Send is a revelation. Many Tk (and even Tcl) programmers claim it as indispensable. Start by reading the distribution documentation mentioned above.

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Can someone sketch up an example of how to use send - demonstrating some of the things one should be certain to consider when working with send?

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Can different Tk bindings communicate through send? Yes--Perl/Tk, Tcl/Tk, and Tkinter are all happy sending messages to each other with built-in send. At some point, we need to present examples of this ... Steve Lidie devotes an entire chapter of his Perl/Tk book to send.

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LV Back in the olden days, there was also an X Windows system library that allowed one to communicate between Xt based applications, with a Tcl interpreter, and Tk applications via send.

However, I don't know of any method for non-GUI applications to communicate via send to Tk applications, nor of a way for Java applications to communicate via send with Tk applications.

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Start small with your send programming. Use a couple of simple Tk instances, to work out the security and application-name issues. Once those are properly settled, it's usually quick work to automate as much as you choose.

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CAVEAT: The Tk send command depends on working with an X server and thus does not work normally on Windows or MacOS. Check out dde for Windows and Tcl's AppleScript support for Mac.

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See also comm (a send look alike based on sockets which should work cross platform and cross machine), dde, and other methods listed in the Inventory of IPC methods.

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PT writes on 29 March 2002:

I have released a preliminary version of a send for MS Windows systems that uses COM to do the work of registering interpreters and passing messages around. See http://tclsoap.sourceforge.net/winsend.html for more information and for the package. The source is in the CVS repository there (cvs -d:pserver:[email protected]:/cvsroot/tclsoap co winsend).

At the moment (Nov 2002) I'm not sure how successful the winsend package has been. However, there have been over 200 downloads so I imagine it's been of some use to someone :)

DKF: A version of this (based on the spec in TIP#150[1]) is going to be in Tk 8.5a1 when that is released.

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Pat Thoyts gave this example for a send substitute with DDE in the Tcl chatroom:

 # Emulate the 'send' command using the dde package if available.
 if {[info command send] == {}} {
    if {![catch {package require dde}]} {
        dde servername TkInspect-[pid]
        proc send {app args} {
            eval dde eval [list $app] $args
        }
    }
 }

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PT writes:

This is a suggestion for a generalisation of the send functionality. This is derived from the tkinspect send handling and should help a little with the use of send in a cross-platform application.

 # send.tcl - Copyright (C) 2002 Pat Thoyts <[email protected]>
 #
 # General send command
 #
 # This provides an abstraction of send, appname and interps that works for
 # all available send mechanisms. This is especially useful for Windows where
 # the usual Tk send is not available.
 #
 # We attempt to use the most efficient transport available. If the normal Tk
 # send command is present then this will be used. Otherwise we try winsend,
 # then dde for windows before defaulting to use comm.
 #
 # The interps command will try and build a list from all available transports.
 #
 # The appname command will try to apply the new name to all transports and
 # will read it from them all as well.
 #

 namespace eval send {
     variable version 1.0
     variable rcsid {$Id: 1055,v 1.23 2007-02-03 07:00:07 jcw Exp $}

     namespace export send interps appname
 }

 # Try using Tk send first, then look for a winsend interp,
 # then try dde and finally have a go at comm
 proc send::send {args} {
     array set opts [list displayof {} async 0]
     while {[string match -* [lindex $args 0]]} {
         switch -exact -- [lindex $args 0] {
             -displayof { set opts(displayof) [Pop args 1] }
             -async     { set opts(async) 1 }
             -- { Pop args ; break }
             default {
                 return -code error "bad option \"[lindex $args 0]\":\
                     should be -displayof, -async or --"
             }
         }
         Pop args
     }
     set app [Pop args]

     if {[info command ::winfo] != {}
         && [lsearch -exact [::winfo interps] $app] > -1} {
         set cmd ::send
         if {$opts(async) == 1} {append cmd " -async"}
         if {$opts(displayof) != {}} {append cmd " -displayof $opts(displayof)" }
         eval $cmd [list $app] $args
     } elseif {[info command ::winsend] != {}
               && [lsearch -exact [::winsend interps] $app] > -1} {
         eval ::winsend send [list $app] $args
     } elseif {[info command ::dde] != {}
               && [lsearch -exact [dde services TclEval {}] \
                       [list TclEval $app]] > -1} {
         eval ::dde eval [list $app] $args
     } elseif {[package provide comm] != {}
               && [string is integer [lindex $app 0]]} {
         if {$opts(displayof) != {} && [llength $app] == 1} {
             lappend app $opts(displayof)
         }
         eval ::comm::comm send [list $app] $args
     } else {
         return -code error "bad interp: \"$app\" could not be found"
     }
 }

 proc send::interps {args} {
     array set opts [list displayof {}]
     while {[string match -* [lindex $args 0]]} {
         switch -exact -- [lindex $args 0] {
             -displayof { set opts(displayof) [Pop args 1] }
             --         { Pop args ; break }
             default {
                 return -code error "bad option \"[lindex $args 0]\":\
                     should be -displayof or --"
             }
         }
         Pop args
     }

     set interps {}
     if {[info command ::winfo] != {}} {
         set cmd "::winfo interps"
         if {$opts(displayof) != {}} {
             append cmd " -displayof $opts(displayof)"
         }
         set interps [concat $interps [eval $cmd]]
     }
     if {[info command ::winsend] != {}} {
         set interps [concat $interps [::winsend interps]]
     }
     if {[info command ::dde] != {}} {
         set servers {}
         foreach server [::dde services TclEval {}] {
             lappend servers [lindex $server 1]
         }
         set interps [concat $interps $servers]
     }
     if {[package provide comm] != {}} {
         set interps [concat $interps [::comm::comm interps]]
     }
     return $interps
 }

 proc send::appname {{newname {}}} {
     set appname {}
     if {[info command ::tk] != {}} {
         set appname [eval ::tk appname $newname]
     }
     if {[info command ::winsend] != {}} {
         set appname [concat $appname [eval ::winsend appname $newname]]
     }
     if {[info command ::dde] != {}} {
         set appname [concat $appname [eval ::dde servername $newname]]
     }
     # comm? can set port num and local/global interface.
     return [lsort -unique $appname]
 }

 proc send::Pop {varname {nth 0}} {
     upvar $varname args
     set r [lindex $args $nth]
     set args [lreplace $args $nth $nth]
     return $r
 }

 package provide send $send::version

 # Local variables:
 #   mode: tcl
 #   indent-tabs-mode: nil
 # End:

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LV Has anyone investigated what would be necessary for non-Tk applications to send info to Tk apps in the same manner as the Tk send? For instance, if I have a Java application or some other language without a Tk binding...

Well, one thing that could be done is to write a Tclblend wrapper for the Java code. Then write some code that talks to the Java code and then acts as a send manager.

PT 26Jun2003: There is an Xt based library - tclXtSend [2] [3] which permits non-Tk applications to use send. It may be out of date though.

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LV Sometimes a Tk application dies and its interpreter is left registered with the window manager. When this happens, one can see things like this:

 $ wish
 % send Display.tcl puts $args
 can't read "args": no such variable
 % send Display.tcl info vars
 no application named "Display.tcl"
 % winfo interps
 wish Driver.tcl pilotManager xfilewatch.tcl
 % send Driver.tcl puts abc
 target application died or uses a Tk version before 4.0
 % winfo interps
 wish Driver.tcl pilotManager xfilewatch.tcl

Has anyone a tip on how to clear out that interpreter name?

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LV What are some techniques that a Tk application can use to discover to what applications it can send commands?

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kostix (17-Sep-2007) notes that you must enter event loop to use send -async properly. I don't know Tk internals but it looks like the first call to send -async in a chain always succeeds but if a series of these commands is fired in a quick succession, second and following commands get queued and won't be dispatched unless the event loop is running. If it's not running each new send -async will "pop" the command entered that "queue" first while your current command will be added to the end.

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LV So, does anyone have code demonstrating the starting of two or more tcl/tk threads and then the tk's sending information between one another? A simple illustration bit of code (that could be used to verify that send is working in one's environment) would be wonderful - an actually useful bit of code would be amazing.

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apptalk - simple front end to the Tk send command