed. This however has its own perils, as this makes it possible
to both overflow the Tcl stack with a large number of event loop, and to have a newer requests block the
return of older ones, as the eventloop have to be unwound in the order of their creation.
The proper solution is to have the invoked command indicate to comm that it cannot or will not deliver an
immediate, synchronous result, but will do so later. At that point the framework can put sending the
actual result on hold and continue processing requests using the main event loop. No blocking, no nesting
of event loops. At some future date the long running operation delivers the result to comm, via the
future object, which is then forwarded to the invoker as usual.
The necessary support for this solution has been added to comm since version 4.5, in the form of the new
method return_async.
::comm::comm return_async
This command is used by a remotely invoked script to notify the comm channel which invoked it that
the result to send back to the invoker is not generated synchronously. If this command is not
called the default/standard behaviour of comm is to send the synchronously generated result of the
script itself to the invoker.
The result of return_async is an object. This object, called a future is where the result of the
script has to be delivered to when it becomes ready. When that happens it will take all the
necessary actions to deliver the result to the invoker of the script, and then destroy itself.
Should comm have lost the connection to the invoker while the result is being computed the future
will not try to deliver the result it got, but just destroy itself. The future can be configured
with a command to call when the invoker is lost. This enables the user to implement an early abort
of the long-running operation, should this be supported by it.
An example:
# Procedure invoked by remote clients to run database operations.
proc select {sql} {
# Signal the async generation of the result
set future [::comm::comm return_async]
# Generate an async db operation and tell it where to deliver the result.
set query [db query -command [list $future return] $sql]
# Tell the database system which query to cancel if the connection
# goes away while it is running.
$future configure -command [list db cancel $query]
# Note: The above will work without problem only if the async
# query will nover run its completion callback immediately, but
# only from the eventloop. Because otherwise the future we wish to
# configure may already be gone. If that is possible use 'catch'
# to prevent the error from propagating.
return
}
The API of a future object is:
$future return ?-code code? ?value?
Use this method to tell the future that long-running operation has completed. Arguments are
an optional return value (defaults to the empty string), and the Tcl return code (defaults
to OK).
The future will deliver this information to invoker, if the connection was not lost in the
meantime, and then destroy itself. If the connection was lost it will do nothing but
destroy itself.
$future configure ?-command ?cmdprefix??
$future cget -command
These methods allow the user to retrieve and set a command to be called if the connection
the future belongs to has been lost.
COMPATIBILITY
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:
4.6.3 Fixed ticket [ced0d60fc9]. Added proper detection of eof on a socket, properly closing it.
4.6.2 Fixed bugs 2972571 and 3066872, the first a misdetection of quoted brace after double backslash,
the other a blocking gets making for an obvious (hinsight) DoS attack on comm channels.
4.6.1 Changed the implementation of comm::commCollect to emulate lindex's pre-Tcl 8 behaviour, i.e. it
was given the ability to parse out the first word of a list, even if the whole buffer is not a
well-formed list. Without this change the first word could only be extracted if the whole buffer
was a well-formed list (ever since Tcl 8), and in a ver-high-load situation, i.e. a server sending
lots and/or large commands very fast, this may never happen, eventually crashing the receiver when
it runs out of memory. With the change the receiver is always able to process the first word when
it becomes well-formed, regardless of the structure of the remainder of the buffer.
4.6 Added the option -socketcmd enabling users to override how a socket is opened. The envisioned main
use is the specification of the tls::socket command, see package tls, to secure the communication.
4.5.7 Changed handling of ports already in use to provide a proper error message.
4.5.6 Bugfix in the replacement for vwait, made robust against of variable names containing spaces.
4.5.5 Bugfix in the handling of hooks, typo in variable name.
4.5.4 Bugfix in the handling of the result received by the send method. Replaced an after idle unset
result with an immediate unset, with the information saved to a local variable.
The after idle can spill into a forked child process if there is no event loop between its setup
and the fork. This may bork the child if the next event loop is the vwait of comm's send a few
lines above the after idle, and the child used the same serial number for its next request. In
that case the parent's after idle unset will delete the very array element the child is waiting
for, unlocking the vwait, causing it to access a now missing array element, instead of the
expected result.
4.5.3 Bugfixes in the wrappers for the builtin update and vwait commands.
4.5.2 Bugfix in the wrapper for the builtin update command.
4.5.1 Bugfixes in the handling of -interp for regular scripts. The handling of the buffer was wrong for
scripts which are a single statement as list. Fixed missing argument to new command commSendReply,
introduced by version 4.5. Affected debugging.
4.5 New server-side feature. The command invoked on the server can now switch comm from the standard
synchronous return of its result to an asynchronous (defered) return. Due to the use of snit to
implement the future objects used by this feature from this version on comm requires at least Tcl
8.3 to run. Please read the section Asynchronous Result Generation for more details.
4.4.1 Bugfix in the execution of hooks.
4.4 Bugfixes in the handling of -interp for regular and hook scripts. Bugfixes in channel cleanup.
4.3.1 Introduced -interp and -events to enable easy use of a slave interp for execution of received
scripts, and of event scripts.
4.3 Bugfixes, and introduces -silent to allow the user to force the server/listening side to silently
ignore connection attempts where the protocol negotiation failed.
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).
TLS SECURITY CONSIDERATIONS
This package uses the TLS package to handle the security for https urls and other socket connections.
Policy decisions like the set of protocols to support and what ciphers to use are not the responsibility
of TLS, nor of this package itself however. Such decisions are the responsibility of whichever
application is using the package, and are likely influenced by the set of servers the application will
talk to as well.
For example, in light of the recent POODLE attack
[http://googleonlinesecurity.blogspot.co.uk/2014/10/this-poodle-bites-exploiting-ssl-30.html] discovered
by Google many servers will disable support for the SSLv3 protocol. To handle this change the
applications using TLS must be patched, and not this package, nor TLS itself. Such a patch may be as
simple as generally activating tls1 support, as shown in the example below.
package require tls
tls::init -tls1 1 ;# forcibly activate support for the TLS1 protocol
... your own application code ...
AUTHOR
John LoVerso, John@LoVerso.Southborough.MA.US
http://www.opengroup.org/~loverso/tcl-tk/#comm
LICENSE
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.
BUGS
• 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.
ON USING OLD VERSIONS OF TCL
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).
RELATED WORK
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. The DpTcl package is available at
http://chiselapp.com/user/gwlester/repository/DpTcl.
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.
BUGS, IDEAS, FEEDBACK
This document, and the package it describes, will undoubtedly contain bugs and other problems. Please
report such in the category comm of the Tcllib Trackers [http://core.tcl.tk/tcllib/reportlist]. Please
also report any ideas for enhancements you may have for either package and/or documentation.
When proposing code changes, please provide unified diffs, i.e the output of diff -u.
Note further that attachments are strongly preferred over inlined patches. Attachments can be made by
going to the Edit form of the ticket immediately after its creation, and then using the left-most button
in the secondary navigation bar.
SEE ALSO
send(3tk)
KEYWORDS
comm, communication, ipc, message, remote communication, remote execution, rpc, secure, send, socket,
ssl, tls
CATEGORY
Programming tools
COPYRIGHT
Copyright (c) 1995-1998 The Open Group. All Rights Reserved.
Copyright (c) 2003-2004 ActiveState Corporation.
Copyright (c) 2006-2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>
tcllib 4.7 comm(3tcl)