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Tcl/Tk
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Tcl/Tk
A Developer’s Guide
Third Edition
Clif Flynt
AMSTERDAM•BOSTON•HEIDELBERG•LONDON
NEWYORK•OXFORD•PARIS•SANDIEGO
SANFRANCISCO•SINGAPORE•SYDNEY•TOKYO
MorganKaufmannPublishersisanimprintofElsevier
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Notices
Knowledgeandbestpracticeinthisfieldareconstantlychanging.Asnewresearchandexperiencebroadenourunderstanding,
changesinresearchmethodsorprofessionalpractices,maybecomenecessary.Practitionersandresearchersmustalwaysrely
ontheirownexperienceandknowledgeinevaluatingandusinganyinformationormethodsdescribedherein.Inusingsuch
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Flynt,Clif.
Tcl/Tk:adeveloper’sguide/ClifFlynt.–3rded.
p.cm.–(TheMorganKaufmannseriesinsoftwareengineeringandprogramming)
ISBN978-0-12-384717-1(pbk.)
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QA76.73.T44F552012
005.2’762–dc23
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Foreword
It is a pleasure to introduce you to the first revision of Clif’s opus on the popular Tool Command
Language(Tcl).TclhasbecometheSwissArmyknifeforthebusyprogrammer—youkeepitwithin
easyreachatalltimessimplybecauseithaslotsofusesinmanydifferentenvironments.
Clif’s book is both a pleasure to read and a useful reference. Clif is a strong advocate for Tcl,
explainingthemanydifferentscenarioswhereTclshinesandthemyriadwaysthesamefacilitiescan
beusedoverandoveragain.
In addition to the breadth that Clif brings to the subject, he also knows the details—and, to his
credit, he doesn’t overwhelm you with them. Instead, each chapter starts with the basics and builds
to the fun stuff. There are even self-help questions at the end of each chapter to help you review the
lessonsClifhastaught.
Whetheryou’reusingClif’sbookasanintroductiontoTclorasacomprehensivereference,I’m
sureyou’llfinditanecessaryadditiontoyourprofessionaltoolkit.
MarshallT.Rose
Principal,DoverBeachConsulting,Inc.
xvii
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Preface
TCL/TK: GUI PROGRAMMING IN A GOOEY WORLD
AlanWatts,theEpiscopalpriestwhopopularizedBuddhistthoughtin1960sCalifornia,oncewrotethat
philosophicalthinkerscanbedividedintotwobasictypes,whichhecalled“prickly”and“gooey.”The
pricklypeople,byhisdefinition,“aretough-minded,rigorous,andprecise,andliketostressdifferences
anddivisionsbetweenthings.Theypreferparticlestowaves,anddiscontinuitytocontinuity.Thegooey
peoplearetender-mindedromanticistswholovewidegeneralizationsandgrandsyntheses....Waves
suitthemmuchbetterthanparticlesastheultimateconstituentsofmatter,anddiscontinuitiesjartheir
teethlikeacompressed-airdrill.”1
Wattschosethetermspricklyandgooeycarefully,seekingtoavoidmakingonetermpositiveand
theotherpejorative,andheoffereddelightfulcaricaturesofthestrengthsandweaknessesofeachtype
ofperson.Inhisview,aconstanttensionbetweenthetwoperspectivesishealthyinpromotingprogress
towardamorecompleteunderstandingofreality.
Westernsciencehaslongmadeasimilardistinctionbetweentwocomplementaryapproaches,the-
oreticalandempirical.Pricklytheoreticiansseektounderstandtheworldthroughtheabstractionsof
thought,whereasgooeyempiricistsreturnceaselesslytotherealworldfortheever-more-refineddata
thatmethodicalexperimentationcanyield.Thecomplementarityofthetwoapproachesiswidelyrec-
ognizedbyscientiststhemselves.Althoughmostscientistswouldplacethemselvessquarelyineither
the theoretical or empirical camp, very few would go so far as to argue that the other approach is
without merit. A constant dialectic between empiricism and theory is generally seen to promote the
integrityandhealthofscientificinquiry.
Morerecently,theadventofcomputerprogramminghasbroughtwithitanewroundintheprickly–
gooey dialectic. By temperament, there seem to be two types of programmers, which I will call the
plannersandthedoers.Althoughgoodprogrammerswillofcourseplantheirsoftwarebeforebuilding
it(andwillactuallybuilditafterplanningit),someprogrammers(thepricklyplanners)seeplanning
anddesigningastheprimaryactivity,afterwhichsystembuildingisrelativelyautomatic.Others(the
gooey doers) perceive system design as a necessary and important preparatory stage before the real
workofsystembuilding.
Bothplannersanddoerscanbeexcellentprogrammers,andthebestofthemexcelatbothdesign
and execution. However, they may be categorized as planners or doers by their basic worldview, or
primary orientation. They show different patterns of strengths and weaknesses, and they can with
skilled management play complementary roles in large project teams. Still, certain tasks cry out for
onetypeofprogrammerortheother.Thedesignofsoftwaresystemstocontrolnuclearweapons,for
example,isataskthatdemandsexhaustiveplanningbecausetheriskofunanticipatederrorsistoohigh.
Thedesignofathrowawayprogramthatneedstoberunjustoncetocalculatetheanswertoasingle
question,ontheotherhand,shouldprobablybebuiltbythefastestmethodologypossible,robustness
1Watts,Alan,TheBook:OntheTabooAgainstKnowingWhoYouAre,NY:VintageBooks,1966,1989.
xix
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xx Preface
bedamned.Betweentheseextremes,ofcourse,lienearlyallreal-worldprogrammingprojects,which
iswhybothapproachesareuseful.Theyarenot,however,alwaysequallywellappreciated.
Themodernmanagerialmentalitythrivesoncarefulorganization,planning,anddocumentation.It
shouldbenosurprisethatmostmanagersprefertheplannerstothedoers;iftheywereprogrammers,
theywouldprobablybeplannersthemselves.Sincetheyarenotprogrammers,theymayeasilyfailto
recognizethevalueoftheskillsdoersbringtothetable.Programmers,however,arelesslikelytomake
thismistake.Inthemeager(butgrowing)literaturebyandforseriousprogrammers,theroleofdoers
ismoregenerallyrecognized.Indeed,planning-orientedprogrammersoftenstillperceivethemselves
as the underdogs, and programmer humor shows a deeply rooted skepticism regarding overplanned
projects.
Historically, the dialectic between planners and doers has been a driving force in the evolution
of programming languages. As the planners moved from FORTRAN to Pascal, C, C++, and Java,
the doers have moved from COBOL to Lisp, Basic, Perl, and Tcl. Each new generation of language
reflectedtheinnovationsofbothplannersanddoersinthepreviousgeneration,buteachmademostof
itsowninnovationsinasinglerealm,eitherplanningordoing.
Historically, it has been surprisingly difficult to make money by inventing even the most wildly
successfulprogramminglanguages.Successistypicallyhighlyindirect.Forexample,beingthebirth-
placeofCiscertainlyaclaimBellLabsisproudtomake,butitisdifficulttoarguethatthelanguage
was spectacularly meaningful for AT&T (or Lucent’s) bottom line. ParcPlace has perhaps come the
closesttoagenuinelanguage-inventionsuccessstory,butitisclearlynoMicrosoft.Imagine,then,the
dilemma in which Sun Microsystems found itself, circa 1994, when its research labs were polishing
uptwonewlanguages,onepricklyandonegooey,andeachaplausiblecontenderforleadershipinthe
nextgenerationofprogramminglanguages.Couldanycorporationreallyaffordtwosuchhigh-status,
low-profit“success”stories?
Sun’s decision to promote Java more vigorously than Tcl is difficult to argue with. Because most
decisionmakersaremanagers,andmostmanagersareplanners,Javahasalwaysbeenaneasierproduct
tosell.Sun’scontinuedandparallelsupportforTclwascorrect,evennoble,andprobablywiserthan
one could expect from the average large corporation. Still, in the face of the Java juggernaut, the
promotionofTclhasbeenleft,forthemostpart,tothegooeydoersamongus.Pragmaticprogrammers
withdirtontheirhands,manyofusbecameTcladvocatesforthemostpracticalofreasons:ithelped
usgetourjobsdone,andquickly.
Onesuchgooeydoeristheauthor,aveteranofcountlessprogramminglanguageswhohasfallen
inlovewithTclforalltheneatthingsitletshimdo.Javaadvocateswilloftenpraisethatlanguage’s
abstractions,suchasitsvirtualmachinemodel.Tcladvocatessuchastheauthorprefertograbonto
the nuts and bolts and show you how quickly you can get real work done. That is what this book is
intendedtodo,walkingyouallthewayfromthesimplestbasicstosophisticatedandusefulexamples.
Ifyouareina“doer”stateofmind,youhavepickeduptherightlanguageandtherightbook.Itcan
helpyougetalotofjobsdone.
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Introduction
ThanksforpurchasingthiscopyofTcl/Tk:ADeveloper’sGuide.(Youdidbuythis,didn’tyou?)This
book will help you learn Tcl/Tk and show you how to use these tools to increase your programming
productivity.
By the time you’ve finished reading this book, you’ll have a good idea of what the strengths and
weaknessesofTcl/Tkare,howyoucanusetheTcl/Tklibrariesandinterpreters,andwhatconstitutes
aneffectivestrategyforusingTcl/Tk.
This book is aimed at both computer professionals (from novice to expert level) and students. If
youareexperiencedwithcomputerprogramminglanguages,youwillbeabletoskimthroughthefirst
few chapters getting a feel for how Tcl/Tk works, and then go on to the chapters on techniques and
extensions.Ifyouarelessexperienced,youshouldreadthefirstchaptersfairlycarefullytobesureyou
understandtheTcl/Tksyntax.Tclhassomefeaturesthatmaynotbeobvious.
IfyourprimarygoalistolearntheTcl/Tkbasicsasquicklyaspossible,visitoneofthewebsites
listedbelowforatutorial.TherearealargenumberofTcl/Tktutorialsavailableinalargenumberof
languages.
Everylanguagehassomesimpletricksandtechniquesthatarespecifictothatlanguage.Tcl/Tkis
no exception. I’ll discuss how to best accomplish certain tasks with Tcl/Tk: how to use the list and
associativearraydatastructureseffectively,howtobuildmodularcodeusingTcl’ssoftwareengineer-
ingfeatures,andwhatkindsofprogrammingconstructswillhelpyougetyourprojectfromprototype
toproductwiththeminimalamountofrewriting.
Finally, there are plenty of code snippets, examples, and bits of packages to help you see how
Tcl/Tkcanbeused.Theseexamplesarechosentoprovideyouwithsomeboilerplateproceduresthat
youcanaddtoyourprogramsrightnow,tohelpyougetfromscribblesonpapertoaworkingprogram.
Thisshouldgiveyouagoodideaofwhat’scomingup.Feelfreetoskiparoundthebook.It’swritten
tobeanoverviewandreference,aswellasatutorial.
WHERE TO GET MORE INFORMATION
MuchasIwouldliketocoveralltheaspectsofTcl/Tkinthisbook,itisn’tpossible.HadIbutworld
enoughandtime,Istillwouldn’thaveenoughofeither.TheTcl/TkcommunityandtheTcl/Tktools
aregrowingtooquicklyforabooktodomorethancoverthecentralcoreofthelanguage.
. YoucanlearnmoreaboutTcl/Tkfromthesewebreferences:
.
http://www.tcl.tk/
.
Primarytcl.tkwebsite
.
ThedefinitivesiteforTcl/Tkinformation.
.
http://wiki.tcl.tk/
.
TheTcler’sWiki.
.
AcollectionofbriefarticlesanddiscussionsofTclfeatures,tricks,tips,andexamples.
.
http://www.tclcommunityassociation.org
TclCommunityAssociationhomepage
xxiii
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xxiv Introduction
.
PromotestheuseanddevelopmentofTcl/Tk.TheyruntheannualTcl/TkConferenceintheUS,
administertheTclcontributiontoGoogleSummerofCode,hostthewikiandsourcewebsites
.
andmore.
.
www.purl.org/NET/Tcl-FAQ/
.
TclFAQLaunchpage.
.
URLsforseveralTcl/TkFAQsandcomp.lang.tcl
.
http://core.tcl.tk/
.
Tcl/TkFossilrepository
.
TheofficialrepositoryforTcl/Tkdistributions.
.
http://sourceforge.net/projects/tcl/
.
Tcl/TkArchives
.
ThemirrorrepositoryforTcl/Tkdistributions.
.
http://www.activestate.html
.
ActiveState.
.
Tcl/Tkmaintenance,ActiveTcl,developmenttools,andmore.
.
http://www.tcl-tk.de/
.
Tcl/TkinformationinGerman
.
http://www.geocities.co.jp/SiliconValley/4137/
.
TclScriptingLaboratory
.
Tcl/TkinformationinJapanese
.
http://www.cyut.edu.tw/∼ckhung/olbook/
.
Chao-KueiHung’spages
.
Tcl/TkinformationinChinese
.
http://www.noucorp.com
.
NoumenaCorporationHomePage
Errataandextensionsforthisbook,updatedTclTutor
Foradditionallinksandresourcespleasevisitthecompanionsiteat:
http://www.elsevierdirect.com/9780123847171.
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Acknowledgments
Youmayliketothinkoftheauthorsittingalone,creatingabookinafewweeks.I’mafraidthatthis
idea has as much to do with reality as the Easter Bunny and Santa Claus. The initial revision of this
booktookthebetterpartofayearandalotofhelpfrommyfriends(someofwhomI’dneverheard
ofwhenIstartedtheproject).Thesecondandnowthirdeditionshaveeachtakenoverayeartowrite
andintroducedmetomorenewfriends.
The first acknowledgment goes to my wife, Carol. For more than a year she’s been: putting up
with “I can’t do any work around the house. I’ve got to finish this chapter,” correcting the grammar
onmyroughdraftsbeforeIsendthemoutfortechnicalreview,editinggalleys,makingsurethatIate
occasionally,andkeepinglife(almost)sanearoundhere.Thebookwouldnothavehappenedwithout
herhelp.
I also want to thank Ken Morton and his editorial assistants Samantha Libby and Jenn Vilaga. If
theyhadn’tbeenwillingtotakeachanceonanunprovenauthor’sabilitytofinishthefirsteditionof
thisbook,thiseditionwouldneverexist.
AlexSafonovprovidedthetechnicalreviewforthefirstedition.LarryVirden,andLeamHallkept
mehonestinthesecondedition.ArnulfWiedemann,MassimoManghi,RichardOwlett,BrianStretch,
VirginiaGielincki,andRobertClayprovidedmuchassistancewiththethirdedition.Theircomments
greatlyimprovedthebook,andearnedmorethanksthanIcangivethem.
My heartfelt and sincere thanks to the folks who read and commented on the early drafts. These
folks pulled duty way above the call of friendship by reading the truly wretched first drafts. Their
commentswereallinvaluable.Mythanksto:MargaretBumby,ClarkWierda,TerryGliedt,Elizabeth
Lehmann, Nick DiMasi, Hermann Boeken, Greg Martin, John Driestadt, Daniel Glasser, Theresa
Arzadon, Lee Cave-Berry, William Roper, Phillip Johnson, Don Libes, Jeffrey Hobbs, John Stump,
Laurent Demailly, Mark Diekhans, David Dougherty, Tod More, Hattie Schroeder, Michael Doyle,
Sarah Daniels, Ray Johnson, Melissa Hirschl, Jan Nijtmans, Forest Rouse, Brion Sarachan, Lindsay
F.Marshall,GregCronauandSteveSimmons.
And,finally,mythankstoJohnOusterhoutandhisteamsatUCBerkeley,Sun,ScripticsandAjuba,
and now to the Tcl Core Team and the host of maintainers. Programming in Tcl has been more pro-
ductiveandgivenmemorepleasurethananytoolI’veusedsinceIgotmyfirstchancetoplayLunar
LanderonanIBM1130.
xxi
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CHAPTER
1
Tcl/Tk Features
Your first question is likely to be “What features will Tcl/Tk offer me that other languages won’t?”
ThischaptergivesyouanoverviewoftheTcl/Tkfeatures.ItcoversthebasicsofwhatTcl/Tkcanoffer
youandwhatitsstrengthsarecomparedwithseveralalternatives.
Tcl is a multifaceted language package. You can use Tcl as a command scripting language, as a
powerfulmulti-platforminterpretedlanguage,asarapidprototypingplatform,orasthebackboneof
an application that is primarily written in C or Java. You can use the Tcl and Tk libraries to embed
powerful scripting capabilities into a compiled application like C, Java or FORTRAN. Tcl’s simple
syntaxmakessingle-usescripts(toreplacerepetitivecommandtypingorgraphicaluserinterface(GUI)
clicking)quickandeasytowrite.Tcl’smodularization,encapsulationandobjectorientedfeatureshelp
you develop large (100,000+lines of code) projects. Tcl’s extensibility makes it easy to use Tcl as
thebaselanguageacrossabroadrangeofprojects,frommachinecontroltodatabaseapplicationsto
electronicdesignapplications,networktestdevices,andmore.
Tcl is both free software and a commercially supported package. The core Tcl language is pri-
marily supported by a worldwide group of volunteers. Commercial support can be purchased from
ActiveState,NoumenaCorporation,CygnusSolutions,andothers.
The central site for Tcl/Tk information is http://www.tcl.tk. The source code reposi-
tory and some binary snapshots are maintained at http://sourceforge.net/projects/tcl/.
Tcl/Tk runtime packages are included with the Linux and FreeBSD packages, and with commer-
cial UNIX distributions such as Solaris, HPUX and Mac OS X. The current binaries for selected
systems (including MS-Windows, Linux, Mac OS X, and Solaris) are available from ActiveState
(http://www.activestate.com).
OneofthestrengthsofTclisthenumberofspecial-purposeextensionsthathavebeenaddedtothe
language.Extensionsarecommonlywrittenin“C”andaddhigh-performancespecialpurposefeatures
thatnotallusersneedtothelanguage.TheuseofextensionskeepstheTclbaselanguagesmall,while
supportingawidevarietyofuses.
ThemostpopularTclextensionisTk,whichstandsforToolKit.Thisextensionprovidestoolsfor
graphics programming, including drawing canvases, buttons, menus, and so on. The Tk extension is
consideredpartoftheTclcoreandisincludedinthesourcecodedistributionsatSourceforgeandmost
binarydistributions.
Other popular extensions to Tcl include [incr Tcl] (which adds support for C++ style object-
orientedprogrammingtoTcl)andexpect,anextensionthatsimplifiescontrollingotherapplications
anddevicesandallowsmanycomplextaskstobeeasilyautomated.
Tcl/Tk:ADeveloper’sGuide.DOI:10.1016/B978-0-12-384717-1.00001-4 1
(cid:13)c 2012Elsevier,Inc.Allrightsreserved.
