


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  ?4.6.1?

       ::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 facil-
       ity 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



comm                          4.6.1                             1





comm(n)                Remote communication               comm(n)


       multiple interpreters, works on Windows and Macintosh systems, and pro-
       vides 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  notifi-
              cation  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 com-
              mand; -serial, a unique serial for each command sent to  a  par-
              ticular  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  con-
              nection 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  send-
              ing  a  command.   After  this,  the  remote  id  will appear in



comm                          4.6.1                             2





comm(n)                Remote communication               comm(n)


              ::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  com-
       mand,  try  the same thing with Tk's send and see if the result is dif-
       ferent.  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  dou-
       ble 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  chan-
              nel  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"






comm                          4.6.1                             3





comm(n)                Remote communication               comm(n)


   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



comm                          4.6.1                             4





comm(n)                Remote communication               comm(n)


       -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 direc-
       tions).  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 cur-
       rent 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  com-
       munication channel is destroyed.  Note that reconfiguration of the com-
       munication 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  cur-
       rent 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 config-
       uration  option For such channels, the id parameter takes the form { id
       host }.




comm                          4.6.1                             5





comm(n)                Remote communication               comm(n)


       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  com-
              mands  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 chan-
              nel.

       ::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 outgo-
              ing  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)




comm                          4.6.1                             6





comm(n)                Remote communication               comm(n)


              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 connec-
              tion 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  connec-
              tion  to  id,  allowing arbitrary authentication over the socket
              named by fid.  An error return (via error ) will close the  con-
              nection with the error.  host and port are merely extracted from
              the id; changing any of these will have no effect on the connec-
              tion,  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.



comm                          4.6.1                             7





comm(n)                Remote communication               comm(n)


              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 evalu-
              ated.  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                          4.6.1                             8





comm(n)                Remote communication               comm(n)


                         % ::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



comm                          4.6.1                             9





comm(n)                Remote communication               comm(n)


       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  sup-
       ported, allowing the user of a comm channel to specify which command to



comm                          4.6.1                            10





comm(n)                Remote communication               comm(n)


       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 com-
       mand, 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 block-
       ing 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.   How-
       ever, 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 philos-
       ophy 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 gener-
       ated synchronously, and the server handling the call is blocked for the
       duration of the command.



comm                          4.6.1                            11





comm(n)                Remote communication               comm(n)


       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 per-
       ils, 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 con-
              nection 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 imple-
              ment 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.



comm                          4.6.1                            12





comm(n)                Remote communication               comm(n)


                  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 com-
                     mand 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 com-
       mand 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:

       4.6.1  Changed  the  implementation  of  comm::commCollect  to  emulate



comm                          4.6.1                            13





comm(n)                Remote communication               comm(n)


              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  pro-
              cess  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  com-
              munication.

       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 wait-
              ing 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  com-
              mands.

       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 comm-
              SendReply, 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.



comm                          4.6.1                            14





comm(n)                Remote communication               comm(n)


       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



comm                          4.6.1                            15





comm(n)                Remote communication               comm(n)


              to avoid the use of comm:* procedures; this also cut the invoca-
              tion 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 pro-
              tocol  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 ver-
              sion 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, com-
              mDied 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.



comm                          4.6.1                            16





comm(n)                Remote communication               comm(n)


BUGS
       o      If  there  is  a  failure  initializing  a  channel created with
              ::comm::comm new, then the channel should  be  destroyed.   Cur-
              rently, it is left in an inconsistent state.

       o      There  should be a way to force a channel to quiesce when chang-
              ing 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 recom-
       mended you use Tcl8.0p1 (or Tcl7.6p2).

RELATED WORK
       Tcl-DP provides an RPC-based remote execution interface, but is a  com-
       piled    Tcl    extension.    See   http://www.cs.cornell.edu/Info/Pro-
       jects/zeno/Projects/Tcl-DP.html.




comm                          4.6.1                            17





comm(n)                Remote communication               comm(n)


       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://source-
       forge.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 execu-
       tion, 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                          4.6.1                            18


