'\"
'\" Generated from file 'comm_wire\&.man' by tcllib/doctools with format 'nroff'
'\" Copyright (c) 2005 Docs\&. Andreas Kupries <andreas_kupries@users\&.sourceforge\&.net>
'\"
.TH "comm_wire" n 3 tcllib "Remote communication"
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.SH NAME
comm_wire \- The comm wire protocol
.SH SYNOPSIS
package require \fBcomm\fR
.sp
.BE
.SH DESCRIPTION
.PP
The \fBcomm\fR command provides an inter-interpreter remote
execution facility much like Tk's \fBsend(n)\fR, except that it uses
sockets rather than the X server for the communication path\&.  As a
result, \fBcomm\fR works with multiple interpreters, works on
Windows and Macintosh systems, and provides control over the remote
execution path\&.
.PP
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 \fBcomm\fR, 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\&.
.SH "WIRE PROTOCOL VERSION 3"
.SS "BASIC LAYER"
The basic encoding for \fIall\fR 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\&.
.SS "BASIC MESSAGE LAYER"
On top of the \fBBasic Layer\fR we have a
\fImessage oriented\fR exchange of data\&.
The totality of all characters written to the channel is a Tcl list,
with each element a separate \fImessage\fR, 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\&.
.PP
EOL is signaled through the linefeed character, i\&.e \fBLF\fR, or,
hex \fB0x0a\fR\&. This is following the unix convention for
line-endings\&.
.PP
As a list each message is composed of \fIwords\fR\&. Their meaning
depends on when the message was sent in the overall exchange\&. This is
described in the upcoming sections\&.
.SS "NEGOTIATION MESSAGES - INITIAL HANDSHAKE"
The command protocol is defined like this:
.IP \(bu
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 \fB0\fR 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\&.
.IP \(bu
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 \fBvers\fR as its first element, and the version of the
wire protocol the server has selected from the offered versions as the
second\&.
.PP
.SS "SCRIPT/COMMAND MESSAGES"
All messages coming after the \fBinitial handshake\fR
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\&.
.TP
\fBsend\fR
.TP
\fBasync\fR
.TP
\fBcommand\fR
The payload is the Tcl script to execute on the server\&. It is actually
a list containing the script fragments\&. These fragment are
\fBconcat\fRenated 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\&.
.sp
Examples:
.sp
.CS


    (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

.CE
.sp
Note that (a), generated by (a'), is the usual form, if only single
commands are sent by the client\&.
For example constructed using \fBlist\fR, if the command contains
variable arguments\&. Like
.sp
.CS


    send [list array get $the_variable]

.CE
.sp
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\&.
.RS
.TP
\fBsend\fR
A reply is expected\&. The sender is waiting for the result\&.
.TP
\fBasync\fR
No reply is expected, the sender has no interest in the result and is
not waiting for any\&.
.TP
\fBcommand\fR
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\&.
.RE
.TP
\fBreply\fR
Like the previous three command, however the tcl script in the payload
is highly restricted\&.
It has to be a syntactically valid Tcl \fBreturn\fR command\&. This
contains result code, value, error code, and error info\&.
.sp
Examples:
.sp
.CS


    {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}}}

.CE
.PP
.SH "BUGS, IDEAS, FEEDBACK"
This document, and the package it describes, will undoubtedly contain
bugs and other problems\&.
Please report such in the category \fIcomm\fR of the
\fITcllib Trackers\fR [http://core\&.tcl\&.tk/tcllib/reportlist]\&.
Please also report any ideas for enhancements you may have for either
package and/or documentation\&.
.PP
When proposing code changes, please provide \fIunified diffs\fR,
i\&.e the output of \fBdiff -u\fR\&.
.PP
Note further that \fIattachments\fR are strongly preferred over
inlined patches\&. Attachments can be made by going to the \fBEdit\fR
form of the ticket immediately after its creation, and then using the
left-most button in the secondary navigation bar\&.
.SH "SEE ALSO"
comm
.SH KEYWORDS
comm, communication, ipc, message, remote communication, remote execution, rpc, socket
.SH CATEGORY
Programming tools
.SH COPYRIGHT
.nf
Copyright (c) 2005 Docs\&. Andreas Kupries <andreas_kupries@users\&.sourceforge\&.net>

.fi
