ESSENTIAL Linux For EMBEDDED Developers · 2018. 9. 20. · • The usual development tools...

ESSENTIALLinuxForEMBEDDEDDevelopers

Theopensourceway!

AnoverviewofLinuxenvironment

LinuxEvolutionWhatisLinux?Distributions

LinuxVirtualizationLinuxEverywhere

TheLinuxkernelarchives

http://en.wikipedia.org/wiki/Linux

GNU/Linux

• Recursiveacronym“GNU’snotUNIX”• http://www.gnu.org/ nothttp://www.gnu.com/

• RichardStallman(1983)GoalafreeUnix• KnownforFreeSoftwaremovement,GNU,Emacs,gcc• NeverreallyreleasedGNUoperatingsystem

• Linus1992opensourcedLinuxKernelandnamedGNU/Linux

• FreeSoftwareFoundation• http://www.fsf.org/

http://en.wikipedia.org/wiki/GNU

TypicalLinuxSYSTEMLayout

SystemReserved

Rootfs

Kernel

Bootloader

Hardware

Linux Distributions

•Redhat/fedora/Centos• Most popular, good all around choice• Fedora – community supported• Enterprise Redhat – corporate supported

•Debian/Ubuntu/Mint• Completely noncommercial• Massive package selection and easy management• Not as user friendly, but improving

•SuSe• IBM Preferred Linux – z/Series Linux of choice

TheBlackfin uClinux DistributionbyAnalogDevices–aforkoftheuClinux distributionforBlackfin processors

EmbeddedAlley- seehttp://www.embeddedalley.com/

Lineo SolutionsuLinux

MontaVista Linux- seehttp://www.mvista.com/products_services.php

Pengutronix - seehttp://www.pengutronix.de/oselas/bsp/index_en.html

RidgeRun Linux- seehttp://www.ridgerun.com/sdk.shtml

TimeSys Linux- seehttp://www.timesys.com/embedded-linux/linuxlink

WindRiver- seehttp://www.windriver.com/products/linux/

Digi EmbeddedLinuxforDigi's ARMbasedmodules

EmbeddedLINUX

Virtualization

GuestOperatingSystemVirtualization SharedKernelVirtualization

KernelLevelVirtualization HypervisorVirtualizationXen,VMwareESXServerandMicrosoft'sHyper-Vtechnology(UML)and(KVM).

LinuxVServer,SolarisZonesandContainers,andOpenVZVMwareServerandVirtualBox.

SeeL4LinuxCoLinuxMkLinux

LinuxKERNELTODAY

Kernel.org

Linux Kernel Information

l The Linux kernel version numbers consist of three numbers separated by decimals, such as2.2.14. The first number is the major version number. The second number is the minor revisionnumber. The third number is the patch level version

l There are two stages of kernel releases: “stable” and “development”. Development kernels endin an odd number (2.3, 2.5, …), stable or production kernels end in an even number (2.4,2.6,3.0).

l Once a kernel is deemed stable, it will move from an odd to even second number for release(e.g., from 2.3.51 to 2.4.0).

l You can get a good sense of what the future production state of Linux will be by looking at thedevelopment kernel.

l http://www.kernel.org

ApplicationinteractionsinLinuxTheKernelArchitectureoverviewandaddressspaceview

Applicationandmodesofoperations,KernelandUseraddressSpacesSystemcall,entryandexitpoints,lowlevelview,strace,

parameterpassingandkernelimplementations

Structure:The“Core” LinuxKernel

Applications

SystemLibraries(libc)

SystemCallInterface

Hardware

Architecture-DependentCode

I/ORelated ProcessRelatedScheduler

MemoryManagement

IPC

FileSystems

Networking

DeviceDrivers

Mod

ules

OperativeModes

• To avoid having applications that constantly crashed,newer OSs were designed with 2 different operativemodes:

• Kernel Mode:• the machine operates with critical data structure, directhardware (IN/OUT or memory mapped), direct memory,IRQ, DMA, and so on.

• User Mode:• users can run applications.

OperativeModes

• Kernel Mode "prevents" User Mode applications fromdamaging the system or its features.

•Modern microprocessors implement in hardware atleast 2 different states. For ex. under Intel, 4 statesdetermine the PL (Privilege Level). It is possible to use0,1,2,3 states, with 0 used in Kernel Mode.

• Linux/UnixOSrequiresonly2privilegelevels,andwewillusesuchaparadigmaspointofreference

SwitchingfromUserModetoKernelMode-1

• Whendoweswitch?• Onceweunderstandthatthereare2differentmodes,wehavetoknow

whenweswitchfromonetotheother.

• Typically,thereare2pointsofswitching:

1. WhencallingaSystemCall:2. WhenanIRQ(orexception)comes

Let’sobservetheuser/kernelspace

• usevmstat tograsprecentprocesscontext

Librarycallvs SystemCallinLinuxWrapperandWrapperbased

Systemcall,EntryandExitPointsExampleofwrappertoasystemcallflow

Systemcalls

• Themaininterfacebetweenthekernelanduserspace isthesetofsystemcalls

• About~300systemcallsthatprovidesthemainkernelservices• Fileanddeviceoperations,networkingoperations,inter-

processcommunication,processmanagement,memorymapping,timers,threads,synchronizationprimitives,etc.

• Thisinterfaceisstableovertime:onlynewsystemcallscanbeaddedbythekerneldevelopers

• ThissystemcallinterfaceiswrappedbytheClibrary,anduserspace applicationsusuallynevermakeasystemcalldirectlybutratherusethecorrespondingClibraryfunction

SystemCalls:read• Cexample:

count=read(fd,buffer,nbyte)

• pushparametersonstack

• calllibrarycode

• putsystemcallnumberinregister

• callkernel(TRAP)• kernelexaminessystemcallnumber• findsrequestedsystemcallhandler• executerequestedoperation

• returntolibraryandcleanup• increaseinstructionpointer• removeparametersfromstack

• resumeprocess

application

user spacekernel space

count = read (fd , buffer , nbytes)

read libraryprocedure

register

memory (stack) buffer

nbytesbufferfd

X (read)

system callhandler

X

sys_read()

KernelEntryandExit

Kernel

Devices

Library Code

System Call Interface

appapp

trap /interrupt

table

systemcalltable

schedulerboot

80h

exceptions(error traps)

interrupt devicedialog

trap

page faults

SystemCallsvs.LibraryCalls

•man2• historicalevolutionof#ofcalls

• Unix6e(~50),Solaris7(~250)• Linux2.0(~160),Linux2.2(~190),Linux2.4(~220)

• librarycallsvs.systemcallpossibilities:• librarycallneverinvokessystemcall• librarycallsometimesinvokessystemcall• librarycallalwaysinvokessystemcall• systemcallnotavailablevialibrary

• caninvokesystemcall“directly” viaassemblycode

CostofCrossingthe“KernelBarrier”

•Morethanaprocedurecall• Lessthanacontextswitch• Costs:

• Establishingkernelstack• Validatingparameters• Kernelmappedtouseraddressspace?

ImplementationExample:“Hello,world!”

.data#sectiondeclaration

msg:.string"Hello,world!\n"#ourdearstringlen =.- msg #lengthofourdearstring

.text #sectiondeclaration

#wemustexporttheentrypointtotheELFlinkeror.global_start#loader.Theyconventionallyrecognize_startastheir

#entrypoint.Useld -efootooverridethedefault._start:#writeourstringtostdout

movl $len,%edx #thirdargument:messagelengthmovl $msg,%ecx #secondargument:pointertomessagetowritemovl $1,%ebx#firstargument:filehandle(stdout)movl $4,%eax#systemcallnumber(sys_write)int $0x80#callkernel

#andexit

movl $0,%ebx#firstargument:exitcodemovl $1,%eax#systemcallnumber(sys_exit)int $0x80#callkernel

LinuxSystemCalls(1)

Invokedbyexecutingint $0x80.• Programmedexceptionvectornumber128.• CPUswitchestokernelmode&executesakernelfunction.

• Callingprocesspassessyscall number identifyingsystemcallineax register(onIntelprocessors).

• Syscallhandlerresponsiblefor:• Savingregistersonkernelmodestack.• Invokingsyscallserviceroutine.• Exitingbycallingret_from_sys_call().

ParameterPassing

• Onthe32-bitIntel80x86:• 6registersareusedtostoresyscall parameters.

• eax (syscall number).• ebx,ecx,edx,esi,edi storeparameterstosyscall serviceroutine,identifiedbysyscallnumber.

InteractingwithModules

App_1 App_2 App_N

/dev/device_nodes

Vmlinuxmodule.ko

Basicutility,filter&developercommandessentials

UnderstandingRootFileSystemHierarchy

LinuxFilesystemsondesktop

• HistoricallyLinuxhadnofs ofitsownandformallyhadminix fs runningonit

• Lateradpoted thethesecondextendedfilesystemformallyknownasext2fs

• Hasbeenenhancedtoext3fs,ext4fs• NowtheLinux3.XislookingtohaveBtrfs

• Btrfs isanewcopyonwritefilesystem forLinuxaimedatimplementingadvancedfeatureswhilefocusingonfaulttolerance,repairandeasyadministration

LinuxTreeHierarchy

/

/root /bin /proc /usr /sbin /dev /opt /home

/home/ram

/home/sita

/src

linux

/ - (unnamed) the actual root

/root – a home area for root

/bin – utility commands

/proc – a bogus fs for Linux kernel

/usr – usr utilities and application/sbin – special commands /privileged /dev – device node for external devices /opt – optional applications /home – usually all you do is here/var – spool , log messages etc

TheShell

• CommandinterpreterinoriginalUnix

• Readcommand• Perhapspre-processcommand

• Fork/execute• Returnexitstatusofcommand

• Alittlehistoryrevisited• BourneShell(sh)• Cchell• Korn shell• BourneAgainshell(bash)

Entrylevelcommands

• man• which• whatis• info• apropos

• date• cal• who• w• id• Write• mesg• bc

PiPEs (|)&FILTER

• ps• nice• sleep• at• nohup• kill• ctrl+c• ctrl+z• fg• bg• top• vmstat

• grep• sort• tr• cut• paste• more• less• head• tail• nl• tee• wc

PROCESS

FILEs/misc

• pwd• mkdir• cd• Ls• rmdir• cat• cp• ln• mv• rm

• chmod• chgrp• Chown• comm• cmp• diff• Uniq• touch• stat• file• compgen

DIR/FILE

CSCOPE

• Lookatthefunctiondefinitionsstraightwayfromthefunctioncall• Lookoutforalltheplacesfromwhereaparticularfunctionisbeing

called.• Searchingstringsandpatterns• Determiningsourceswhichincludeaspecificheader• Besides,itdoeseditortaskslikeviewingthesourcefile,replacing

strings,etc

CSCOPE• $cscope-help• Usage:cscope[-bcCdehklLqRTuUvV][-ffile][-Ffile][-ifile][-Idir][-sdir]

• [-pnumber][-Ppath][-[0-8]pattern][sourcefiles]

• -bBuildthecross-referenceonly.• -CIgnorelettercasewhensearching.• -cUseonlyASCIIcharactersinthecross-reffile(don’tcompress).

• -dDonotupdatethecross-reference.• -eSuppressthe-ecommandpromptbetweenfiles.

• -Fsymfile Readsymbolreferencelinesfromsymfile.

• -freffile Usereffile ascross-reffilenameinsteadofcscope.out.

• -hThishelpscreen.• -Iincdir Lookinincdir forany#includefiles.• -i namefile Browsethroughfileslistedinnamefile,insteadofcscope.files

• -kKernelMode– don’tuse/usr/includefor#includefiles.

-LDoasinglesearchwithline-orientedoutput.

-lLine-orientedinterface.

-num patternGotoinputfieldnum (countingfrom0)andfindpattern.

-PpathPrependpathtorelativefilenamesinpre-builtcross-reffile.

-pnDisplaythelastnfilepathcomponents.

-qBuildaninvertedindexforquicksymbolsearching.

-RRecurse directoriesforfiles.

-sdir Lookindir foradditionalsourcefiles.

-TUseonlythefirsteightcharacterstomatchagainstCsymbols.

-UCheckfiletimestamps.

-uUnconditionallybuildthecross-referencefile.

-vBemoreverboseinlinemode.

-VPrinttheversionnumber.

SHELL• Internalcommandsvs externalcommands• Variables- local,exported,ENV• Arithmetic- expr [+,- ,\*,/,%]• Controls

• if[expression];then;…else…fi• caseexpressionin;1);;2);;*);;esac

• Looping:• while[expression]do…done• until[expression]do…done• forindexinexpressiondo…done

• CommandLine• $1,…$9• shift• $*,$#,$$,• Set

• Funtion:• hidesthecommandline• mustassigntoshellvars

BuildingApplicationsonLinuxToolChainOverviewandComponent

Binutils,Kernel-header,GCC,Library,Debugger,Vendor&PreBuilt

Definition

• TheusualdevelopmenttoolsavailableonaGNU/Linuxworkstationisanativetoolchain

• Thistoolchainrunsonyourworkstationandgeneratescodeforyourworkstation,usuallyx86

• Forembeddedsystemdevelopment,itisusuallyimpossibleornotinterestingtouseanativetoolchain• Thetargetistoorestrictedintermsofstorageand/ormemory• Thetargetisveryslowcomparedtoyourworkstation• Youmaynotwanttoinstallalldevelopmenttoolsonyourtarget.

• Therefore,cross-compilingtoolchains aregenerallyused.Theyrunonyourworkstationbutgeneratecodeforyourtarget.

Environment

x86

ARMx86

Compilationmachine

Execution machine

Sourcecode

NativetoolchainCross-compiling

toolchain

x86binary ARMbinary

Components

Binutils Kernelheaders

C/C++libraries GCCcompiler

GDBdebugger(optional)

Binutils

• Binutils isasetoftoolstogenerateandmanipulatebinariesforagivenCPUarchitecture• as,theassembler,thatgeneratesbinarycodefrom

assemblersourcecode• ld,thelinker• ar, ranlib,togenerate.a archives,usedfor

libraries• objdump, readelf, size, nm, strings,

toinspectbinaries.Veryusefulanalysistools!• strip,tostripuselesspartsofbinariesinorderto

reducetheirsize• http://www.gnu.org/software/binutils/• GPLlicense

Kernelheaders(1)

• TheClibraryandcompiledprogramsneedstointeractwiththekernel• Availablesystemcallsandtheir

numbers• Constantdefinitions• Datastructures,etc.

• Therefore,compilingtheClibraryrequireskernelheaders,andmanyapplicationsalsorequirethem.

Kernel

Kernelheaders

CLibrary

Applications

• Available in <linux/...> and <asm/...> and a few other directories corresponding to the ones visible in include/ in the kernel sources

Kernelheaders(2)

• Systemcallnumbers,in<asm/unistd.h>

• Constant definitions, here in <asm-generic/fcntl.h>, included from <asm/fcntl.h>, included from <linux/fcntl.h>

• Data structures, here in <asm/stat.h>

#define __NR_exit 1#define __NR_fork 2#define __NR_read 3

#define O_RDWR 00000002

struct stat {unsigned long st_dev;unsigned long st_ino;

[...]};

Kernelheaders(3)

• Thekernel-to-userspace ABIisbackwardcompatible• Binariesgeneratedwithatoolchain usingkernelheadersolder

thantherunningkernelwillworkwithoutproblem,butwon'tbeabletousethenewsystemcalls,datastructures,etc.

• Binariesgeneratedwithatoolchain usingkernelheadersnewerthantherunningkernelmightworkoniftheydon'tusetherecentfeatures,otherwisetheywillbreak

• Usingthelatestkernelheadersisnotnecessary,unlessaccesstothenewkernelfeaturesisneeded

• Thekernelheadersareextractedfromthekernelsourcesusingtheheaders_install kernelMakefile target

GCCcompiler

• GNUCCompiler,thefamousfreesoftwarecompiler

• CancompileC,C++,Ada,Fortran,Java,Objective-C,Objective-C++,andgeneratecodeforalargenumberofCPUarchitectures,includingARM,AVR,Blackfin,CRIS,FRV,M32,MIPS,MN10300,PowerPC,SH,v850,i386,x86_64,IA64,Xtensa,etc.

• http://gcc.gnu.org/• AvailableundertheGPLlicense,

librariesundertheLGPL.

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Clibrary

• TheClibraryisanessentialcomponentofaLinuxsystem• Interfacebetweentheapplications

andthekernel• Providesthewell-knownstandardC

APItoeaseapplicationdevelopment

• SeveralClibrariesareavailable:glibc,uClibc,eglibc,dietlibc,newlib,etc.

• ThechoiceoftheClibrarymustbemadeatthetimeofthecross-compilingtoolchaingeneration,astheGCCcompileriscompiledagainstaspecificClibrary.

Kernel

CLibrary

Applications

glibc

• http://www.gnu.org/software/libc/• License:LGPL• ClibraryfromtheGNU project• Designedforperformance,standardscomplianceand

portability• FoundonallGNU/Linux hostsystems• Ofcourse,activelymaintained• Quitebigforsmallembeddedsystems:approx2.5MBonarm (version2.9- libc:1.5MB,libm:750KB)

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Using the Toolchain

build host target

Native buildused to build the normal gcc of a

workstation

build host target

Cross buildused to build a toolchain that runs on

your workstation but generates binaries for the target

The most common solution in embeddedThe most common solution in Enterprise

Experimentonfollowing

• Gcc ccompiler• Ld gnulinker• As assembler• Size infooncs/ds• Nm symbolicinfo(var/codecommnets/…)• Readelf infoonfile• Strip removethedebugginginfo• Objdump disassemblyinfo• Strace systemcalltrace• Objcopy exe/binay copy/conversion• Ar createarchive(libraray)• Ranlib indeding ofarchive• Make automatethebuild• Ldd infoonlibraryfortheexe• Ldconfig pathsedits

Somesupportingutilitiescanbealsotriedoutstat,file,diff,cmp,comm,patch,wc,nl,bc,script,sdiff,vimeditorwithcscope isagoodchoicefordevelopers

BuildLifecycleofanImage

SourceProgram

Preprocessorcpp / gcc -E

Compilercc1 / gcc -S

AssemblyCode Assembler

as / gcc -c

ObjectProgram

Linkerld / gcc -l

ExecutablesLoader

ApplicationBuildstyles

• DynamicBuild• defaulttheprocessisbuildwithdynamicdependencies.• Itmeans,itusessharedlibraryfortheexetoloadcertainserviceatloadtime.Example:$gcc sample.c –osample.exe$ldd ./sample.exeLinux-gate.so.1=>(0xb7776000)libc.so.6=>/lib/i386-linux-gnu/libc.so.6(0xb75a9000) /lib/ld-linux.so.2(0xb7777000)

• StaticBuild• Iftheprocessisbuildwithstaticbuild,thereisnodynamicdependency.• Itmeans,itusesstaticlibraryfortheexe.

• Thisisfastbutoccupiedalotofspace.static build

• $gcc -staticsample.c -ossample.exe

ExecutableImageonstaticanddynamicbuild

App1 App2 AppN

lib1.a lib1.a lib1.a

lib2.a lib2.a lib2.a

App1 App2 AppN

linux-gate.so.1

libc.so.6

ld-linux.so.2

Programmer’sviewofProgram

.stack

.heap

Environmentvariables

.bss

.text

.rodata

.data

_end_edata_etext

ExecutableandLinkerFormat

MaintainingSTPsStaticlibrary

DynamiclibraryRealname

LD_LIBRARY_PATHProductionreadywithldconfig

SonameandversioningtechniquesUsingdynamiclinkerasanAPIusinglibdl.so

LD_PRELOADDo’sandDon’tinSharedLibraries

LIBRARIES

• Staticlibraries• special“archivefiles”• haveextension’.a’• createdfromobjectfilesusingGNUarchiver ‘ar’• usedbylinkertoresolvereferencestofunctionsatcompile

time

• Sharedlibraries• haveextension’.so’• preferredoverstaticlibraries• Memoryoptimises• Canbebuildintwoways

• Realname• Soname

LIBRARIES

• GNUar tocreateastaticlibrary:$gcc -Wall-c-ohello.o hello.c

• $ar cr libhello.a hello.o• The‘cr’standsfor“createandreplace”

• The“tableofcontents”option‘t’canlisttheobjectfilesinanexistinglibrary

• $ar tlibhello.ahello_fn.obye_fn.o

• $gcc main.c /tmp/tmp/test/libhello.a -omain• $./main• Hello,everyone!

LIBRARIES

• /*calc.c */• #include<math.h>• #include<stdio.h>

• int main(void){

• doublex=sqrt (2.0);printf ("Thesquarerootof2.0is%f\n",x);

• return0;• }

LIBRARIES

• $gcc -Wallcalc.c -ocalc• /tmp/ccbR6Ojm.o:Infunction‘main’:• /tmp/ccbR6Ojm.o(.text+0x19):undefinedreferenceto‘sqrt’•

• ‘/tmp/ccbR60jm.o’isatemporaryobjectfilecreatedforlinking• Alibraryshouldappearafteranysourcefilesorobjectfiles

• $gcc -Wallcalc.c /usr/lib/libm.a -ocalc

LIBRARIES

• -lNAME willlinkobjectfileswithlibraryfile‘libNAME.a’inthestandardlibrarydirectories

• $gcc -Wallcalc.c -lm-ocalc• Iflibrarylglpk usesanddependsonlmlibrary,itmustappearbeforelm

• $gcc -Walldata.c -lglpk -lm• Notallcompilerssearchalllibraries,soorderlibrariesfromlefttoright

Libraries

•Alibraryisacollectionofsubprogramsusedtodevelopsoftware.•Allowscodeanddatatobereused,shared andchanged inamodularfashion.

•Linking:Alinkerresolvesthereferencesbetweenexecutables andlibraries.

BenefitsofUsingLibraries

•SoftwareEngineeringPerspective:• Increasingthereusability ofcommonroutines.•Easytoupgradebychanging thelibrariesonly.

•SystemUtilizationPerspective:•Thecodesegmentcanbesharingatruntime;decreasetheconsumeofmemoryanddiskspace.

GNUBinaryUtilitiesusedwithlib

• strings:displayallprintablecharacters.• ar:createstatic-linkinglibraries.• nm:listsymbolsfromobjectfiles.• size:listsectionsizesandtotalsizes.• readelf:• objdump:• ldd• ldconfig

PrintSharedLibraryDependencies

• UNIXPlatform> ldd /bin/bash

linux-gate.so.1 => (0xffffe000)

libncurses.so.5 => /lib/libncurses.so.5 (0xb7f1d000)

libdl.so.2 => /lib/libdl.so.2 (0xb7f19000)

libc.so.6 => /lib/libc.so.6 (0xb7dfb000)

/lib/ld-linux.so.2 (0xb7f61000)

• WindowsPlatform• PEViewerorPEExplorer• PE:PortableExecutable

Smallerismoreefficient

objjectfile

functionlibrary

objectfile

pointer

staticlinking

objectfile

functionlibrary

dynamiclinking

objectfile

pointer

sharedfunctionlibrary

executablefiles

FromSourcetoExecution

SourceProgram

Preprocessorcpp / gcc -E

Compilercc1 / gcc -S

AssemblyCode Assembler

as / gcc -c

ObjectProgram

Linkerld / gcc -l

ExecutablesLoader

static-linking

dynamic-linking

CategoriesofLibraries(bylinkingtime)

•Staticlinkinglibraries•Dynamiclinkinglibraries

•Run-TimeEnvironmentlibraries•ProgrammingLanguagelibraries

StaticLinkingLibraries

•Thecodesegmentswillbecopytoeachexecutables

•Pros:•Easytouse;nodependencyproblemaftercompilation.

•Cons:•Theexecutablesizewillbelarger.•Requirere-linkingwhenlibrarieschanged.

DynamicLinkingLibraries(1/2)

• Allowmultipleprocessestosharethesamecodesegment.• Pros:

• Greaterflexibility• Possiblesupportforplugins.

• Cons:• Slowapplicationatstarttime.• Dependentonthelibrarieswhenexecution.

DynamicLinkingLibraries(2/2)

•Thereferencescanberesolvedeitherat:•Load-time•Run-time

LocationofLibraries

•UNIXPlatform• /lib:runtimeenvironmentlibraries• /usr/lib:forprogramdevelopment

LinkingwithCRuntimeLibraries

•StaticLinking• gcc–static –ohello-shelloworld.c/usr/lib/libc.a• hello-s:521,238bytes

•DynamicLinking• gcc–ohello-dhelloworld.c• hello-d:6,020bytes

•TheCRTlibrariesconsume515,218bytes.

Static-LinkingLibraries

•Build•gcc –cmystaticlib.c•ar –rmystaticlib.a mystaticlib.o

•Usage• (sameasusingCruntimelibraries)

Createtheobjectmodulesusing–fPIC

• $gcc -fPIC -g-c-Wallmod1.cmod2.cmod3.c• -fPIC (PositionIndependentCode)causescompilertogeneratecodethatcanbeloadedanywhereinvirtualmemoryatrun-time.

• Createthesharedlibrary:• $gcc -shared-olibfoo.so mod1.omod2.omod3.o

• Usualnamingconvention:libname.so[.version]

• Linkprogramagainstlibrary:• $gcc -g-Wall-oprog prog.c libfoo.so

• or:• $gcc -g-Wall-oprog prog.c -L.-lfoo

• Thisstepembedsnameofsharedlibraryinsideexecutablefile.

Run?

• Butwhenwetrytoexecutetheprogram:

• $./prog• ./prog:errorinloadingsharedlibraries:• libfoo.so:cannotopensharedobjectfile:• Nosuchfileordirectory

• Thelibrary"libfoo.so"couldnotberesolvedatrun-timebythedynamiclinker(aka:dynamiclinkingloader).

TheDynamicLinker

• Examinessharedlibrarydependenciesinexecutablefileand• Loadssharedlibrariesintomemory(iftheyarenotalready);• ItisitselfasharedlibrarywhichispartofeveryELFexecutableusing

sharedlibraries:/lib/ld-linux.so• Itappliesasetofrulestofindlibrariesreferencedbytheprogram.• Therulesspecifyasetofstandarddirectories.

• (Forexample,manysharedlibrariesresidein/liband/usr/lib.)

• Theerrormessageearlieroccurredbecauseourlibraryisinthecurrentdirectory,whichisnotpartofstandardlist

LD_LIBRARY_PATH

• Weneedtogivethedynamiclinkerinformationonhowtofindoursharedlibraryatrun-time

• Onesolution:LD_LIBRARY_PATH.• Setthisenvironmentvariabletoalistofcolon-separateddirectories.• Dynamiclinkersearchesthesedirectoriesbeforelookinginstandardlibrarydirectories.

• $LD_LIBRARY_PATH=../prog• Calledmod1-x1• Calledmod2-x2

• Note:productionapplicationsshouldnotuseLD_LIBRARY_PATHas• Usersshouldnothavetoknowwherethelibraryislocated

Thesharedlibrarysoname

• Intheearlierexample,weembeddedtheactualname(therealname)ofthesharedlibraryinanexecutablefile.

• Itispossibletocreateanalias,calledthesoname,whichwillbeembeddedinanexecutablefileinsteadoftherealname.

• Atrun-time,thedynamiclinkerwillusethesoname whensearchingforthelibrary.

• Thepurposeofthesoname istoprovidealevelofindirection.• Atrun-time,executablecanuseaversionofthesharedlibrarythat

isdifferent(butcompatible)fromthatagainstwhichitwaslinked.

howtouseasoname

• Specifysoname whencreatingsharedlibrary:• $gcc -fPIC -c-Wall-gmod1.cmod2.cmod3.c• $gcc -shared-Wl,-soname,libbar.so -olibfoo.so \• mod1.omod2.omod3.o• -Wl,-soname,libbar.so instructslinkertomarkthesharedlibrarylibfoo.sowiththesoname libbar.so.

• Createexecutable:• $gcc -g-Wall-oprog prog.c libfoo.so

• Linkerdetectsthatlibfoo.so containsthesoname libbar.so andembedsthelatternameinsidetheexecutable.

Runtheprogram

• $LD_LIBRARY_PATH=. ./prog• ./prog:errorinloadingsharedlibraries:• libbar.so:cannotopensharedobjectfile:• Nosuchfileordirectory• Dynamiclinkercannotfindanythingnamedlibbar.so.

• Createasymboliclinkfromthesoname totherealnameofthelibrary:

• $ln -slibfoo.so libbar.so• $LD_LIBRARY_PATH=. ./prog

• Calledmod1-x1• Calledmod2-x2

• Atrun-timethislinkcanpointtoaversionofthelibrarywhichisdifferentfromtheversionagainstwhichlinkingwasperformed.

Sharedlibraryversionsandnaming

• Ifanewversionofasharedlibraryiscompatible withanexistinglibrary,wecanmakeanewminorversion ofthelibrary.

• Ifanewversionofasharedlibraryisincompatible withanexistinglibrary,wemustmakeanewmajorversion ofthelibrary.

• Constraint:itmustbepossibletocontinuerunningprogramsrequiringtheolderversionofthelibrary.

• Solution:anamingconvention isusedforsharedlibraryrealnamesandsonames.

RealnameREALNAMEisNameofthefilecontaininglibrarycode.Format:libname.so.major-id.minor-id

• Majorversionidentifierisanumberwhichissequentiallyincrementedwitheachincompatiblereleaseofthelibrary.

• Minorversionidentifierdistinguishesdifferentcompatibleminorversionsofalibrarywithinthesamemajorversion.

• Usuallyeitheranumber,ortwonumbersseparatedbyadot,withfirstnumberidentifyingminorversion,andsecondnumberindicatingapatchlevelorrevisionnumberwithintheminorversion.

Examples:• libdemo.so.1.0.1• libdemo.so.1.0.2• libdemo.so.2.0.1• libreadline.so.4.0

SonameFormat:libname.so.major-id• soname includesthesamemajorversionidentifier ascorrespondingrealname,

butdoesnotincludeminorversion identifier.

Purpose:run-timeloadingisdependentonlyonmajorversionnumberofthelibrary• soname iscreatedasasymboliclink (usuallyinsamedirectoryasrealname).• Thesoname foreachmajorlibraryversionpointstomostrecentminorversion.• Sinceitisthesoname (nottherealname)thatisembeddedinexecutableby

linker:Atanytime,wecanchangesoname symboliclinktopointtoanewerminorversion.

• Differentmajorversionscanco-exist andbeaccessedbytheprogramsthatrequirethem.

Examplesofsonames (alongwiththerealnamestowhichtheymightbesymbolicallylinked):

• libdemo.so.1->libdemo.so.1.0.2• libdemo.so.2->libdemo.so.2.0.1• libreadline.so.4->libreadline.so.4.0

Linkername

LinkernameFormat:libname.so

Purpose:allowsustoconstructversion-independentlinkcommands whichautomaticallyoperatewiththeright(i.e.mostuptodate)versionofthesharedlibrary.

Createdasasymboliclink toeitherrealname orsoname ofmostrecentmajorversionofthelibrary.

Moreconvenienttohaveitpointtosoname.

Examples:

libdemo.so ->libdemo.so.2Libreadline.so ->libreadline.so.4

createasharedlibraryusingstandardnamingconventions

Createthesharedlibrarywithrealnamelibdemo.so.1.0.1andsoname libdemo.so.1.$gcc -fPIC -g-c-Wallmod1.cmod2.cmod3.c$gcc -shared-Wl,-soname,libdemo.so.1-olibdemo.so.1.0.1\

mod1.omod2.omod3.oCreatesymboliclinksforthesoname andlinkername:$ln -slibdemo.so.1.0.1libdemo.so.1$ln -slibdemo.so.1libdemo.so$ls -llibdemo.so*|cut-c1-11,55- #Verifythesetup

lrwxrwxrwx libdemo.so ->libdemo.so.1lrwxrwxrwx libdemo.so.1->libdemo.so.1.0.1-rwxr-xr-xlibdemo.so.1.0.1

Buildexecutableusingthelinkername:$gcc -g-Wall-o./prog prog.c -L.-ldemoRuntheprogramasusual:$LD_LIBRARY_PATH=../progCalledmod1-x1Calledmod2-x2

DynamicloadingwithLinux

Function Descriptiondlopen Makesanobjectfileaccessibletoaprogramdlsym Obtainstheaddressofasymbolwithinadlopened objectfiledlerror Returnsastringerrorofthelasterrorthatoccurreddlclose Closesanobjectfile

Description

#include<dlfcn.h>

void*dlopen(const char*file,int mode);WithahandletotheELFobject,youcanidentifyaddressestosymbolswithinthisobjectusingthedlsym call.Thisfunctiontakesasymbolname,suchasthenameofafunctioncontainedwithintheobject.Thereturnvalueisaresolvedaddresstothesymbolwithintheobject:void *dlsym(void *restrict handle,const char *restrict name );IfanerroroccursduringacallwiththisAPI,youcanusethedlerror functiontoreturnahuman-readablestringrepresentingtheerror.ThisfunctionhasnoargumentsandreturnsastringifapriorerroroccurredorreturnsNULLifnoerroroccurred:char *dlerror();Finally,whennoadditionalcallstothesharedobjectarenecessary,theapplicationcancalldlclose toinformtheoperatingsystemthatthehandleandobjectreferencesarenolongernecessary.Thisisproperlyreference-counted,sothatmultipleusersofasharedobjectdonotconflictwithoneanother(itremainsinmemoryaslongasthereisauserforit).Anysymbolsresolvedthroughdlsym fortheclosedobjectwillnolongerbeavailable.char*dlclose(void*handle);

DynamicallyLoaded(DL)Libraries

• Dynamicallyloaded(DL)librariesarelibrariesthatareloadedattimesotherthanduringthestartupofaprogram.

• They'reparticularlyusefulforimplementingpluginsormodules,becausetheypermitwaitingtoloadthepluginuntilit'sneeded.

• Forexample,thePluggableAuthenticationModules(PAM)systemusesDLlibrariestopermitadministratorstoconfigureandreconfigureauthentication.

Usingreadelf forinfo

$ldd$readelf -ldl

ElffiletypeisEXEC(Executablefile)Entrypoint0x8048618Thereare7programheaders,startingatoffset52

ProgramHeaders:TypeOffsetVirtAddr PhysAddr FileSiz MemSiz Flg AlignPHDR0x0000340x080480340x080480340x000e00x000e0RE0x4INTERP0x0001140x080481140x080481140x000130x00013R0x1[Requesting programinterpreter:/lib/ld-linux.so.2]

LOAD0x0000000x080480000x080480000x009580x00958RE0x1000LOAD0x0009580x080499580x080499580x001200x00128RW0x1000DYNAMIC0x00096c0x0804996c0x0804996c0x000d00x000d0RW0x4NOTE0x0001280x080481280x080481280x000200x00020R0x4GNU_STACK0x0000000x000000000x000000000x000000x00000RW0x4

$

TestingEmbeddedApplication

QEMUCrossApplications

Qemu

Greattoolforembeddeddevelopers

Whatisqemu

• QuickEMUlator”isaprocessoremulatorthatreliesondynamicbinarytranslationtoachieveareasonablespeedwhilebeingeasytoporttonewhostCPUarchitectures• Whenusedasamachineemulator,QEMUcanrunOSes andprograms

madeforonemachine• EX:ARMboardonadifferentmachine(e.g.yourownPC).• Byusingdynamictranslation,itachievesverygoodperformance.

• Whenusedasavirtualizer,QEMUachievesnearnativeperformancesbyexecutingtheguestcodedirectlyonthehostCPU.• WhenusingKVM,QEMUcanvirtualizex86,serverandembedded

PowerPC,andS390guests.

TypicalSetup

GetQEMUConfigureQEMUforKVM

Build&InstallTesttheQEMU

GettoolchainConfiguretoolchain

Build&InstallTesttoolchain

Requirement/BoardspecificconfigurationConfigure,build&testyourkernelConfigure,build,&testfilesystem

Build&testbareapps

1 2

3

TestApplicationonemulator

• arm-none-linux-gnueabi-gcc sample.c –osample.arm

• Qemu-arm–L/path_to_cross_sdk/arm…/libc/libsample.arm

ESSENTIAL Linux For EMBEDDED Developers · 2018. 9. 20. · • The usual development tools...

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