Subject: comp.lang.perl.tk FAQ part4 of 5
From: [email-masked] (Peter Prymmer)
Date: Mon, 07 Oct 1996 17:23:26 GMT
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Perl/Tk FAQ part 4 of 5 - More Perl/Tk     
************************************** 

 ______________________________________________________________________

 
 12. What are some of the primary differences between Tcl/Tk and
 Perl/Tk? 
 
 Considering that both interpreters/(compiler) for Tcl and Perl were
 written in C for original use on Unix computers it is not surprising that
 there are some similarities between the two languages. 
 
 Nevertheless, there are a large number of differences between the Tcl
 language and the Perl language. 
 
 Tom Christiansen (a definite perl proponent) has put up a web page that
 elucidates some critical technical differences between Tcl and Perl at: 
 
     http://www.perl.com/perl/versus/tcl-discussion.html
 
 Within each language there is Tk - a widget Toolkit. One must be careful
 that some of the Tcl/Tk widget names and options have been modified
 slightly in the perl/Tk language. 
 
 With Tk-b9.01 (and higher) a great many functions (method calls
 actually) start with an upper case letter and continue with all lower case
 letters (e.g. there is a perl/Tk Entry widget but no entry widget), and
 many configuration options are all lower case (e.g. there is a perl/Tk 
 highlightthickness option but no highlightThickness option).
 Thus if you are having trouble converting a script check your typing. (there
 is a script b9names to help). There is also a tcl2perl script (discussed
 later). 
 
 While the following table does not cover all the differences it is hoped that
 it will prove useful, especially to those people coming from a primarily
 Tcl/Tk background. These are some of the common Tcl->Perl stumbling
 points: 

 what              Tcl/Tk                 Perl/Tk
 variable          set a 123              $a = 123; or $a = '123';
  initialization
 re-assignment     set b $a               $b = $a;
 
 lists/arrays      set a {1 2 fred 7.8}   @a = (1,2,'fred',7.8);
 re-assignment     list set b $a          @b = @a;
 
 associative       set a(Jan) 456.02      %a = ('Jan',456.02,'Feb',534.96);
  arrays           set a(Feb) 534.96
 re-assignment     foreach i \            %b = %a;
                    [array names a] {
                    set b($i) = $a($i) }
 
 Note on the above examples:
 In Tcl the scalar, list, and array variable 'a' will overwrite each 
 previous assignment.
 In Perl $a, @a, %a are all distinct (occupy separate namespaces).
 
 expressions       set a [expr $b+$c]     $a = $b+$c;
 
 increment         incr i                 $i++; or ++$i;
 
 declare           proc plus {a b} {      sub plus { my($a,$b) = @_;
  subroutines       expr $a + $b }         $a+$b; }
 
 variable scope    local default          global default
                   override w/ "global"   override w/ "my" (or "local")
 
 call              plus 1 2               &plus(1,2); #or
  subroutines                             plus(1,2);  #OK after sub plus
 
 statement sep     newline or at ";"      ";" required
 
 statement         "\" - newline          none required
  continuation
 
 verbatim strings  {}                     ''
  e.g.             {a \ lot@ of $stuff}   'a \ lot@ of $stuff'
 
 escaped strings   ""                     ""
  e.g.             "Who\nWhat\nIdunno"    "Who\nWhat\nIdunno"
 
 STDOUT            puts "Hello World!"    print "Hello World!\n"
                   puts stdout "Hello!"   print STDOUT "Hello!\n"
 
 Note also that Tcl/Tk has a built-in abbreviation completion mechanism
 that lets you specify short hand, e.g. 
 
    canvas .frame.canvas -yscrollcommand ".frame.scroll set" ; #Tcl/Tk OK
    canvas .frame.canvas -yscroll ".frame.scroll set" ;        #Tcl/Tk also OK
    $canvas=$main->Canvas(-yscroll => ['set',$scroll]);  #ERROR perl/Tk
    $canvas=$main->Canvas(-yscrollcommand => ['set',$scroll]); #perl/Tk OK
 
 You may get around this with the perl abbrev.pl package in certain
 circumstances. For example: 
 
    require 'abbrev.pl';
    %foo = ();
    &abbrev(*foo,'-yscrollcommand');
  ...
    $canvas=$main->Canvas($foo{'-yscroll'} => ['set',$scroll]); #perl/Tk OK
 
 In Perl you can emulate the Tcl unknown proc (through the perl 
 AUTOLOAD mechanism) as follows: 
 
     use Shell;
     print($p = man(-k => bitmap));
 
 Which is equivalent to what you would get if you typed: 
 
     man -k bitmap
 
 From within tclsh or wish. (Thanks to Ilya Zakharevich 
 <[email-masked]> for pointing out this feature. ;-) 
 
 ______________________________________________________________________

 
 How do I install new scripts | modules | extensions? 
 
 (Thanks to Ilya Zakharevich <[email-masked]> for pointing out
 that perl code comes in a variety of flavors and some code requires more
 work than others to install. Hence I have expanded this topic and will refer
 to three distinct categories here: Scripts Modules and Extensions:) 
 
 Scripts
 -------
 
 A "self-contained" script needs little modification (in principle!) to run. It
 is a good idea to check the #! line at the very top of the file to reflect your
 local perl setup (e.g. #!/usr/bin/perl -w (change to) 
 #!/usr/gnu/local/perl -w or what have you). There are allegedly
 "more portable" ways to invoke the perl interpretor as well - they are
 more fully documented in the perl FAQ and the perlrun(1) man page,
 however. 
 
 Other things you do not want to forget when trying to run a perl script
 include giving yourself permission to do so, e.g.: 
 
     chmod u+x newscriptname
 
 You also want to be sure your DISPLAY environment variable is set up
 properly when attempting to run a perl/Tk script. You may also need to
 look at the xhost(1) or the xauth(1) man pages for setting up your
 X-display properly. 
 
 If you are still experiencing difficulty check to be sure that extraneous
 /newsgroup|e-mail|HTML headers|footers|markup//; are not in the file and
 that you have on hand all that is requireed or useed by the script (if not
 you may need to install a module - or even a perl4 style lib.pl file). 
 
 Modules
 -------
 
 Check out the module - make sure it is OK and will run on your system -
 does it require a specific location? For testing purposes (always a good
 idea) or if you do not have root priveleges set the file in a directory that
 you do have write access to and try to include it in a test script. Assuming
 you have a module to test called "Foo.pm" and are simply running the test
 script in the same directory as the module begin by adding to the @INC
 array like so: 
 
     #!/usr/bin/perl -w
      BEGIN { @INC = ("$ENV{'PWD'}",@INC); }
      use Tk;
      use Foo;
 
 or 
 
     #!/usr/bin/perl -w
      use lib $ENV{PWD};
      use Tk;
      use Foo;
 
 Another approach is to set either the PERLLIB or PERL5LIB
 environment variable from your shell. This method allows invoking the
 test script from within a number of different directories without having to
 edit a hard coded use lib or push(@INC,".") kind of statement within
 the script. Yet another way to do it is with the -I switch on the command
 line like so: 
 
     perl -Ipath/to/Foo -e fooscriptname
 
 After a successful test; if you are a system administrator, or have root
 priveleges, or are modifying your own copy of perl; then copy it to the 
 perl5/Tk directory. Depending on how the module was written it should
 be possible to use it either with the use Tk; statement itself or with an
 explicit use Tk::Foo; (for module perl5/Tk/Foo.pm). 
 
 Extensions (Overgrown Modules)
 ------------------------------
 
 These may come as a multi-file kit (tape archive usually) and may require
 a C compiler for part of the installation (perl/Tk itself falls into this
 category). You know you have an Overgrown Module (Extension) when
 there is one or more files with an .xs extension (perl->C meta code) and
 a Makefile.PL (perl->make meta code). One invokes the perl 
 MakeMaker on the file called Makefile.PL in order to create a 
 Makefile via: 
 
     perl Makefile.PL
 
 You may now run make on the resultant Makefile - but the details of
 this process are module dependent and should be documented in a README
 or an INSTALL file. A very standard perl extension requires 4 (or 5 if
 making static) standard commands to make and install: 
 
     perl Makefile.PL
     make
     make test
     make install
 
 If you have the appropriate CPAN and FTP modules already installed you
 can retrieve a module from CPAN and carry out all of the above steps with
 a perl one-liner like this: 
 
     perl -MCPAN -e 'install "Foo"'
 
 ______________________________________________________________________

 
 14. How do I write new modules? 
 
 You might want to start by poking around your perl/Tk build directory. Is
 there something there that already does what you want? Is there
 something that is reasonably close - but only requires minor modification?
 
 Next go through the various perl documents - including the FAQ as well
 as the various relevant man pages: perlmod(1), perlobj(1), 
 perlbot(1), (and please don't forget: perlpod(1)!) 
 
 Post your idea to comp.lang.perl.tk and discuss it with others - there might
 very well be someone working on an approach already. A clear explanation
 of all the stuff that gets put into a module was posted to the mailing list and
 can be found in the archive at: 
 
     http://sun20.ccd.bnl.gov/~ptk/archive/ptk.1995.10/0012.html
 
 Also, be sure to check out a recent version of the official Module List that
 Tim Bunce <[email-masked]> and Andreas Koenig 
 <[email-masked]> maintain and post to 
 comp.lang.perl.announce periodically. The list is also available at any
 CPAN ftp site as well as: 
 
     http://www.perl.com/CPAN/modules/00modlist.long.html
     ftp://rtfm.mit.edu/pub/usenet/news.answers/perl-faq/module-list
     ftp://ftp.demon.co.uk/pub/perl/db/mod/module-list.txt
     ftp://ftp.wpi.edu/perl5/Modules/module_list.txt
 
 Finally ready to ship? Small (perl/Tk) modules have been posted directly to
 comp.lang.perl.tk. Big modules may require ftp distribution (see upload
 info at one of the CPAN sites) then make your announcement to 
 comp.lang.perl.tk and possibly to comp.lang.perl.announce. 
 
 ______________________________________________________________________

 
 15. Composite Widgets. 
 
 Composite widgets combine the functions of two or more widget
 primitives into something that is not quite a stand alone program but is
 something that may prove very useful for inclusion in your own scripts. A
 variety of composite widgets have been written and many are still being
 worked on. Many come bundled with your perl/Tk distribution kit, and
 some are simply posted to comp.lang.perl.tk. It is quite common to have
 composite widgets written in perl modules - usually in terms of the Tk
 widget primitives. Graphical examples of some of the composites
 discussed here can be seen by GUI browsers at: 
 
     http://w4.lns.cornell.edu/~pvhp/ptk/ptkIMG.html
 
 ______________________________________________________________________

 
 15.1. How do I get a Dialog box? 
 
 For things like a simple "are you sure?" dialog box you might want to take
 a look at perl5/Tk/Dialog.pm. This module may be invoked with 
 require Tk::Dialog; etc. - there are much more extensive directions
 inside the comment fields at the top of the Dialog.pm file itself. The
 module has a lot of options and has a tutorial driver script in 
 perl5/Tk/demos/dialog. Dialog.pm is also used by the 
 perl5/Tk/demos/widget demo. In particular look at 
 perl5/Tk/demos/widget_lib/dialog1.pl and dialog2.pl for
 examples of how one makes use of Tk::Dialog. A snippet of a script that
 uses this module could look like: 
 
     require Tk::Dialog;
 
     my $mw = MainWindow->new;
     my $D = $mw->Dialog(
                  -title => 'Are you sure?',
                  -text  => "You have requested rm \*\nAre you sure?",
                  -default_button => 'No',
                  -buttons        => ['No','yes']
                        );
     my $choice = $D->Show;  # use Show for Tk-b9.01
 # if using Tk-b8:    my $choice = $D->show;
 
     print " you chose $choice \n";
 
 A question concerning configuration of the Subwidgets on the Dialogs
 came up recently: 
 
 <[email-masked]> wrote:
 ! I want to reconfigure the colors of the Dialog and
 ! ErrorDialog buttons.  How do I do this?
 ! Thanks in advance.
 
    $dialog_widget->configure(-background => 'purple'); 
 
  Since these two widgets are composites you manage them like any 'ol
  widget. If the default delegate subwidget(s) aren't to your liking you
  can always get to individual component widgets of the composite via
  the ->Subwidget() method. 
 
  I see these subwidgets: 
 
 Dialog
    'message' is the label subwidget with the dialog text, and 'bitmap' is
    the label subwidget showing the dialog bitmap
 ErrorDialog
    'error_dialog' is Dialog subwidget, 'text' is text subwidget
 
  You can even do things like this: 
 
  $error_dialog->Subwidget('error_dialog')->Subwidget('bitmap')->configure(..);
 
  to "get to" the label widget of the dialog component of the
  error_dialog widget..... 
 
  Be sure to also check out the "dialog" demo. 
 
 ______________________________________________________________________

 
 15.2. Is there a file selector? 
 
 Yes, there may be several eventually... 
 
 One distributed with the perl/Tk code kit itself is called FileSelect.pm and
 was written by Frederick L. Wagner - (based on an original by Klaus
 Lichtenwalder). 
 
 Another module called SelFile.pm was adapted by Alan Louis Scheinine
 from Wagner's FileSelect.pm. It is available from: 
 
     http://sun20.ccd.bnl.gov/~ptk/archive/ptk.1995.11/0122.html
 or
     http://w4.lns.cornell.edu/~pvhp/ptk/etc/SelFile.pm
 
 A module called FileSave.pm allows one to type in a new (non-existant)
 filename for "Save as..." type operations. It was posted by Mark Elston on
 12 Oct 1995 to the mailing list and is available from: 
 
     http://sun20.ccd.bnl.gov/~ptk/archive/ptk.1995.10/0093.html
 or
     http://w4.lns.cornell.edu/~pvhp/ptk/etc/FileSave.pm
 
 A slightly different behaviour is to be had with Brent B. Powers' 
 FileDialog.pm that was posted to the mailing list on 12 Jan 1996 and
 available from: 
 
     http://sun20.ccd.bnl.gov/~ptk/archive/ptk.1995.12/0201.html
 or
     http://w4.lns.cornell.edu/~pvhp/ptk/etc/FileDialog.pm
 
 Harry Bochner chimed in with SaveAs.pm. It is available from: 
 
     http://w4.lns.cornell.edu/~pvhp/ptk/etc/SaveAs.pm
 
 In general, if there is a feature that you want missing from one of these, or
 some behaviour that you would like to see modified then by all means cp
 the source code to your area and start hacking ;-) 
 
 ______________________________________________________________________

 
 15.3. Is there a color editor? 
 
 There is. Please see 
 
     perldoc ColorEditor.pm
 
 or run the Tk/demos/color_editor demo script for more information. 
 
 ______________________________________________________________________

 
 15.4. Is there a round Scale? 
 
 It is not quite a "round Scale" but Roy Johnson has written "Dial.pm" for
 round dial (or speedometer) -like settable widgets. It is available from: 
 
     http://sun20.ccd.bnl.gov/~ptk/archive/ptk.1995.08/0431.html
 or
     http://w4.lns.cornell.edu/~pvhp/ptk/etc/Dial.pm
 
 As well as from the Contrib/ sub-directory of your perl/Tk build
 directory. 
 
 ______________________________________________________________________

 
 15.5. Is there something equivalent to tkerror? 
 
 Yes there is. Please see the Tk/ErrorDialog.pm module for further
 information. 
 
 ______________________________________________________________________

 
 15.6. Are there Tables? 
 
 There are least two: 
 
 Nick's Table
 ------------
 
 Nick Ing-Simmons has distributed his own Table widget package with
 Tk-b9.01 (and higher). It is used through a use TK::Table; and 
 $top->Table(); calls. A rather detailed demo of this widget/geometry
 manager's capabilities can be found in the table_demo script (in your 
 Tk-b9.01/ build directory). There is also pod in the 
 perl5/Tk/Table.pm file. You may also browse the perl Tk::Table man
 page on the web at 
 
     http://w4.lns.cornell.edu/~pvhp/ptk/etc/Table.pm.html
 
 Guy Decoux's BLT_Table
 ----------------------
 
 Guy Decoux <[email-masked]> has ported the popular
 BLT_Table Tcl/Tk tabular geometry manager to perl/Tk. It was known to
 work with Tk-b8. You may obtain the latest version of it either from 
 
     ftp://moulon.inra.fr/pub/pTk/
 
 or from a CPAN site in the authors/id/GUYDX/ directory. You may also
 browse the perl BLT_Table man page on the web at 
 
     http://w4.lns.cornell.edu/~pvhp/ptk/etc/Table.html
 
 ______________________________________________________________________

 
 16. Programming/development tools. 
 
 There are a number of tools and methods to help you with your perl/Tk
 scripting and development. It is worthwhile to note here that the -w switch
 is recommended as is the use strict; statement near the top of your
 script/program. If it dies and you still cannot decrypt the error message
 that these generate take a look though man perldiag(1). 
 
 ______________________________________________________________________

 
 16.1 Is there a Tcl/Tk to perl/Tk translator? 
 
 Nick Ing-Simmons has written a (rather lengthy) tcl2perl script. It is
 distributed with the perl/Tk build kit. Please handle carefully! (translation:
 do not expect it to translate arbitrary tcl code accurately nor even into the
 most efficient perl/Tk equivalent. Do go over the converted script with
 care - and do not forget -w and use strict;.) Thanks Nick :-) 
 
 ______________________________________________________________________

 
 16.2 Is there something equivalent to wish in perl/Tk? 
 
 The answer is yes. 
 
 The idea of wish is that you read from <STDIN> and evaluate each
 statement. The standard way to do this in perl/Tk is to use the tkpsh
 script that comes in your perl/Tk build directory. Another elegant way to
 get wish like behavior in perl/Tk is to use rmt which you can find in 
 perl5/Tk/demos in your perl/Tk distribution. When you run rmt you
 already have Tk.pm set up for you so you can start typing things like $mmm
 = new MainWindow; etc. at the rmt: prompt. (This use belies the power
 of rmt which is derived from Ousterhout's Tcl/Tk version of rmt [see
 section 27.2 of his book]. rmt is capable of "inserting Tk code" into
 simultaneously running Tk applications.) 
 
 A cruder way to get wish-like behaviour with perl/Tk is to run a "perl
 shell" and type in your usual commands, including use Tk; etc. There is a
 script distributed with perl called perlsh which is written quite simply as: 
 
     #!/usr/bin/perl
      $/ = '';        # set paragraph mode
      $SHlinesep = "\n";
      while ($SHcmd = <>) {
          $/ = $SHlinesep;
          eval $SHcmd; print $@ || "\n";
          $SHlinesep = $/; $/ = ''; 
      }
 
 You can use this during code development to test out little snippets of
 code. It helps to be an accurate typist and the use strict; is optional
 here :-) 
 
 KOBAYASI Hiroaki has a more sophisticated wish like perl/Tk "shell"
 that is called EVA. It is available from: 
 
     ftp://ftp.sowa.is.uec.ac.jp/pub/Lang/perl5/Tk/eva-*.tar.gz
 
 ______________________________________________________________________

 
 16.3. Is there a debugger specifically for perl/Tk? 
 
 Not for the latest version - but the -w switch and use strict; are
 always helpful with debugging as they provide informative error messages. 
 
 You can, of course, run under the regular perl debugger using: 
 
     perl -d myscript
 
 Gurusamy Sarathy <[email-masked]> has built a PERL5DB file
 called Tkperldb (which despite the name is for pTk not Tk/perl). One
 must install an early de-bugger then apply a patch to bring the debugger up
 to date. The early debugger kit is available from: 
 
     ftp://ftp.perl.com/pub/perl/ext/TK/Tkperldb-a1.tar.gz
 
 And Gurusamy Sarathy notes that the patch to bring the debugger up to
 date is available at: 
 
  You need a post 5.001m perl that has support for debugging closures. 
  Or you can simply apply:
 
    http://www-personal.umich.edu/~gsar/perl5.001m-bugs.patch
 
  to 5.001m. (5.002beta includes all the fixes in the above patch).
 
 Note that a perl debugger may be invoked within your script with a line
 like: 
 
     $ENV{'PERL5DB'} = 'BEGIN { require Tkperldb }';
 
 See man perldebug(1) for more help. 
 
 Keep in mind that you are programming in perl after all. The perl debug
 line mode is available to you through executing the following from your
 shell: 
 
     perl -de 0
 
 Whereupon you must enter all the lines of a script including use Tk;.
 (Fancier file reads & evals are possible - but if you are getting that
 sophisticated why not create your own custom PERL5DB file?) When
 using perl -dwe 0 beware of the emacs like line editing under this
 debugger, and be forewarned that as soon as you type in the MainLoop;
 statement perl will no longer read from <STDIN>. 
 
 Ilya Zakharevich <[email-masked]> points out that very recent
 perldb versions will allow for simultaneous X and STDIN reads. He also
 points out: 
 
 Note that you may use 
 
     sub myLoop {
       if (defined &DB::DB) {
         while (1) {             # MainWindow->Count
           Tk::DoOneEvent(0);
         }
       } else {
         MainLoop;
       }
     }
 
 (and I hope the analogous provision will be in MainLoop in 
  tk-b9 - hi, Nick ;-)
 
 ______________________________________________________________________

 
 16.4. Is there a GUI builder in perl/Tk? 
 
 Work has reputedly (January 1996) started on porting a Tcl/Tk GUI
 builder known as SpecTcl for use with perl/Tk. For the Tcl/Tk SpecTcl kit
 see: 
 
     ftp://ftp.sunlabs.com/pub/tcl/SpecTcl-*.tar.[gz|Z]
 
 and address questions about SpecTcl to <[email-masked]>. 
 
 In <news:[email-masked]> Andreas
 Koschinsky <[email-masked]> announced a perl script for use with
 SpecTcl that has some interesting capabilies: 
 
  24 Mar 1996 22:45:21 GMT
  ... So i wrote a perl-script that can convert project-file (.ui-files)
  which spectcl writes. The script reads the .ui-file and generates an
  equivalent perl-module. 
 
 The URL for ui2perl should be something like: 
 
     ftp://susan.logware.de/pub/incoming/ui2perl*.tar.gz
 
 Somewhat more removed from SpecTcl there is also SWIG. 
 
 ______________________________________________________________________

 
 17. Processes & Inter-Process Communication under Perl/Tk. 
 
 Inter-Process Communication (IPC) is the subject of spawning and
 controlling other programs or "processes" from within perl (sometimes
 using sockets to do so). The subject is briefly discussed in the perlipc(1)
 man page, and was addressed towards the end of Chapter 6 of The Camel.
 The subject is also discussed in the perl FAQ and at Tom Christiansen's
 ftp site (in the various perlipc* files) at: 
 
     ftp://ftp.perl.com/perl/info/everything_to_know/    199.45.129.30
 
 as well as the web site at: 
 
     http://www.perl.com/perl/everything_to_know/ipc/index.html
 
 In addition to the usual perl IPC routines Tk allows (at least) three more
 special functions: fileevent (for handling I/O events), send (for
 inter-widget communication), and after (for time control like a sleep
 expressly for widgets). 
 
 Remember:
 
  If a packet hits a pocket on a socket on a port,
  And the bus is interrupted as a very last resort,
  And the address of the memory makes your floppy disk abort,
  Then the socket packet pocket has an error to report!
  -Ken Burchill(?) 
 
 ______________________________________________________________________

 
 17.1. How does one get Perl/Tk to act on events that are not coming from
 X? 
 
 On 22 Nov 1995 (Yaniv Bargury) [email-masked] wrote: 
 
  I need to write a GUI monitor, that displays the status and controls
  a set of processes running in the background. The idea is to have the
  GUI application start a few child processes, command the children
  through pipes from the GUI to the children, and display the children
  status coming on pipes from the children to the GUI in real time. 
 
  The GUI must not be busy waiting, because the CPU resources are
  limited. This excludes using the Tk_DoWhenIdle as explained in
  the manual. 
 
  The usual way to do this is to for the GUI process to have one 
  select() in its main loop. That select() should wait for X events
  or input from the pipes leading from the children. 
 
  How do you do this? 
 
 To which Nick Ing-Simmons <[email-masked]> replied: 
 
  fileevent - it is the hook into the select() in the mainloop. 
 
 In addition Avi Deitcher <[email-masked]> replied with: 
 
  I wrote something similar to effectively do a tail -f on multiple
  hosts, displaying the result on separate text widgets. Do the following:
 
     parent
      child
      child
      child
      ..
 
  with a one-way pipe from each child to the parent. Set up the
  following: 
 
     $main->fileevent(FILEHANDLE,status,subroutine);
 
  for each pipe that you have. This will cause pTk to monitor the 
  FILEHANDLE and call 'subroutine' when an event happens on
  that handle. In this case: FILEHANDLE = pipename status =
  'readable' or 'writable' or 'exception' and subroutine =
  any subroutine that you want. 
 
 To provide a concrete example of fileevent usage Stephen O. Lidie
 wrote a wonderful little GUI tail monitor he calls tktail: 
 
     #!/usr/local/bin/perl -w
     #
     # tktail pathname
     
     use English;
     use Tk;
     
     open(H, "tail -f -n 25 $ARGV[0]|") or die "Nope: $OS_ERROR";
     
     $mw = MainWindow->new;
     $t = $mw->Text(-width => 80, -height => 25, -wrap => 'none');
     $t->pack(-expand => 1);
     $mw->fileevent(H, 'readable', [\&fill_text_widget, $t]);
     MainLoop;
     
     sub fill_text_widget {
     
         my($widget) = @ARG;
     
         $ARG = <H>;
         $widget->insert('end', $ARG);
         $widget->yview('end');
     
     } # end fill_text_widget
 
 An example of how one might use such a script would be to create and
 monitor a file foo like so: 
 
     echo Hello from foo! > foo
     tktail foo &
     echo \"A ship then new they built for him/of mithril and of elven glass\" --Bilbo \
      >> foo
 
 ______________________________________________________________________

 
 17.2. Is there a send and do I need xauth? 
 
 There is a Tk::send, but to use it own must write one's own version of 
 Tk::receive. An example of this may be found in the rmt program
 distributed with perl/Tk. Note that as of Tk-b12 (including the released
 version Tk400.200) the script that receives from a Tk::send must run with
 taint chcecking turned on (i.e. with the -T switch thrown) and it must
 untaint all commands received from the other process. 
 
 The Tk::send <-> Tk::receive process will work under xhost +
 authority. The security this affords comes from the fact that anyone who
 would want to exploit it would have to know how to write a Tk::receive
 custom tailored to your application (in addition to all the other protocol
 hacking). 
 
 Please note that while you may not need xauth authorization it is
 nevertheless always a good idea. 
 
 ______________________________________________________________________

 
 17.3. How can I do animations using after? 
 
 There is a "toggling button" demo script supplied with Tk called 
 after_demo that makes effective use of after(). 
 
 Terry Greenlaw <[email-masked]> of Encompass Technologies
 posted a character cell animator for the really bored. Here it is in a slightly
 modified form that allows string input from the command line (note too
 the recursive call that doesn't sop up system memory): 
 
 #!/usr/bin/perl
 
 =head1 NAME
 
 From: [email-masked] "Terry Greenlaw"  Thu Feb 1 12:02:24 EST 1996
 To: [email-masked]
 Subj: A code sample for the REALLY bored
 
 For everyone with a case of Browser envy after using Microsoft's Internet
 Explorer, here's a perl/tk script only slightly more useful than a script
 to do <BLINK>. Don't know why I wrote it. Don't know why you'd run it.
 Maybe if you were writing a ticker tape application. Or had a weird thing
 for Times Square. Anyway....
 
 tog
 Terry Greenlaw (on-site @ Lockheed Martin)      Encompass Technologies
 [email-masked]                    [email-masked]
 
 ##################################################################
 
 =cut
 
     #!/usr/bin/perl
     
     #use strict;
     use Tk;
     
     $message=join(' ',@ARGV,''); 
     if (!$message) {
         $message="THIS IS A VERY LONG SCROLLING MESSAGE...      ";
         $topmssg="This is the top of the screen";
         $botmssg="This is the bottom of the screen";
     }
     else {
         $topmssg=$message;
         $botmssg=$message;
     }
     $top = MainWindow->new;
     $l1 = $top->Label(-fg => 'White', -text => $topmssg);
     $l1->pack(-fill => 'both', -expand => 1 );
     $m1 = $top->Label(-fg=>'Red', -bg=>'black',
                       -textvariable => \$message, 
                       -width => 15 
                       );
     $m1->pack();
     $m2 = $top->Label(-wrap=>1, 
                       -fg=>'Green', -bg=>'black',
                       -textvariable => \$message2, 
                       -width=>1, -height=>8 
                       );
     $m2->pack(-anchor=>'w');
     $l2 = $top->Label(-fg => 'White', -text => $botmssg);
     $l2->pack(-fill => 'both', -expand => 1 );
     
     after(100, \&scroll_it);
     
     $top->MainLoop;
     
     sub scroll_it {
         $message =~ /(.)(.*)/;
         $message="$2$1";
         ($message2 = $message) =~ s/ /  /g;
         after(100, \&scroll_it);
     }
     __END__
 
 (Please note that a script like this is now distributed as "TickerTape" in
 your Tk*/Contrib/ directory.) 
 
 ______________________________________________________________________

 
 17.4. How do I update widgets while waiting for other processes to
 complete? 
 
 The short answer is either 
 
     $widget -> update;
 or
     $widget -> DoOneEvent;
 
 Here is a script that makes use of the first of these methods. Note that
 instead of actually doing something useful the "long running process" is
 simply a call to the perl sleep() function for illustrative purposes: 
 
     #!/usr/bin/perl -w
     
     use Tk;
     
     my $m = MainWindow->new();
     my $l = $m -> Listbox();
     $l -> bind('<Double-1>' => sub{sleepy($l)} );
     my @nuts   = qw(Almond Brazil Chestnut Doughnut Elmnut Filbert);
     for (@nuts) { $l -> insert('end',$_); }
     $l -> pack;
     MainLoop;
     
     sub sleepy {
         my $widget = shift;
         print "before 1st sleep \n";
         sleep(10);
         print "after 1st sleep before delete \n";
         $widget -> delete('active');
         $widget -> update;             # try [un]*commenting this
         print "after delete before 2nd sleep \n";
         sleep(10);
         print "after 2nd sleep \n";
     }
     __END__
 
 ______________________________________________________________________

 
 17.5. How do you fork on System V (HP)? 
 
 Kraegeloh Martin <[email-masked]> originally asked: 

 ! Subj: signal handling difference on HP vs. SUN
 !
 ! the following code will fork an xterm with vi in it, and it
 ! will refuse to do so while the first xterm is still running.
 ! works fine on my sun.
 ! On HP however, the second time an xterm is started, NO handler
 ! is called when the child dies.
 !
 ! the code:
 ! ===================== 8< ===============================
 ! $SIG{CHLD}=\&w;
 !
 ! sub w{
 !    $pid=wait;
 !    print STDERR "died: $pid\n";
 !    if ( $have == $pid ) { $have = 0; }
 ! }
 
 To which a part of Nick Ing-Simmons' response was: 
 
  I suspect HPUX is SysV-ish not BSD or POSIX. So every time a
  signal fires, it removes the handler - you need to reset it in the
  handler: 
 
     sub w{
         $SIG{CHLD}=\&w;
         $pid=wait;
         print STDERR "died: $pid\n";
         if ( $have == $pid ) { $have = 0; }
      }

  Whether you reset it before/after the wait may be very important ... 
 
 Then Bjarne Steinsbo <[email-masked]> followed up with: 
 
  That's not the whole story... Another problem is that SIGCLD
  interrupts the read system call on SysV-ish (I like that word! :-)
  systems. This means that you have to test why "" fails, and act
  accodingly. A program that works on both Sun and HP is: 
 
     $SIG{CHLD}=\&w;
     while(1){
        $_ = ;
        $! = 0, next if $! =~ /Interrupted/;
        last if $! or !defined $_;
        if($have){
             print STDERR "child still alive\n";
        }
        else{
             if(($pid=fork()) != 0){
                $have=$pid;
                print STDERR "forked $pid\n";
             }
             else {
                exec("xterm -e vi") 
             }
        }
     }
 
     sub w{
        $pid=wait;
        print STDERR "died: $pid\n";
        if ( $have == $pid ) { $have = 0; }
        $SIG{CHLD}=\&w;
     }