1comm_wire(n) Remote communication comm_wire(n)
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8 comm_wire - The comm wire protocol
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11 package require comm
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16 The comm command provides an inter-interpreter remote execution facil‐
17 ity much like Tk's send(n), except that it uses sockets rather than the
18 X server for the communication path. As a result, comm works with mul‐
19 tiple interpreters, works on Windows and Macintosh systems, and pro‐
20 vides control over the remote execution path.
21 This document contains a specification of the various versions of the
23 wire protocol used by comm internally for the communication between its
24 endpoints. It has no relevance to users of comm, only to developers who
25 wish to modify the package, write a compatible facility in a different
26 language, or some other facility based on the same protocol.
27
29 BASIC LAYER
30 The basic encoding for all data is UTF-8. Because of this binary data,
31 including the NULL character, can be sent over the wire as is, without
32 the need for armoring it.
33 BASIC MESSAGE LAYER
35 On top of the Basic Layer we have a message oriented exchange of data.
36 The totality of all characters written to the channel is a Tcl list,
37 with each element a separate message, each itself a list. The messages
38 in the overall list are separated by EOL. Note that EOL characters can
39 occur within the list as well. They can be distinguished from the mes‐
40 sage-separating EOL by the fact that the data from the beginning up to
41 their location is not a valid Tcl list.
42 EOL is signaled through the linefeed character, i.e LF, or, hex 0x0a.
44 This is following the unix convention for line-endings.
45 As a list each message is composed of words. Their meaning depends on
47 when the message was sent in the overall exchange. This is described in
48 the upcoming sections.
49 NEGOTIATION MESSAGES - INITIAL HANDSHAKE
51 The command protocol is defined like this:
52 · The first message send by a client to a server, when opening the
54 connection, contains two words. The first word is a list as
55 well, and contains the versions of the wire protocol the client
56 is willing to accept, with the most preferred version first. The
57 second word is the TCP port the client is listening on for con‐
58 nections to itself. The value 0 is used here to signal that the
59 client will not listen for connections, i.e. that it is purely
60 for sending commands, and not receiving them.
61 · The first message sent by the server to the client, in response
63 to the message above contains only one word. This word is a
64 list, containing the string vers as its first element, and the
65 version of the wire protocol the server has selected from the
66 offered versions as the second.
67 SCRIPT/COMMAND MESSAGES
69 All messages coming after the initial handshake consist of three words.
70 These are an instruction, a transaction id, and the payload. The valid
71 instructions are shown below. The transaction ids are used by the
72 client to match any incoming replies to the command messages it sent.
73 This means that a server has to copy the transaction id from a command
74 message to the reply it sends for that message.
75 send
77 async
79 command
81 The payload is the Tcl script to execute on the server. It is
82 actually a list containing the script fragments. These fragment
83 are concatenated together by the server to form the full script
84 to execute on the server side. This emulates the Tcl "eval"
85 semantics. In most cases it is best to have only one word in
86 the list, a list containing the exact command.
87 Examples:
89 (a) {send 1 {{array get tcl_platform}}}
92 (b) {send 1 {array get tcl_platform}}
93 (c) {send 1 {array {get tcl_platform}}}
94 are all valid representations of the same command. They are
96 generated via
97 (a') send {array get tcl_platform}
99 (b') send array get tcl_platform
100 (c') send array {get tcl_platform}
101 respectively
103 Note that (a), generated by (a'), is the usual form, if only
106 single commands are sent by the client. For example constructed
107 using list, if the command contains variable arguments. Like
108 send [list array get $the_variable]
111 These three instructions all invoke the script on the server
114 side. Their difference is in the treatment of result values, and
115 thus determines if a reply is expected.
116 send A reply is expected. The sender is waiting for the
118 result.
119 async No reply is expected, the sender has no interest in the
121 result and is not waiting for any.
122 command
124 A reply is expected, but the sender is not waiting for
125 it. It has arranged to get a process-internal notifica‐
126 tion when the result arrives.
127 reply Like the previous three command, however the tcl script in the
129 payload is highly restricted. It has to be a syntactically
130 valid Tcl return command. This contains result code, value,
131 error code, and error info.
132 Examples:
134 {reply 1 {return -code 0 {}}}
137 {reply 1 {return -code 0 {osVersion 2.4.21-99-default byteOrder littleEndian machine i686 platform unix os Linux user andreask wordSize 4}}}
138
141 This document, and the package it describes, will undoubtedly contain
142 bugs and other problems. Please report such in the category comm of
143 the Tcllib SF Trackers [http://source‐
144 forge.net/tracker/?group_id=12883]. Please also report any ideas for
145 enhancements you may have for either package and/or documentation.
146
148 comm
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151 comm, communication, ipc, message, remote communication, rpc, socket
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154 Copyright (c) 2005 Docs. Andreas Kupries <andreas_kupries@users.sourceforge.net>
155comm 3 comm_wire(n)