comm_wire(3) The comm wire protocol


package require comm


The comm command provides an inter-interpreter remote execution facility much like Tk's send(3tk), except that it uses sockets rather than the X server for the communication path. As a result, comm works with multiple interpreters, works on Windows and Macintosh systems, and provides control over the remote execution path.

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.



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.


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 message-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.


The command protocol is defined like this:
  • 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 connections to itself. The value 0 is used here to signal that the client will not listen for connections, i.e. that it is purely for sending commands, and not receiving them.
  • 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.


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.
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.


    (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}

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]

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.

A reply is expected. The sender is waiting for the result.
No reply is expected, the sender has no interest in the result and is not waiting for any.
A reply is expected, but the sender is not waiting for it. It has arranged to get a process-internal notification when the result arrives.
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.


    {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}}}


This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category comm of the Tcllib Trackers []. Please also report any ideas for enhancements you may have for either package and/or documentation.


comm, communication, ipc, message, remote communication, remote execution, rpc, socket


Programming tools


Copyright (c) 2005 Docs. Andreas Kupries <[email protected]>