


comm_wire(n)           Remote communication          comm_wire(n)


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NAME
       comm_wire - The comm wire protocol

SYNOPSIS
       package require comm

_________________________________________________________________

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 mul-
       tiple interpreters, works on Windows and Macintosh  systems,  and  pro-
       vides control over the remote execution path.

       This  document  contains a specification of the various versions of the
       wire protocol used by comm internally for the communication between its
       endpoints. It has no relevance to users of comm, only to developers who
       wish to modify the package, write a compatible facility in a  different
       language, or some other facility based on the same protocol.

WIRE PROTOCOL VERSION 3
   BASIC LAYER
       The  basic encoding for all data is UTF-8. Because of this binary data,
       including the NULL character, can be sent over the wire as is,  without
       the need for armoring it.

   BASIC MESSAGE LAYER
       On  top of the Basic Layer we have a message oriented exchange of data.
       The totality of all characters written to the channel is  a  Tcl  list,
       with  each element a separate message, each itself a list. The messages
       in the overall list are separated by EOL. Note that EOL characters  can
       occur  within the list as well. They can be distinguished from the mes-
       sage-separating EOL by the fact that the data from the beginning up  to
       their location is not a valid Tcl list.

       EOL  is  signaled through the linefeed character, i.e LF, or, hex 0x0a.
       This is following the unix convention for line-endings.

       As a list each message is composed of words. Their meaning  depends  on
       when the message was sent in the overall exchange. This is described in
       the upcoming sections.

   NEGOTIATION MESSAGES - INITIAL HANDSHAKE
       The command protocol is defined like this:

       o      The first message send by a client to a server, when opening the
              connection,  contains  two  words.  The  first word is a list as
              well, and contains the versions of the wire protocol the  client
              is willing to accept, with the most preferred version first. The
              second word is the TCP port the client is listening on for  con-
              nections  to itself. The value 0 is used here to signal that the



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comm_wire(n)           Remote communication          comm_wire(n)


              client will not listen for connections, i.e. that it  is  purely
              for sending commands, and not receiving them.

       o      The  first message sent by the server to the client, in response
              to the message above contains only one  word.  This  word  is  a
              list,  containing  the string vers as its first element, and the
              version of the wire protocol the server has  selected  from  the
              offered versions as the second.


   SCRIPT/COMMAND MESSAGES
       All messages coming after the initial handshake consist of three words.
       These are an instruction, a transaction id, and the payload. The  valid
       instructions  are  shown  below.  The  transaction  ids are used by the
       client to match any incoming replies to the command messages  it  sent.
       This  means that a server has to copy the transaction id from a command
       message to the reply it sends for that message.

       send

       async

       command
              The payload is the Tcl script to execute on the  server.  It  is
              actually  a list containing the script fragments. These fragment
              are concatenated together by the server to form the full  script
              to  execute  on  the  server side.  This emulates the Tcl "eval"
              semantics.  In most cases it is best to have only  one  word  in
              the list, a list containing the exact command.

              Examples:


                  (a)     {send 1 {{array get tcl_platform}}}
                  (b)     {send 1 {array get tcl_platform}}
                  (c)     {send 1 {array {get tcl_platform}}}

                  are all valid representations of the same command. They are
                  generated via

                  (a')    send {array get tcl_platform}
                  (b')    send array get tcl_platform
                  (c')    send array {get tcl_platform}

                  respectively


              Note  that  (a),  generated  by (a'), is the usual form, if only
              single commands are sent by the client.  For example constructed
              using list, if the command contains variable arguments. Like


                  send [list array get $the_variable]




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comm_wire(n)           Remote communication          comm_wire(n)


              These  three  instructions  all  invoke the script on the server
              side. Their difference is in the treatment of result values, and
              thus determines if a reply is expected.

              send   A  reply  is  expected.  The  sender  is  waiting for the
                     result.

              async  No reply is expected, the sender has no interest  in  the
                     result and is not waiting for any.

              command
                     A  reply  is  expected, but the sender is not waiting for
                     it. It has arranged to get a  process-internal  notifica-
                     tion when the result arrives.

       reply  Like  the  previous three command, however the tcl script in the
              payload is highly restricted.  It  has  to  be  a  syntactically
              valid  Tcl  return  command.  This  contains result code, value,
              error code, and error info.

              Examples:


                  {reply 1 {return -code 0 {}}}
                  {reply 1 {return -code 0 {osVersion 2.4.21-99-default byteOrder littleEndian machine i686 platform unix os Linux user andreask wordSize 4}}}



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
       comm

KEYWORDS
       comm, communication, ipc, message, remote communication, remote  execu-
       tion, rpc, socket

CATEGORY
       Programming tools

COPYRIGHT
       Copyright (c) 2005 Docs. Andreas Kupries <andreas_kupries@users.sourceforge.net>










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