comm_wire(3)

comm_wire(3tcl) Remote communication comm_wire(3tcl)

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NAME
comm_wire - The comm wire protocol

SYNOPSIS
package require comm

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DESCRIPTION
The comm command provides an inter-interpreter remote execution facil-
ity 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 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
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" se-
mantics. 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]

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

send A reply is expected. The sender is waiting for the re-
sult.

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, er-
ror 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 Trackers [http://core.tcl.tk/tcllib/reportlist]. Please
also report any ideas for enhancements you may have for either package
and/or documentation.

When proposing code changes, please provide unified diffs, i.e the out-
put of diff -u.

Note further that attachments are strongly preferred over inlined
patches. Attachments can be made by going to the Edit form of the
ticket immediately after its creation, and then using the left-most
button in the secondary navigation bar.