[//000000001]: # (comm \- Remote communication) [//000000002]: # (Generated from file 'comm\.man' by tcllib/doctools with format 'markdown') [//000000003]: # (Copyright © 1995\-1998 The Open Group\. All Rights Reserved\.) [//000000004]: # (Copyright © 2003\-2004 ActiveState Corporation\.) [//000000005]: # (Copyright © 2006\-2009 Andreas Kupries ) [//000000006]: # (comm\(n\) 4\.7 tcllib "Remote communication")
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# NAME comm \- A remote communication facility for Tcl \(8\.5 and later\) # Table Of Contents - [Table Of Contents](#toc) - [Synopsis](#synopsis) - [Description](#section1) - [Commands](#subsection1) - [Eval Semantics](#subsection2) - [Multiple Channels](#subsection3) - [Channel Configuration](#subsection4) - [Id/port Assignments](#subsection5) - [Execution Environment](#subsection6) - [Remote Interpreters](#subsection7) - [Closing Connections](#subsection8) - [Callbacks](#subsection9) - [Unsupported](#subsection10) - [Security](#subsection11) - [Blocking Semantics](#subsection12) - [Asynchronous Result Generation](#subsection13) - [Compatibility](#subsection14) - [TLS Security Considerations](#section2) - [Author](#section3) - [License](#section4) - [Bugs](#section5) - [On Using Old Versions Of Tcl](#section6) - [Related Work](#section7) - [Bugs, Ideas, Feedback](#section8) - [See Also](#seealso) - [Keywords](#keywords) - [Category](#category) - [Copyright](#copyright) # SYNOPSIS package require Tcl 8\.5 package require comm ?4\.7? [__::comm::comm send__ ?\-async? ?\-command *callback*? *id* *cmd* ?*arg arg \.\.\.*?](#1) [__::comm::comm self__](#2) [__::comm::comm interps__](#3) [__::comm::comm connect__ ?*id*?](#4) [__::comm::comm new__ *chan* ?*name value \.\.\.*?](#5) [__::comm::comm channels__](#6) [__::comm::comm config__](#7) [__::comm::comm config__ *name*](#8) [__::comm::comm config__ ?*name* *value* *\.\.\.*?](#9) [__::comm::comm shutdown__ *id*](#10) [__::comm::comm abort__](#11) [__::comm::comm destroy__](#12) [__::comm::comm hook__ *event* ?__\+__? ?*script*?](#13) [__::comm::comm remoteid__](#14) [__::comm::comm\_send__](#15) [__::comm::comm return\_async__](#16) [__$future__ __return__ ?__\-code__ *code*? ?*value*?](#17) [__$future__ __configure__ ?__\-command__ ?*cmdprefix*??](#18) [__$future__ __cget__ __\-command__](#19) # DESCRIPTION The __comm__ command provides an inter\-interpreter remote execution facility much like Tk's __send\(n\)__, 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\. These commands work just like __[send](\.\./\.\./\.\./\.\./index\.md\#send)__ and __winfo interps__ : ::comm::comm send ?-async? id cmd ?arg arg ...? ::comm::comm interps This is all that is really needed to know in order to use __comm__ ## Commands The package initializes __::comm::comm__ as the default *chan*\. __comm__ names communication endpoints with an *id* unique to each machine\. Before sending commands, the *id* of another interpreter is needed\. Unlike Tk's send, __comm__ doesn't implicitly know the *id*'s of all the interpreters on the system\. The following four methods make up the basic __comm__ interface\. - __::comm::comm send__ ?\-async? ?\-command *callback*? *id* *cmd* ?*arg arg \.\.\.*? This invokes the given command in the interpreter named by *id*\. The command waits for the result and remote errors are returned unless the __\-async__ or __\-command__ option is given\. If __\-async__ is given, send returns immediately and there is no further notification of result\. If __\-command__ is used, *callback* specifies a command to invoke when the result is received\. These options are mutually exclusive\. The callback will receive arguments in the form *\-option value*, suitable for __array set__\. The options are: *\-id*, the comm id of the interpreter that received the command; *\-serial*, a unique serial for each command sent to a particular comm interpreter; *\-chan*, the comm channel name; *\-code*, the result code of the command; *\-errorcode*, the errorcode, if any, of the command; *\-errorinfo*, the errorinfo, if any, of the command; and *\-result*, the return value of the command\. If connection is lost before a reply is received, the callback will be invoked with a connection lost message with \-code equal to \-1\. When __\-command__ is used, the command returns the unique serial for the command\. - __::comm::comm self__ Returns the *id* for this channel\. - __::comm::comm interps__ Returns a list of all the remote *id*'s to which this channel is connected\. __comm__ learns a new remote *id* when a command is first issued it, or when a remote *id* first issues a command to this comm channel\. __::comm::comm ids__ is an alias for this method\. - __::comm::comm connect__ ?*id*? Whereas __::comm::comm send__ will automatically connect to the given *id*, this forces a connection to a remote *id* without sending a command\. After this, the remote *id* will appear in __::comm::comm interps__\. ## Eval Semantics The evaluation semantics of __::comm::comm send__ are intended to match Tk's __[send](\.\./\.\./\.\./\.\./index\.md\#send)__ *exactly*\. This means that __comm__ evaluates arguments on the remote side\. If you find that __::comm::comm send__ doesn't work for a particular command, try the same thing with Tk's send and see if the result is different\. If there is a problem, please report it\. For instance, there was had one report that this command produced an error\. Note that the equivalent __[send](\.\./\.\./\.\./\.\./index\.md\#send)__ command also produces the same error\. % ::comm::comm send id llength {a b c} wrong # args: should be "llength list" % send name llength {a b c} wrong # args: should be "llength list" The __eval__ hook \(described below\) can be used to change from __[send](\.\./\.\./\.\./\.\./index\.md\#send)__'s double eval semantics to single eval semantics\. ## Multiple Channels More than one __comm__ channel \(or *listener*\) can be created in each Tcl interpreter\. This allows flexibility to create full and restricted channels\. For instance, *[hook](\.\./\.\./\.\./\.\./index\.md\#hook)* scripts are specific to the channel they are defined against\. - __::comm::comm new__ *chan* ?*name value \.\.\.*? This creates a new channel and Tcl command with the given channel name\. This new command controls the new channel and takes all the same arguments as __::comm::comm__\. Any remaining arguments are passed to the __config__ method\. The fully qualified channel name is returned\. - __::comm::comm channels__ This lists all the channels allocated in this Tcl interpreter\. The default configuration parameters for a new channel are: "-port 0 -local 1 -listen 0 -silent 0" The default channel __::comm::comm__ is created with: "::comm::comm new ::comm::comm -port 0 -local 1 -listen 1 -silent 0" ## Channel Configuration The __config__ method acts similar to __fconfigure__ in that it sets or queries configuration variables associated with a channel\. - __::comm::comm config__ - __::comm::comm config__ *name* - __::comm::comm config__ ?*name* *value* *\.\.\.*? When given no arguments, __config__ returns a list of all variables and their value With one argument, __config__ returns the value of just that argument\. With an even number of arguments, the given variables are set to the given values\. These configuration variables can be changed \(descriptions of them are elsewhere in this manual page\): - __\-listen__ ?*0|1*? - __\-local__ ?*0|1*? - __\-port__ ?*port*? - __\-silent__ ?*0|1*? - __\-socketcmd__ ?*commandname*? - __\-interp__ ?*interpreter*? - __\-events__ ?*eventlist*? These configuration variables are read only: - __\-chan__ *chan* - __\-serial__ *n* - __\-socket__ sock*In* When __config__ changes the parameters of an existing channel \(with the exception of __\-interp__ and __\-events__\), it closes and reopens the listening socket\. An automatically assigned channel *id* will change when this happens\. Recycling the socket is done by invoking __::comm::comm abort__, which causes all active sends to terminate\. ## Id/port Assignments __comm__ uses a TCP port for endpoint *id*\. The __interps__ \(or __ids__\) method merely lists all the TCP ports to which the channel is connected\. By default, each channel's *id* is randomly assigned by the operating system \(but usually starts at a low value around 1024 and increases each time a new socket is opened\)\. This behavior is accomplished by giving the __\-port__ config option a value of 0\. Alternately, a specific TCP port number may be provided for a given channel\. As a special case, comm contains code to allocate a a high\-numbered TCP port \(>10000\) by using __\-port \{\}__\. Note that a channel won't be created and initialized unless the specific port can be allocated\. As a special case, if the channel is configured with __\-listen 0__, then it will not create a listening socket and will use an id of __0__ for itself\. Such a channel is only good for outgoing connections \(although once a connection is established, it can carry send traffic in both directions\)\. As another special case, if the channel is configured with __\-silent 0__, then the listening side will ignore connection attempts where the protocol negotiation phase failed, instead of throwing an error\. ## Execution Environment A communication channel in its default configuration will use the current interpreter for the execution of all received scripts, and of the event scripts associated with the various hooks\. This insecure setup can be changed by the user via the two options __\-interp__, and __\-events__\. When __\-interp__ is set all received scripts are executed in the slave interpreter specified as the value of the option\. This interpreter is expected to exist before configuration\. I\.e\. it is the responsibility of the user to create it\. However afterward the communication channel takes ownership of this interpreter, and will destroy it when the communication channel is destroyed\. Note that reconfiguration of the communication channel to either a different interpreter or the empty string will release the ownership *without* destroying the previously configured interpreter\. The empty string has a special meaning, it restores the default behaviour of executing received scripts in the current interpreter\. *Also of note* is that replies and callbacks \(a special form of reply\) are *not* considered as received scripts\. They are trusted, part of the internal machinery of comm, and therefore always executed in the current interpreter\. Even if an interpreter has been configured as the execution environment for received scripts the event scripts associated with the various hooks will by default still be executed in the current interpreter\. To change this use the option __\-events__ to declare a list of the events whose scripts should be executed in the declared interpreter as well\. The contents of this option are ignored if the communication channel is configured to execute received scripts in the current interpreter\. ## Remote Interpreters By default, each channel is restricted to accepting connections from the local system\. This can be overridden by using the __\-local 0__ configuration option For such channels, the *id* parameter takes the form *\{ id host \}*\. *WARNING*: The *host* must always be specified in the same form \(e\.g\., as either a fully qualified domain name, plain hostname or an IP address\)\. ## Closing Connections These methods give control over closing connections: - __::comm::comm shutdown__ *id* This closes the connection to *id*, aborting all outstanding commands in progress\. Note that nothing prevents the connection from being immediately reopened by another incoming or outgoing command\. - __::comm::comm abort__ This invokes shutdown on all open connections in this comm channel\. - __::comm::comm destroy__ This aborts all connections and then destroys the this comm channel itself, including closing the listening socket\. Special code allows the default __::comm::comm__ channel to be closed such that the __::comm::comm__ command it is not destroyed\. Doing so closes the listening socket, preventing both incoming and outgoing commands on the channel\. This sequence reinitializes the default channel: "::comm::comm destroy; ::comm::comm new ::comm::comm" When a remote connection is lost \(because the remote exited or called __shutdown__\), __comm__ can invoke an application callback\. This can be used to cleanup or restart an ancillary process, for instance\. See the *lost* callback below\. ## Callbacks This is a mechanism for setting hooks for particular events: - __::comm::comm hook__ *event* ?__\+__? ?*script*? This uses a syntax similar to Tk's __[bind](\.\./\.\./\.\./\.\./index\.md\#bind)__ command\. Prefixing *script* with a __\+__ causes the new script to be appended\. Without this, a new *script* replaces any existing script\. When invoked without a script, no change is made\. In all cases, the new hook script is returned by the command\. When an *event* occurs, the *script* associated with it is evaluated with the listed variables in scope and available\. The return code \(*not* the return value\) of the script is commonly used decide how to further process after the hook\. Common variables include: * __chan__ the name of the comm channel \(and command\) * __id__ the id of the remote in question * __fid__ the file id for the socket of the connection These are the defined *events*: - __connecting__ Variables: __chan__, __id__ This hook is invoked before making a connection to the remote named in *id*\. An error return \(via __[error](\.\./\.\./\.\./\.\./index\.md\#error)__\) will abort the connection attempt with the error\. Example: % ::comm::comm hook connecting { if {[string match {*[02468]} $id]} { error "Can't connect to even ids" } } % ::comm::comm send 10000 puts ok Connect to remote failed: Can't connect to even ids % - __connected__ Variables: __chan__, __fid__, __id__, __host__, and __port__\. This hook is invoked immediately after making a remote connection to *id*, allowing arbitrary authentication over the socket named by *fid*\. An error return \(via __[error](\.\./\.\./\.\./\.\./index\.md\#error)__ \) will close the connection with the error\. *host* and *port* are merely extracted from the *id*; changing any of these will have no effect on the connection, however\. It is also possible to substitute and replace *fid*\. - __incoming__ Variables: __chan__, __fid__, __addr__, and __remport__\. Hook invoked when receiving an incoming connection, allowing arbitrary authentication over socket named by *fid*\. An error return \(via __[error](\.\./\.\./\.\./\.\./index\.md\#error)__\) will close the connection with the error\. Note that the peer is named by *remport* and *addr* but that the remote *id* is still unknown\. Example: ::comm::comm hook incoming { if {[string match 127.0.0.1 $addr]} { error "I don't talk to myself" } } - __eval__ Variables: __chan__, __id__, __cmd__, and __buffer__\. This hook is invoked after collecting a complete script from a remote but *before* evaluating it\. This allows complete control over the processing of incoming commands\. *cmd* contains either __send__ or __async__\. *buffer* holds the script to evaluate\. At the time the hook is called, *$chan remoteid* is identical in value to *id*\. By changing *buffer*, the hook can change the script to be evaluated\. The hook can short circuit evaluation and cause a value to be immediately returned by using __[return](\.\./\.\./\.\./\.\./index\.md\#return)__ *result* \(or, from within a procedure, __return \-code return__ *result*\)\. An error return \(via __[error](\.\./\.\./\.\./\.\./index\.md\#error)__\) will return an error result, as is if the script caused the error\. Any other return will evaluate the script in *buffer* as normal\. For compatibility with 3\.2, __break__ and __return \-code break__ *result* is supported, acting similarly to __return \{\}__ and __return \-code return__ *result*\. Examples: 1. augmenting a command % ::comm::comm send [::comm::comm self] pid 5013 % ::comm::comm hook eval {puts "going to execute $buffer"} % ::comm::comm send [::comm::comm self] pid going to execute pid 5013 1. short circuiting a command % ::comm::comm hook eval {puts "would have executed $buffer"; return 0} % ::comm::comm send [::comm::comm self] pid would have executed pid 0 1. Replacing double eval semantics % ::comm::comm send [::comm::comm self] llength {a b c} wrong # args: should be "llength list" % ::comm::comm hook eval {return [uplevel #0 $buffer]} return [uplevel #0 $buffer] % ::comm::comm send [::comm::comm self] llength {a b c} 3 1. Using a slave interpreter % interp create foo % ::comm::comm hook eval {return [foo eval $buffer]} % ::comm::comm send [::comm::comm self] set myvar 123 123 % set myvar can't read "myvar": no such variable % foo eval set myvar 123 1. Using a slave interpreter \(double eval\) % ::comm::comm hook eval {return [eval foo eval $buffer]} 1. Subverting the script to execute % ::comm::comm hook eval { switch -- $buffer { a {return A-OK} b {return B-OK} default {error "$buffer is a no-no"} } } % ::comm::comm send [::comm::comm self] pid pid is a no-no % ::comm::comm send [::comm::comm self] a A-OK - __reply__ Variables: __chan__, __id__, __buffer__, __ret__, and __return\(\)__\. This hook is invoked after collecting a complete reply script from a remote but *before* evaluating it\. This allows complete control over the processing of replies to sent commands\. The reply *buffer* is in one of the following forms * return result * return \-code code result * return \-code code \-errorinfo info \-errorcode ecode msg For safety reasons, this is decomposed\. The return result is in *ret*, and the return switches are in the return array: * *return\(\-code\)* * *return\(\-errorinfo\)* * *return\(\-errorcode\)* Any of these may be the empty string\. Modifying these four variables can change the return value, whereas modifying *buffer* has no effect\. - __callback__ Variables: __chan__, __id__, __buffer__, __ret__, and __return\(\)__\. Similar to *reply*, but used for callbacks\. - __lost__ Variables: __chan__, __id__, and __reason__\. This hook is invoked when the connection to __id__ is lost\. Return value \(or thrown error\) is ignored\. *reason* is an explanatory string indicating why the connection was lost\. Example: ::comm::comm hook lost { global myvar if {$myvar(id) == $id} { myfunc return } } ## Unsupported These interfaces may change or go away in subsequence releases\. - __::comm::comm remoteid__ Returns the *id* of the sender of the last remote command executed on this channel\. If used by a proc being invoked remotely, it must be called before any events are processed\. Otherwise, another command may get invoked and change the value\. - __::comm::comm\_send__ Invoking this procedure will substitute the Tk __[send](\.\./\.\./\.\./\.\./index\.md\#send)__ and __winfo interps__ commands with these equivalents that use __::comm::comm__\. proc send {args} { eval ::comm::comm send $args } rename winfo tk_winfo proc winfo {cmd args} { if {![string match in* $cmd]} { return [eval [list tk_winfo $cmd] $args] } return [::comm::comm interps] } ## Security Starting with version 4\.6 of the package an option __\-socketcmd__ is supported, allowing the user of a comm channel to specify which command to use when opening a socket\. Anything which is API\-compatible with the builtin __::socket__ \(the default\) can be used\. The envisioned main use is the specification of the __tls::socket__ command, see package __[tls](\.\./\.\./\.\./\.\./index\.md\#tls)__, to secure the communication\. # Load and initialize tls package require tls tls::init -cafile /path/to/ca/cert -keyfile ... # Create secured comm channel ::comm::comm new SECURE -socketcmd tls::socket -listen 1 ... The sections [Execution Environment](#subsection6) and [Callbacks](#subsection9) are also relevant to the security of the system, providing means to restrict the execution to a specific environment, perform additional authentication, and the like\. ## Blocking Semantics There is one outstanding difference between __comm__ and __[send](\.\./\.\./\.\./\.\./index\.md\#send)__\. When blocking in a synchronous remote command, __[send](\.\./\.\./\.\./\.\./index\.md\#send)__ uses an internal C hook \(Tk\_RestrictEvents\) to the event loop to look ahead for send\-related events and only process those without processing any other events\. In contrast, __comm__ uses the __[vwait](\.\./\.\./\.\./\.\./index\.md\#vwait)__ command as a semaphore to indicate the return message has arrived\. The difference is that a synchronous __[send](\.\./\.\./\.\./\.\./index\.md\#send)__ will block the application and prevent all events \(including window related ones\) from being processed, while a synchronous __::comm::comm send__ will block the application but still allow other events to get processed\. In particular, __after idle__ handlers will fire immediately when comm blocks\. What can be done about this? First, note that this behavior will come from any code using __[vwait](\.\./\.\./\.\./\.\./index\.md\#vwait)__ to block and wait for an event to occur\. At the cost of multiple channel support, __comm__ could be changed to do blocking I/O on the socket, giving send\-like blocking semantics\. However, multiple channel support is a very useful feature of comm that it is deemed too important to lose\. The remaining approaches involve a new loadable module written in C \(which is somewhat against the philosophy of __comm__\) One way would be to create a modified version of the __[vwait](\.\./\.\./\.\./\.\./index\.md\#vwait)__ command that allow the event flags passed to Tcl\_DoOneEvent to be specified\. For __comm__, just the TCL\_FILE\_EVENTS would be processed\. Another way would be to implement a mechanism like Tk\_RestrictEvents, but apply it to the Tcl event loop \(since __comm__ doesn't require Tk\)\. One of these approaches will be available in a future __comm__ release as an optional component\. ## Asynchronous Result Generation By default the result returned by a remotely invoked command is the result sent back to the invoker\. This means that the result is generated synchronously, and the server handling the call is blocked for the duration of the command\. While this is tolerable as long as only short\-running commands are invoked on the server long\-running commands, like database queries make this a problem\. One command can prevent the processing requests of all other clients for an arbitrary period of time\. Before version 4\.5 of comm the only solution was to rewrite the server command to use the Tcl builtin command __[vwait](\.\./\.\./\.\./\.\./index\.md\#vwait)__, or one of its relatives like __tkwait__, to open a new event loop which processes requests while the long\-running operation is executed\. This however has its own perils, as this makes it possible to both overflow the Tcl stack with a large number of event loop, and to have a newer requests block the return of older ones, as the eventloop have to be unwound in the order of their creation\. The proper solution is to have the invoked command indicate to __comm__ that it cannot or will not deliver an immediate, synchronous result, but will do so later\. At that point the framework can put sending the actual result on hold and continue processing requests using the main event loop\. No blocking, no nesting of event loops\. At some future date the long running operation delivers the result to comm, via the future object, which is then forwarded to the invoker as usual\. The necessary support for this solution has been added to comm since version 4\.5, in the form of the new method __return\_async__\. - __::comm::comm return\_async__ This command is used by a remotely invoked script to notify the comm channel which invoked it that the result to send back to the invoker is not generated synchronously\. If this command is not called the default/standard behaviour of comm is to send the synchronously generated result of the script itself to the invoker\. The result of __return\_async__ is an object\. This object, called a *future* is where the result of the script has to be delivered to when it becomes ready\. When that happens it will take all the necessary actions to deliver the result to the invoker of the script, and then destroy itself\. Should comm have lost the connection to the invoker while the result is being computed the future will not try to deliver the result it got, but just destroy itself\. The future can be configured with a command to call when the invoker is lost\. This enables the user to implement an early abort of the long\-running operation, should this be supported by it\. An example: # Procedure invoked by remote clients to run database operations. proc select {sql} { # Signal the async generation of the result set future [::comm::comm return_async] # Generate an async db operation and tell it where to deliver the result. set query [db query -command [list $future return] $sql] # Tell the database system which query to cancel if the connection # goes away while it is running. $future configure -command [list db cancel $query] # Note: The above will work without problem only if the async # query will nover run its completion callback immediately, but # only from the eventloop. Because otherwise the future we wish to # configure may already be gone. If that is possible use 'catch' # to prevent the error from propagating. return } The API of a future object is: * __$future__ __return__ ?__\-code__ *code*? ?*value*? Use this method to tell the future that long\-running operation has completed\. Arguments are an optional return value \(defaults to the empty string\), and the Tcl return code \(defaults to OK\)\. The future will deliver this information to invoker, if the connection was not lost in the meantime, and then destroy itself\. If the connection was lost it will do nothing but destroy itself\. * __$future__ __configure__ ?__\-command__ ?*cmdprefix*?? * __$future__ __cget__ __\-command__ These methods allow the user to retrieve and set a command to be called if the connection the future belongs to has been lost\. ## Compatibility __comm__ exports itself as a package\. The package version number is in the form *major \. minor*, where the major version will only change when a non\-compatible change happens to the API or protocol\. Minor bug fixes and changes will only affect the minor version\. To load __comm__ this command is usually used: package require comm 3 Note that requiring no version \(or a specific version\) can also be done\. The revision history of __comm__ includes these releases: - 4\.6\.3 Fixed ticket \[ced0d60fc9\]\. Added proper detection of eof on a socket, properly closing it\. - 4\.6\.2 Fixed bugs 2972571 and 3066872, the first a misdetection of quoted brace after double backslash, the other a blocking gets making for an obvious \(hinsight\) DoS attack on comm channels\. - 4\.6\.1 Changed the implementation of __comm::commCollect__ to emulate lindex's pre\-Tcl 8 behaviour, i\.e\. it was given the ability to parse out the first word of a list, even if the whole buffer is not a well\-formed list\. Without this change the first word could only be extracted if the whole buffer was a well\-formed list \(ever since Tcl 8\), and in a ver\-high\-load situation, i\.e\. a server sending lots and/or large commands very fast, this may never happen, eventually crashing the receiver when it runs out of memory\. With the change the receiver is always able to process the first word when it becomes well\-formed, regardless of the structure of the remainder of the buffer\. - 4\.6 Added the option __\-socketcmd__ enabling users to override how a socket is opened\. The envisioned main use is the specification of the __tls::socket__ command, see package __[tls](\.\./\.\./\.\./\.\./index\.md\#tls)__, to secure the communication\. - 4\.5\.7 Changed handling of ports already in use to provide a proper error message\. - 4\.5\.6 Bugfix in the replacement for __[vwait](\.\./\.\./\.\./\.\./index\.md\#vwait)__, made robust against of variable names containing spaces\. - 4\.5\.5 Bugfix in the handling of hooks, typo in variable name\. - 4\.5\.4 Bugfix in the handling of the result received by the __send__ method\. Replaced an *after idle unset result* with an immediate __unset__, with the information saved to a local variable\. The __after idle__ can spill into a forked child process if there is no event loop between its setup and the fork\. This may bork the child if the next event loop is the __[vwait](\.\./\.\./\.\./\.\./index\.md\#vwait)__ of __comm__'s __send__ a few lines above the __after idle__, and the child used the same serial number for its next request\. In that case the parent's __after idle unset__ will delete the very array element the child is waiting for, unlocking the __[vwait](\.\./\.\./\.\./\.\./index\.md\#vwait)__, causing it to access a now missing array element, instead of the expected result\. - 4\.5\.3 Bugfixes in the wrappers for the builtin __[update](\.\./\.\./\.\./\.\./index\.md\#update)__ and __[vwait](\.\./\.\./\.\./\.\./index\.md\#vwait)__ commands\. - 4\.5\.2 Bugfix in the wrapper for the builtin __[update](\.\./\.\./\.\./\.\./index\.md\#update)__ command\. - 4\.5\.1 Bugfixes in the handling of \-interp for regular scripts\. The handling of the buffer was wrong for scripts which are a single statement as list\. Fixed missing argument to new command __commSendReply__, introduced by version 4\.5\. Affected debugging\. - 4\.5 New server\-side feature\. The command invoked on the server can now switch comm from the standard synchronous return of its result to an asynchronous \(defered\) return\. Due to the use of snit to implement the *future* objects used by this feature from this version on comm requires at least Tcl 8\.3 to run\. Please read the section [Asynchronous Result Generation](#subsection13) for more details\. - 4\.4\.1 Bugfix in the execution of hooks\. - 4\.4 Bugfixes in the handling of \-interp for regular and hook scripts\. Bugfixes in channel cleanup\. - 4\.3\.1 Introduced \-interp and \-events to enable easy use of a slave interp for execution of received scripts, and of event scripts\. - 4\.3 Bugfixes, and introduces \-silent to allow the user to force the server/listening side to silently ignore connection attempts where the protocol negotiation failed\. - 4\.2 Bugfixes, and most important, switched to utf\-8 as default encoding for full i18n without any problems\. - 4\.1 Rewrite of internal code to remove old pseudo\-object model\. Addition of send \-command asynchronous callback option\. - 4\.0 Per request by John LoVerso\. Improved handling of error for async invoked commands\. - 3\.7 Moved into tcllib and placed in a proper namespace\. - 3\.6 A bug in the looking up of the remoteid for a executed command could be triggered when the connection was closed while several asynchronous sends were queued to be executed\. - 3\.5 Internal change to how reply messages from a __[send](\.\./\.\./\.\./\.\./index\.md\#send)__ are handled\. Reply messages are now decoded into the *value* to pass to __[return](\.\./\.\./\.\./\.\./index\.md\#return)__; a new return statement is then cons'd up to with this value\. Previously, the return code was passed in from the remote as a command to evaluate\. Since the wire protocol has not changed, this is still the case\. Instead, the reply handling code decodes the __reply__ message\. - 3\.4 Added more source commentary, as well as documenting config variables in this man page\. Fixed bug were loss of connection would give error about a variable named __pending__ rather than the message about the lost connection\. __comm ids__ is now an alias for __comm interps__ \(previously, it an alias for __comm chans__\)\. Since the method invocation change of 3\.0, break and other exceptional conditions were not being returned correctly from __comm send__\. This has been fixed by removing the extra level of indirection into the internal procedure __commSend__\. Also added propagation of the *errorCode* variable\. This means that these commands return exactly as they would with __[send](\.\./\.\./\.\./\.\./index\.md\#send)__: comm send id break catch {comm send id break} comm send id expr 1 / 0 Added a new hook for reply messages\. Reworked method invocation to avoid the use of comm:\* procedures; this also cut the invocation time down by 40%\. Documented __comm config__ \(as this manual page still listed the defunct __comm init__\!\) - 3\.3 Some minor bugs were corrected and the documentation was cleaned up\. Added some examples for hooks\. The return semantics of the __eval__ hook were changed\. - 3\.2 A new wire protocol, version 3, was added\. This is backwards compatible with version 2 but adds an exchange of supported protocol versions to allow protocol negotiation in the future\. Several bugs with the hook implementation were fixed\. A new section of the man page on blocking semantics was added\. - 3\.1 All the documented hooks were implemented\. __commLostHook__ was removed\. A bug in __comm new__ was fixed\. - 3\.0 This is a new version of __comm__ with several major changes\. There is a new way of creating the methods available under the __comm__ command\. The __comm init__ method has been retired and is replaced by __comm configure__ which allows access to many of the well\-defined internal variables\. This also generalizes the options available to __comm new__\. Finally, there is now a protocol version exchanged when a connection is established\. This will allow for future on\-wire protocol changes\. Currently, the protocol version is set to 2\. - 2\.3 __comm ids__ was renamed to __comm channels__\. General support for __comm hook__ was fully implemented, but only the *lost* hook exists, and it was changed to follow the general hook API\. __commLostHook__ was unsupported \(replaced by __comm hook lost__\) and __commLost__ was removed\. - 2\.2 The *died* hook was renamed *lost*, to be accessed by __commLostHook__ and an early implementation of __comm lost hook__\. As such, __commDied__ is now __commLost__\. - 2\.1 Unsupported method __comm remoteid__ was added\. - 2\.0 __comm__ has been rewritten from scratch \(but is fully compatible with Comm 1\.0, without the requirement to use obTcl\)\. # TLS Security Considerations This package uses the __[TLS](\.\./\.\./\.\./\.\./index\.md\#tls)__ package to handle the security for __https__ urls and other socket connections\. Policy decisions like the set of protocols to support and what ciphers to use are not the responsibility of __[TLS](\.\./\.\./\.\./\.\./index\.md\#tls)__, nor of this package itself however\. Such decisions are the responsibility of whichever application is using the package, and are likely influenced by the set of servers the application will talk to as well\. For example, in light of the recent [POODLE attack](http://googleonlinesecurity\.blogspot\.co\.uk/2014/10/this\-poodle\-bites\-exploiting\-ssl\-30\.html) discovered by Google many servers will disable support for the SSLv3 protocol\. To handle this change the applications using __[TLS](\.\./\.\./\.\./\.\./index\.md\#tls)__ must be patched, and not this package, nor __[TLS](\.\./\.\./\.\./\.\./index\.md\#tls)__ itself\. Such a patch may be as simple as generally activating __tls1__ support, as shown in the example below\. package require tls tls::init -tls1 1 ;# forcibly activate support for the TLS1 protocol ... your own application code ... # Author John LoVerso, John@LoVerso\.Southborough\.MA\.US *http://www\.opengroup\.org/~loverso/tcl\-tk/\#comm* # License Please see the file *comm\.LICENSE* that accompanied this source, or [http://www\.opengroup\.org/www/dist\_client/caubweb/COPYRIGHT\.free\.html](http://www\.opengroup\.org/www/dist\_client/caubweb/COPYRIGHT\.free\.html)\. This license for __comm__, new as of version 3\.2, allows it to be used for free, without any licensing fee or royalty\. # Bugs - If there is a failure initializing a channel created with __::comm::comm new__, then the channel should be destroyed\. Currently, it is left in an inconsistent state\. - There should be a way to force a channel to quiesce when changing the configuration\. The following items can be implemented with the existing hooks and are listed here as a reminder to provide a sample hook in a future version\. - Allow easier use of a slave interp for actual command execution \(especially when operating in "not local" mode\)\. - Add host list \(xhost\-like\) or "magic cookie" \(xauth\-like\) authentication to initial handshake\. The following are outstanding todo items\. - Add an interp discovery and name\->port mapping\. This is likely to be in a separate, optional nameserver\. \(See also the related work, below\.\) - Fix the *\{id host\}* form so as not to be dependent upon canonical hostnames\. This requires fixes to Tcl to resolve hostnames\! This man page is bigger than the source file\. # On Using Old Versions Of Tcl Tcl7\.5 under Windows contains a bug that causes the interpreter to hang when EOF is reached on non\-blocking sockets\. This can be triggered with a command such as this: "comm send $other exit" Always make sure the channel is quiescent before closing/exiting or use at least Tcl7\.6 under Windows\. Tcl7\.6 on the Mac contains several bugs\. It is recommended you use at least Tcl7\.6p2\. Tcl8\.0 on UNIX contains a socket bug that can crash Tcl\. It is recommended you use Tcl8\.0p1 \(or Tcl7\.6p2\)\. # Related Work Tcl\-DP provides an RPC\-based remote execution interface, but is a compiled Tcl extension\. See [http://www\.cs\.cornell\.edu/Info/Projects/zeno/Projects/Tcl\-DP\.html](http://www\.cs\.cornell\.edu/Info/Projects/zeno/Projects/Tcl\-DP\.html)\. Michael Doyle has code that implements the Tcl\-DP RPC interface using standard Tcl sockets, much like __comm__\. The DpTcl package is available at [http://chiselapp\.com/user/gwlester/repository/DpTcl](http://chiselapp\.com/user/gwlester/repository/DpTcl)\. Andreas Kupries uses __comm__ and has built a simple nameserver as part of his Pool library\. See [http://www\.purl\.org/net/akupries/soft/pool/index\.htm](http://www\.purl\.org/net/akupries/soft/pool/index\.htm)\. # 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 output 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\. # SEE ALSO send\(n\) # KEYWORDS [comm](\.\./\.\./\.\./\.\./index\.md\#comm), [communication](\.\./\.\./\.\./\.\./index\.md\#communication), [ipc](\.\./\.\./\.\./\.\./index\.md\#ipc), [message](\.\./\.\./\.\./\.\./index\.md\#message), [remote communication](\.\./\.\./\.\./\.\./index\.md\#remote\_communication), [remote execution](\.\./\.\./\.\./\.\./index\.md\#remote\_execution), [rpc](\.\./\.\./\.\./\.\./index\.md\#rpc), [secure](\.\./\.\./\.\./\.\./index\.md\#secure), [send](\.\./\.\./\.\./\.\./index\.md\#send), [socket](\.\./\.\./\.\./\.\./index\.md\#socket), [ssl](\.\./\.\./\.\./\.\./index\.md\#ssl), [tls](\.\./\.\./\.\./\.\./index\.md\#tls) # CATEGORY Programming tools # COPYRIGHT Copyright © 1995\-1998 The Open Group\. All Rights Reserved\. Copyright © 2003\-2004 ActiveState Corporation\. Copyright © 2006\-2009 Andreas Kupries