COO3077151

CourantMathematicsand

ComputingLaboratory

U.S.DepartmentofEnergy

ThePL/IProgrammingLanguage

PaulAbrahams

ResearchandDevelopmentReport

PreparedunderContractEY76C023077

withtheOfficeofEnergyResearch

MathematicsandComputing

March1978

NewYorkUniversity

UNCLASSIFIED

CourantMathematicsandComputingLaboratory

NewYorkUniversity

MathematicsandComputingCOO3077151

THEPL/IPROGRAMMINGLANGUAGE

PaulAbrahams

March1978

U.S.DepartmentofEnergy

ContractEY76C023077

UNCLASSIFIED

ThisreportistoappearasanarticleonthePL/I

programminglanguageintheEncyclopediaofComputer

ScienceandTechnologypublishedbyMarcelDekker,Inc,

ii

TableofContents

INTRODUCTION..............................................................1

SyntacticConventions...................................................6

DATATYPES................................................................9

ArithmeticTypes........................................................9StringTypes...........................................................11PicturedTypes.........................................................13Pointers,AreasandOffsets............................................19Files..................................................................20Labels.................................................................22Entries................................................................23Formats................................................................24Arrays.................................................................25Structures.............................................................26

DECLARATIONS.............................................................29

Manifest,Explicit,Contextual,andImplicitDeclarations..............29DeclarationsofStatementNames........................................31AttributeConsistencyandCompleteness.................................32StandardandUserDefinedDefaults.....................................38TheLIKEAttribute.....................................................41

EXPRESSIONS,TYPECONVERSION,ANDASSIGNMENT.............................43

PrefixandInfixExpressions...........................................45BuiltinFunctions......................................................51TypeConversion........................................................64Promotion..............................................................68TheAssignmentStatement...............................................69

STORAGETYPES............................................................72

StaticStorage.........................................................72AutomaticStorage......................................................73ControlledStorage.....................................................73BasedStorage..........................................................74TheReferOption.......................................................77LefttoRightCorrespondence...........................................79AllocationinAreas....................................................80ParameterStorage......................................................81DefinedStorage........................................................82Alignment..............................................................84Initialization.........................................................86

PROCEDURES,SCOPES,ANDENVIRONMENTS.....................................88

iii

TheRETURNStatement...................................................90ArgumentsandParameters...............................................91Options................................................................93Recursion..............................................................94TheGENERICAttribute..................................................95BlocksandScopes......................................................96InternalandExternalScope............................................99EntryValuesandEnvironments.........................................100

ONUNITSANDONSTATEMENTS..............................................102

TheONStatement,REVERTStatement,andSIGNALStatement..............105EnablementandDisablement............................................108BuiltinFunctionsforONConditions...................................109CategorizationoftheONConditions...................................110

OTHERSTATEMENTSAFFECTINGFLOWOFCONTROL..............................115

ConditionalStatements................................................115TheDOStatement......................................................116TheGOTOstatement....................................................119TheSTOPStatementandtheNullStatement.............................121

FILESANDRECORDINPUTOUTPUT...........................................122

FileAttributes.......................................................122FileOpeningandAttributeDetermination..............................123FileClosing..........................................................127OperationsonRecordFiles............................................127

STREAMINPUTOUTPUT.....................................................135

DataLists............................................................137ListDirectedInputOutput............................................138DataDirectedInputOutput............................................140EditDirectedInputOutput............................................142

BIBLIOGRAPHY............................................................150

EDITORIALNOTES.....................................................NOTES1

Acknowledgement...................................................NOTES1Errata............................................................NOTES1AbouttheAuthor..................................................NOTES2

iv

INTRODUCTION

PL/Iisalargeandpowerfulmultipurposeprogramming

language.TheintentofthedesignersofPL/Iwastocreate

alanguagethatcouldbeusedinbusinessandinscientific

applications,aswellasinsystemsprogrammingapplications

suchaswritingoperatingsystems.Theoriginaldesignwas

developedin1963byacommitteeofpeopledrawnfromIBM

andfromSHARE,anIBMusergroup.Foralongtimetheonly

importantimplementationsofPL/IwerethosedevelopedbyIBM

onthe360and370computers,andtheimplementationonthe

GE645attheMULTICSProjectatMIT.However,duringthe

early1970'sanumberofotherimplementationsarose.The

implementationofPL/Ibyotherorganizationswasgivenimpetus

bythedevelopmentofanationalandinternationalstandard

forPL/IbyasubcommitteeoftheAmericanNationalStandards

Institute,inconjunctionwithasimilarsubcommitteeofthe

EuropeanComputerManufacturers'Association.

ThedefinitionofStandardPL/Iwasformallyreleased

latein1976,butthecontentofthestandardwaspubliclyknown

wellbeforethen.Thestandarditselfwaswritteninanovel

mannerasasetofalgorithms,expressedinhighlystylized

English,fortheoperationofahypotheticalPL/Imachine.

TheversionofPL/IdescribedinthisarticleisStandardPL/I.

ThedesignofPL/Idrewheavilyonthemajorlanguages

thatexistedin1963:Fortran,Cobol,andAlgol60.The

syntaxofPL/ImostresemblesthatofFortran,butwithout

1

Fortran'srigidrulesforprogramformatting.Thenotionof

blockstructurewastakenfromAlgol60,whilePL/Istructures

weretakenfromtherecorddescriptionsofCobol.However,a

greatmanyfeatureswereaddedtoPL/Ithathavenocounter

partinitsancestorlanguages.

AnexampleofaPL/Iprogramisgiveninfigure1.A

programiswrittenasasequenceofexternalprocedures,which

aredefinedinsuchawaythattheycanbecompiledseparately

andthenlinkedtogetherwhentheprogramisexecuted.Within

anexternalprocedure,therecanbeinternalprocedures.

Inthisexample,thereisoneinternalprocedure,named

GET_DIGRAM.Eachprocedureintheprogramconstitutesablock,

Inaddition,ablockcanbedelimitedbythePL/Istatements

BEGINandEND(asinAlgol60).

TheinternalprocedureGET_DIGRAMcommunicateswiththe

outerprocedureDIGRAMSviaargumentspassedtoGET_DIGRAM

bytheCALLstatement.FromtheviewpointofGET_DIGRAM,

theseargumentsappearasparametersandarelistedinthe

PROCEDUREstatement.

Thevariablesusedinthisprogramaregiveninthe

DECLAREstatements.Ingeneral,avariable(orotheruseof

anidentifier)isdescribedbyasetofattributes.Notall

oftheseneedbegivenintheDECLAREstatement;thosethat

arenotgivenarededucedthroughtheapplicationofasetof

defaultingrules.Infact,defaultingisappliedinagreat

manycontextswithinPL/I.

2

Onaccountofitscomprehensivenature,PL/Iisadiffi

cultlanguagetolearninitsentirety.Forthatreasonit

wasdesignedsothatausercouldlearnjustthosepartsof

thelanguagethatheneeded,andignoretherestofituntil

theoccasionarosetousesomepreviouslyuntriedfeature.

Theextensivedefaultingconventionswereincluded,fora

largepart,tomakeitpossibletowriteprogramswithout

havingtolearnaboutobscureandirrelevantattributes.For

instance,onecanwritebusinessprogramsinPL/Iwithout

everrealizingthatthelanguageincludescomplexnumbersand

anextensiverepertoireofmathematicalbuiltinfunctions.

SinceStandardPL/Iisintendedtobeimplementedona

varietyofmachines,thestandardprovidesthatanumberof

characteristicsofthelanguageareimplementationdefined.

Forexample,machinesdifferintheirwordlengths;therefore

themaximumnumberofdigitsthatneedbecarriedina

floatingpointcomputationisleftimplementeddefined.Inthe

descriptionofPL/Igiveninthisarticle,implementationdefined

featuresofthelanguagearereferredtofrequently.