Hank Childs, University of OregonOctober 17, 2019

Lecture 6:Memory, part 4

__. __ .__ __. __ ___ ___ ______ ___ .__ __. _______ _______ ___ .___________. ___| | | | | \ | | | | \ \ / / / | / \ | \ | | | \ | \ / \ | | / \| | | | | \| | | | \ V / | ,----' / ^ \ | \| | | .--. | | .--. | / ^ \ `---| |----` / ^ \| | | | | . ` | | | > < | | / /_\ \ | . ` | | | | | | | | | / /_\ \ | | / /_\ \| `--' | | |\ | | | / . \ __ | `----.__ / _____ \ | |\ | | '--' | | '--' | / _____ \ | | / _____ \\______/ |__| \__| |__| /__/ \__\ (_ ) \______(_ ) /__/ \__\ |__| \__| |_______/ |_______/ /__/ \__\ |__| /__/ \__\

_______.___________..______ __ __ ______ .___________. __ __ .______ _______ _______. / | || _ \ | | | | / || || | | | | _ \ | ____| / | | (----`---| |----`| |_) | | | | | | ,----'`---| |----`| | | | | |_) | | |__ | (----` \ \ | | | / | | | | | | | | | | | | | / | __| \ \

.----) | | | | |\ \----.| `--' | | `----. | | | `--' | | |\ \----.| |____.----) ||_______/ |__| | _| `._____| \______/ \______| |__| \______/ | _| `._____||_______|_______/

C&Memory• Callowsyoutodirectlyaccessmemory• Thereisno“babysitter”thatmonitorsyourmemoryaccesses– Unlessyouwanderoutsidea“segment,”inwhichcaseyourprogramcrashes

• Thishelpsperformance• Butmakesprogrammingdifficult– noseatbelts• HOWEVER:thereareprogramsthatwillmonitoryourmemoryaccessesandletyouknowwhenyouhavedonebadthings– valgrind &gdb

Thisweek’slab…

• Ifyou“domemorywrong,”thenyourprogramwillcrash

• Howtofix?• Specialprogram:gdb (GNUdebugger)– gdb cangiveyoulotsofhintsaboutwhyyourprogramcrashed!

NOCLASSTUESDAY

• InsteadYouTubelecture• Postedthisweekend• WillbeentirelyabouthowtodoProject2D

• (2Cdiscussedlaterthislecture)

Whatshouldyoubedoing?

• Duetoday:2B• 2Cmaybepostedtonightifwemakeitfarenoughinlecture

• Readingthetextbook,ifyouhavenotdoneitalready– Chapter2.1,2.2,2.3,2.4.,2.5,and2.6.1– Chapter4.1(butnot4.1.2),4.2-4.4–à willbeanupcomingquizondetailsinthebook(thingsIhavenotlecturedonwillbefairgame)

Review

Pointers• Pointer:pointstomemorylocation– Denotedwith‘*’– Example:“int *p”

• pointertoaninteger– Youneedpointerstogettoheapmemory

• Addressof:getstheaddressofmemory– Operator:‘&’– Example:

int x;int *y=&x;ß thisexampleispointingtoanautomaticvariable,notadynamicvariable

Whatisanarray?

• Blockofcontiguousmemory• IfelementseachhavesizeNbytesandthereareMelements,themN*Mcontiguousbytes

• LetAbeaddressofthebeginningofthearray• ThenA[0]isat“A”• AndA[1]isat“A+N”• A[2]isat“A+2*N”• andsoon…

Dynamicmemoryworksdifferently

• Youallocateit,itstaysarounduntilyoude-allocateitortheprogramends

• Important:youneedawaytokeeptrackofmemory– Ifnot,thememorywillbe“leaked.”

• Soweneedawayofmanagingdynamicmemory.

• TheconceptfordoingthisinCisPOINTERS

DynamicMemoryAllocation

• Specialbuilt-infunctiontoallocatememoryfromheap:malloc– InteractswithOperatingSystem– Argumentformalloc ishowmanybytesyouwant

• Alsobuilt-infunctiontodeallocate memory:free

free/malloc exampleEnablescompilertoseefunctionsthataren’tinthisfile.Moreonthisnextweek.

sizeof isabuiltinfunctioninC.Itreturnsthenumberofbytesforatype(4bytesforint).

don’thavetosayhowmanybytestofree…theOSknows

AutomaticvsDynamic

• Automaticmemorylivesonlyforitscurrentscope

• Dynamicmemorylivesuntilyoufreeit,oruntiltheprogramends

Thisisjustfine…

Thisisnotfine…

Andhereitgoeswrong…

Pauseanddoanexampleandfilmit

MemorySegments• Yourprogramisdividedinto4segments:– Codesegment:wherethemachinecodeinstructionsare

– Datasegment:wheretheglobalvariableslive…andotherthings

– “Stack”:whereautomaticmemorylives– “Heap”:wheredynamicmemorylives– Note:stack&heaparedatastructuresandwewilllearnmoreaboutthemlater.

Ifyouaccessmemoryaddressesoutsideyoursegments,yougeta“segmentationfault”…whichcausesacrash

“StackOverflow”

Stack Heap

Andinsomecasesitdoesn’thavetohittheheap.Justtakemorememorythanintended.

CharacterStrings

ASCIICharacterSet

imagesource:granneman.com

TherehavebeenvariousextensionstoASCII…nowmorethan128characters

Manyspecialcharactersarehandledoutsidethisconvention

SomeSpecialCharacters

New:%cprintscharacter

characterstrings

• Acharacter“string”is:– anarrayoftype“char”– thatisterminatedbytheNULLcharacter

• TheClibraryhasmultiplefunctionsforhandlingstrings

buildingacharacterstring

New:%sprints“strings”(arraysofcharsterminatedby\0)

buildingacharacterstring

Compilerautomaticallyaddsa\0foryou!

Didyounotice

• CharactersinCaresinglequotes:‘A’• StringsinCaredoublequotes:“helloworld”• Astringofasinglecharacterisstilldoublequotes:“A”– (Andthisisactually“A\0”)

Characterstringsexample

UsefulClibrarystringfunctions• strcpy:stringcopy• strncpy:stringcopy,butjustfirstNcharacters• strlen:lengthofastring

UsefulClibrarystringfunctions• strcpy:stringcopy• strncpy:stringcopy,butjustfirstNcharacters• strlen:lengthofastring

Whathappenedhere?

MoreusefulClibrarystringfunctions

source:cplusplus.com

argc &argv

• twoargumentstoeveryCprogram• argc:howmanycommandlinearguments• argv:anarraycontainingeachofthearguments

• “./a.out hankchilds”• à argc ==3• à argv[0]=“a.out”,argv[1]=“hank”,argv[2]=“childs”

Project2C• Assignedtoday• DueFridayOct25• Parsingfloatingpointnumbers(e.g.,-3.68)

• Lotsofhintsinprompt

Enums,structs,typedef,union

Enums

• Enums makeyourowntype– Typeis“listofkeywords”

• Enums areusefulforcodeclarity– Alwayspossibletodothesamethingwithintegers

• Becarefulwithenums– …youcan“contaminate”abunchofusefulwords

enum example

Ckeyword“enum”–

meansenumdefinitionis

coming

Thisenumcontains6differentstudenttypes

semi-colon!!!

enum example

enums translatetointegers…andyoucansettheirrange

Butenums canbeeasiertomaintainthanintegers

Ifyouhadusedintegers,thenthisisabiggerchangeandlikelytoleadtobugs.

SimpleDataTypes

• float• double• int• char• unsignedchar

Allofthesearesimpledatatypes

Structs:acomplexdatatype

• Structs:mechanismprovidedbyCprogramminglanguagetodefineagroupofvariables– Variablesmustbegroupedtogetherincontiguousmemory

• Alsomakesaccessingvariableseasier…theyareallpartofthesamegrouping(thestruct)

struct syntaxCkeyword“struct”–

meansstructdefinitionis

coming

Thisstructcontains6doubles,meaningitis48bytes

semi-colon!!!

“.”accessesdatamembersforastruct

Declaringaninstance

Nestedstructs

accessesdirpartofRay

accessesdirectionZpartofDirection(partofRay)

typedef

• typedef:tellcompileryouwanttodefineanewtype

savesyoufromhavingtotype“struct”everytimeyoudeclareastruct.

Otherusesfortypedef

• Declareanewtypeforcodeclarity– typedef int MilesPerHour;• MakesanewtypecalledMilesPerHour.• MilesPerHour worksexactlylikeanint.

• Alsousedforenums &unions– sametrickasforstructs …typedef savesyouaword

Soimportant:struct datamemberaccessisdifferentwithpointers

Pointers:use“->”Instances(i.e.,notpointers):use“.”

Unions

• Union:specialdatatype– storemanydifferentmemorytypesinonememorylocation

Whendealingwiththisunion,youcantreatitasafloat,asanint,oras

4characters.

Thisdatastructurehas4bytes

Unions

Whyareunionsuseful?

UnionsExample

UnionsExample

BonusSlides

Importantnoteondebugging:bufferingandprintf

• Important:printf isbuffered• So:– printf putsstringinbuffer– otherthingshappen– bufferiseventuallyprinted

• Butwhataboutacrash?– printf putsstringinbuffer– otherthingshappen…includingacrash– bufferisneverprinted!

Onesolution:fflush(stdout)

Let’spauseanddoavideoonfflush

mv:Unixcommandforrenamingafile