comm(n)                      Remote communication                      comm(n)



______________________________________________________________________________

NAME
       comm - A remote communication facility for Tcl (8.3 and later)

SYNOPSIS
       package require Tcl  8.3

       package require comm  ?4.6.2?

       ::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(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

   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

              o      return result

              o      return -code code result

              o      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:

              o      return(-code)

              o      return(-errorinfo)

              o      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:

       4.6.2  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).

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
       o      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.

       o      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.

       o      Allow easier use of a slave interp for actual command execution
              (especially when operating in "not local" mode).

       o      Add host list (xhost-like) or "magic cookie" (xauth-like)
              authentication to initial handshake.

       The following are outstanding todo items.

       o      Add an interp discovery and name->port mapping.  This is likely
              to be in a separate, optional nameserver.  (See also the related
              work, below.)

       o      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.

       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 SF Trackers
       [http://sourceforge.net/tracker/?group_id=12883].  Please also report
       any ideas for enhancements you may have for either package and/or
       documentation.

SEE ALSO
       send(n)

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>




comm                                 4.6.2                             comm(n)
