Hank Childs, University of OregonApril 18th, 2018

CIS 330:_ _ _ _ ______ _ _____

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Lecture 7: file I/O, more Unix

Announcements

• Projects– 4AdueINCLASSFriday(nolate)– 2Bassignedtoday,dueMonday– 3A,2Ctobeassignedsoon

• Nextweek:– LectureMondayandFriday– LabWednesday– Also,willbeaYouTubeshortlecture

• (Wearegettingaheadofschedule… tryinghardtopreventworkfromthisclasstobebackloaded)

Outline

• Review• FileI/O• Project2B• Redirection• Pipes

Outline

• Review• FileI/O• Project2B• Redirection• Pipes

Makefile example:multiplierlib

Unixcommand:tar

• tarcvf 330.tarfile1file2file3– puts3files(file1,file2,file3)intoanewfilecalled330.tar

• scp 330.tar@ix:~• ssh ix• tarxvf 330.tar• ls

file1file2file

MemoryErrors

• Arrayboundsread

• Arrayboundswrite

MemoryErrors

• Freememoryread/freememorywrite

Vocabulary:“danglingpointer”:pointerthatpointstomemorythathasalreadybeenfreed.

MemoryErrors

• Freeingunallocatedmemory

MemoryErrors

• Freeingnon-heapmemory

MemoryErrors

• NULLpointerread/write

• NULLisneveravalidlocationtoreadfromorwriteto,andaccessingthemresultsina“segmentationfault”– ….rememberthosememorysegments?

MemoryErrors

• Uninitializedmemoryread

NEW:IPR,IPW

• int *X=0xDEADBEEF:• int Y=*X;

• whaterrorcodefits?

• Imissedtwo:– InvalidPointerRead(IPR)– InvalidPointerWriter(IPW)

Memoryerrorinaction

Project4A

• Postednow• Youwillpracticedebugging&usingadebugger– Thereare3programsyouneedtodebug

• Inthiscase,“debug”meansidentifythebug– Doesnotmeanfixthebug

• Canusegdb orlldb• Maywanttorunonix

• WorksheetdueinclassFriday• P3.c:4errors

Outline

• Review• FileI/O• Project2B• Redirection• Pipes

FileI/O:streamsandfiledescriptors

• Twowaystoaccessfiles:– Filedescriptors:• Lowerlevelinterfacetofilesanddevices

– Providescontrolstospecificdevices

• Type:smallintegers(typically20total)

– Streams:• Higherlevelinterfacetofilesanddevices

– Providesuniforminterface;easytodealwith,butlesspowerful

• Type:FILE*

Streamsaremoreportable,andmoreaccessibletobeginningprogrammers.(Iteachstreamshere.)

FileI/O

• Processforreadingorwriting– Openafile• TellsUnixyouintendtodofileI/O• Functionreturnsa“FILE*

– Usedtoidentifythefilefromthispointforward

• Checkstoseeifpermissionsarevalid

– Readfromthefile/writetothefile– Closethefile

Openingafile

• FILE*handle=fopen(filename,mode);

Example:FILE*h=fopen(“/tmp/330”,“wb”);

Note:#include<stdio.h>

Closewhenyouaredonewith“fclose”

Reading/Writing

Example

FilePositionIndicator

• Filepositionindicator:thecurrentlocationinthefile

• IfIreadonebyte,theonebyteyougetiswherethefilepositionindicatorispointing.– Andthefilepositionindicatorupdatestopointatthenextbyte

– Butitcanbechanged…

fseek

ftell

Wehaveeverythingweneedtomakeacopycommand…

• fopen• fread• fwrite• fseek• ftell

Canwedothistogetherasaclass?

argc &argv

• twoargumentstoeveryCprogram• argc:howmanycommandlinearguments• argv:anarraycontainingeachofthearguments

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

Returnvaluesinshells

$?isthereturnvalueofthelastexecutedcommand

Printingtoterminalandreadingfromterminal

• InUnix,printingtoterminalandreadingfromterminalisdonewithfileI/O

• Keyboardandscreenarefilesinthefilesystem!– (atleasttheywere…)

StandardStreams

• Wikipedia:“preconnected inputandoutputchannelsbetweenacomputerprogramanditsenvironment(typicallyatextterminal)whenitbeginsexecution”

• Threestandardstreams:– stdin (standardinput)– stdout (standardoutput)– stderr (standarderror)

WhatmechanismsinCallowyoutoaccessstandardstreams?

printf

• Printtostdout– printf(“helloworld\n”);– printf(“Integersarelikethis%d\n”,6);– printf(“Twofloats:%f,%f”,3.5,7.0);

fprintf

• Justlikeprintf,buttostreams• fprintf(stdout,“helloworld\n”);–à sameasprintf

• fprintf(stderr,“helloworld\n”);– printsto“standarderror”

• fprintf(f_out,“helloworld\n”);– printstothefilepointedtobyFILE*f_out.

bufferingandprintf

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

• Butwhataboutacrash?– printf putsstringinbuffer– otherthingshappen…includingacrash– bufferisneverprinted!Solutions:(1)fflush,(2)fprintf(stderr)alwaysflushed

Outline

• Review• FileI/O• Project2B• Redirection• Pipes

Project2B

0 1

Outline

• Review• FileI/O• Project2B• Redirection• Pipes

Unixshellsallowsyoutomanipulatestandardstreams.

• “>”redirectoutputofprogramtoafile• Example:– ls >output– echo“thisisafile”>output2– catfile1file2>file3

Unixshellsallowsyoutomanipulatestandardstreams.

• “<”redirectfiletoinputofprogram• Example:– python<myscript.py• Note:pythonquitswhenitreadsaspecialcharactercalledEOF(EndofFile)• YoucantypethischaracterbytypingCtrl-D• ThisiswhyPythonquitswhenyoutypeCtrl-D

– (manyotherprogramstoo)

Unixshellsallowsyoutomanipulatestandardstreams.

• “>>”concatenateoutputofprogramtoendofexistingfile– (orcreatefileifitdoesn’texist)

• Example:– echo“Iamstartingthefile”>file1– echo“Iamaddingtothefile”>>file1– catfile1

IamstartingthefileIamaddingtothefile

What’shappeninghere?

ls isoutputtingitserrormessagestostderr

Redirectingstderr inashell

Redirectingstderr tostdout

Convenientwhenyouwantbothtogotothesamestream

Outline

• Review• FileI/O• Project2B• Redirection• Pipes

cfunctions:forkandpipe• fork:duplicatescurrentprogramintoaseparateinstance– Tworunningprograms!– Onlydifferentiatedbyreturnvalueoffork(whichisoriginalandwhichisnew)

• pipe:mechanismforconnectingfiledescriptorsbetweentwoforkedprograms

Throughforkandpipe,youcanconnecttworunningprograms.Onewritestoafiledescriptor,andthe

otherreadstheoutputfromitsfiledescript

Onlyusedonspecialoccasions.(Andoneofthoseoccasionsiswiththeshell.)

pipesinUnixshells

• representedwith“|”• outputofoneprogrambecomesinputtoanotherprogram

Veryusefulprograms

• grep:keeplinesthatmatchpattern,discardlinesthatdon’tmatchpattern

Veryusefulprograms

• sed:replacepattern1withpattern2– sed s/pattern1/pattern2/g• smeanssubstitute• gmeans“global”…everyinstanceontheline

sed isalsoavailablein“vi”:%s/pattern1/pattern2/g(%meansalllines)

:103,133s/p1/p2/g(lines103-133)

Wildcards

• ‘*’isawildcardwithunix shells

‘?’isawildcardthatmatchesexactlyonecharacter

Otherusefulshellthings

• ‘tab’:auto-complete• esc=:showoptionsforauto-complete• Ctrl-A:gotobeginningofline• Ctrl-E:gotoendofline• Ctrl-R:searchthroughhistoryforcommand