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" se‐
85 mantics. In most cases it is best to have only one word in the
86 list, a list containing the exact command.
87 Examples:
89 (a) {send 1 {{array get tcl_platform}}}
93 (b) {send 1 {array get tcl_platform}}
94 (c) {send 1 {array {get tcl_platform}}}
95 are all valid representations of the same command. They are
97 generated via
98 (a') send {array get tcl_platform}
100 (b') send array get tcl_platform
101 (c') send array {get tcl_platform}
102 respectively
104 Note that (a), generated by (a'), is the usual form, if only single
107 commands are sent by the client. For example constructed using list,
108 if the command contains variable arguments. Like
109 send [list array get $the_variable]
113 These three instructions all invoke the script on the server side.
116 Their difference is in the treatment of result values, and thus deter‐
117 mines if a reply is expected.
118 send A reply is expected. The sender is waiting for the re‐
120 sult.
121 async No reply is expected, the sender has no interest in the
123 result and is not waiting for any.
124 command
126 A reply is expected, but the sender is not waiting for
127 it. It has arranged to get a process-internal notifica‐
128 tion when the result arrives.
129 reply Like the previous three command, however the tcl script in the
131 payload is highly restricted. It has to be a syntactically
132 valid Tcl return command. This contains result code, value, er‐
133 ror code, and error info.
134 Examples:
136 {reply 1 {return -code 0 {}}}
140 {reply 1 {return -code 0 {osVersion 2.4.21-99-default byteOrder littleEndian machine i686 platform unix os Linux user andreask wordSize 4}}}
141
144 This document, and the package it describes, will undoubtedly contain
145 bugs and other problems. Please report such in the category comm of
146 the Tcllib Trackers [http://core.tcl.tk/tcllib/reportlist]. Please
147 also report any ideas for enhancements you may have for either package
148 and/or documentation.
149 When proposing code changes, please provide unified diffs, i.e the out‐
151 put of diff -u.
152 Note further that attachments are strongly preferred over inlined
154 patches. Attachments can be made by going to the Edit form of the
155 ticket immediately after its creation, and then using the left-most
156 button in the secondary navigation bar.
157
159 comm
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162 comm, communication, ipc, message, remote communication, remote execu‐
163 tion, rpc, socket
164
166 Programming tools
167
169 Copyright (c) 2005 Docs. Andreas Kupries <andreas_kupries@users.sourceforge.net>
170tcllib 3 comm_wire(n)