programming embedded linux

ProgrammingEmbeddedLinux

Unabletohandlekernelpagingrequestatvirtualaddress4d1b65e8 Unabletohandlekernelpagingrequestatvirtualaddress4d1b65e8 Covers Linux version 2.6.24 pgd=c0280000 pgd=c0280000 Version 1.4 [4d1b65e8]*pgd=00000000[4d1b65e8]*pgd=00000000 Internalerror:Oops:f5[#1] Internalerror:Oops:f5[#1] Moduleslinkedin:Moduleslinkedin:hx4700_udchx4700_udcasic3_baseasic3_base CPU:0 CPU:0 PCisatset_pxa_fb_info+0x2c/0x44 PCisatset_pxa_fb_info+0x2c/0x44 LRisathx4700_udc_init+0x1c/0x38[hx4700_udc] LRisathx4700_udc_init+0x1c/0x38[hx4700_udc] pc:[]lr:[]Nottainted sp:c076df78ip:60000093fp:c076df84 pc:[]lr:[]Nottainted

Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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RightstoCopyThiskitcontainsworkbytheAttributionShareAlike2.0 Youarefree tocopy,distribute,display,andperformthework tomakederivativeworks tomakecommercialuseofthework Underthefollowingconditions Attribution.Youmustgivetheoriginalauthorcredit. ShareAlike.Ifyoualter,transform,orbuilduponthiswork, youmaydistributetheresultingworkonlyunderalicense identicaltothisone. Foranyreuseordistribution,youmustmakecleartoothersthe licensetermsofthiswork. Anyoftheseconditionscanbewaivedifyougetpermissionfrom thecopyrightholder. Yourfairuseandotherrightsareinnowayaffectedbytheabove. Licensetext:http://creativecommons.org/licenses/bysa/2.0/legalcodeCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

followingauthors: Copyright20042006 MichaelOpdenacker [email protected] http://www.freeelectrons.com Copyright20032006 OronPeled [email protected] http://www.actcom.co.il/~oron Copyright20042008 Codefidenceltd. [email protected] http://www.codefidence.com


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WhatisLinux?LinuxisakernelthatimplementsthePOSIXandSingleUnixSpecification standardswhichisdevelopedasanopensourceproject. UsuallywhenonetalksofinstallingLinux,oneisreferringtoaLinux distribution. AdistributionisacombinationofLinuxandotherprogramsandlibrarythat formanoperatingsystem. Thereexistsmanysuchdistributionforvariouspurposes,fromhighend serverstoembeddedsystems. Theyallsharethesameinterface,thankstotheLSBstandard. Linuxrunson24mainplatformsandsupportsapplicationsrangingfrom ccNUMAsuperclusterstocellularphonesandmicrocontrollers. Linuxis15yearsold,butisbasedonthe40yearsoldUnixdesignphilosophy.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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LayersinaLinuxSystem

Kernel KernelModules Clibrary Systemlibraries Applicationlibraries Userprograms

Userprograms

Kernel Clibrary



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Linuxvs.LegacyRTOSProcess Library Process

KernelProcess

Shell

RTOS

Process Process

RTOS are like the Linux kernel: Single program with single memory space that manages memory, scheduling and interrupts. Linux also has user tasks that run in their own memory space. One of them is the shell.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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ThisCourseProcess Library Process Shell

KernelProcess Process Process

In this course we will go from the shell, through the system libraries and applications and unto the kernel.



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TableofContentBasicInteractionFilesandfilesystem Theshell Processes Settingupnetworking

KernelSourcelayout Codingconvention Versions Kernelmodules MemoryManagement Characterdevice Scheduling Softandhardinterrupts NetworkingForfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

ApplicationProgrammingMakefile Processes Threads IPCCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

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TheUnixandGNU/LinuxCommandLine

TheUnixFilesystem



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EverythingisaFileAlmosteverythinginUnixisafile! Regularfiles Directories Directoriesarejustfiles listingasetoffiles Symboliclinks Filesreferringtothenameof anotherfile Devicesandperipherals Readandwritefromdevicesas withregularfiles Pipes Usedtocascadeprograms cat*.log|greperror Sockets Interprocesscommunication



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FilenamesFilenamefeaturessincethebeginningofUnix: Casesensitive. Noobviouslengthlimit. Cancontainanycharacter(includingwhitespace,except/). Filetypesstoredinthefile(magicnumbers). Filenameextensionsnotneededandnotinterpreted.Justusedfor userconvenience. Filenameexamples: README .bashrc index.htm index.htmlCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

WindowsBuglist index.html.old


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FilePathsApathisasequenceofnesteddirectorieswithafileordirectoryat theend,separatedbythe/character. Relativepath:documents/fun/microsoft_jokes.html Relativetothecurrentdirectory Absolutepath:/home/bill/bugs/crash9402031614568 /:rootdirectory. Startofabsolutepathsforallfilesonthesystem(evenforfileson removabledevicesornetworkshared).



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GNU/LinuxFilesystemStructure(1)Notimposedbythesystem.Canvaryfromonesystemtotheother, evenbetweentwoGNU/Linuxinstallations! / /bin/ /boot/ /dev/ /etc/ /home/ /lib/ Rootdirectory Basic,essentialsystemcommands Kernelimages,initrdandconfigurationfiles Filesrepresentingdevices /dev/hda:firstIDEharddisk Systemconfigurationfiles Userdirectories BasicsystemsharedlibrariesForfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license


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GNU/LinuxFilesystemStructure(2)/lost+found /mnt/ /opt/ /proc/ /root/ /sbin/ /sys/ Corruptfilesthesystemtriedtorecover Mountedfilesystems /mnt/usbdisk/,/mnt/windows/... Specifictoolsinstalledbythesysadmin /usr/local/oftenusedinstead Accesstosysteminformation /proc/cpuinfo,/proc/version... rootuserhomedirectory Administratoronlycommands Systemanddevicecontrols (cpufrequency,devicepower,etc.)Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license


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GNU/LinuxFilesystemStructure(3)/tmp/ /usr/ /usr/local/ /var/ Temporaryfiles Regularusertools(notessentialtothesystem) /usr/bin/,/usr/lib/,/usr/sbin... Specificsoftwareinstalledbythesysadmin (oftenpreferredto/opt/) Datausedbythesystemorsystemservers /var/log/,/var/spool/mail(incoming mail),/var/spool/lpd(printjobs)...



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ShellsandFileHandling



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CommandLineInterpretersShells:toolstoexecuteusercommands. Calledshellsbecausetheyhidethedetailsonthe underlyingoperatingsystemundertheshell'ssurface. Commandsareenteredusingatextterminal:eitherawindow inagraphicalenvironment,oratextonlyconsole. Resultsarealsodisplayedontheterminal.Nographicsare neededatall. Shellscanbescripted:providealltheresourcestowrite complexprograms(variable,conditionals,iterations...)Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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lsCommandListsthefilesinthecurrentdirectory,inalphanumericorder, exceptfilesstartingwiththe.character. lsa(all) Listsallthefiles(including.* files) lsl(long) Longlisting(type,date,size, owner,permissions) lst(time) Liststhemostrecentfilesfirst lsS(size) Liststhebiggestfilesfirst lsr(reverse) Reversesthesortorder lsltr(optionscanbe combined) Longlisting,mostrecentfilesat theend



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FilenamePatternSubstitutionsBetterintroducedbyexamples: ls*txt Theshellfirstreplaces*txtbyallthefileanddirectorynames endingbytxt(including.txt),exceptthosestartingwith., andthenexecutesthelscommandline. lsd.* Listsallthefilesanddirectoriesstartingwith. dtellslsnottodisplaythecontentsofdirectories. ls?.log Listsallthefileswhichnamesstartby1characterandendby .logCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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SpecialDirectories(1)./ Thecurrentdirectory.Usefulforcommandstakingadirectory argument.Alsosometimesusefultoruncommandsinthecurrent directory(seelater). So./readme.txtandreadme.txtareequivalent. ../ Theparent(enclosing)directory.Alwaysbelongstothe.directory (seelsa).Onlyreferencetotheparentdirectory. Typicalusage: cd..Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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ThecdandpwdCommandscd Changethecurrentdirectoryto. pwd Displaysthecurrentdirectory("workingdirectory").



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ThecpCommandcp Copiesthesourcefiletothetarget. cpfile1file2file3...dir Copiesthefilestothetargetdirectory(lastargument). cpi(interactive) Asksforuserconfirmationifthetargetfilealreadyexists cpr(recursive) Copiesthewholedirectory.



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ThemvandrmCommandsmv (move) Renamesthegivenfileordirectory. mvi(interactive) Ifthenewfilealreadyexits,asksforuserconfirm rmfile1file2file3... (remove) Removesthegivenfiles. rmi(interactive) Alwaysaskforuserconfirm. rmrdir1dir2dir3(recursive) Removesthegivendirectorieswithalltheircontents.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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CreatingandRemovingDirectoriesmkdirdir1dir2dir3...(makedir) Createsdirectorieswiththegivennames. rmdirdir1dir2dir3...(removedir) Removesthegivendirectories Safe:onlyworkswhendirectoriesandempty. Alternative:rmr(doesn'tneedemptydirectories).



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DisplayingFileContentsSeveralwaysofdisplayingthecontentsoffiles. catfile1file2file3...(concatenate) Concatenatesandoutputsthecontentsofthegivenfiles. morefile1file2file3... Aftereachpage,askstheusertohitakeytocontinue. Canalsojumptothefirstoccurrenceofakeyword (/command). lessfile1file2file3... Doesmorethanmorewithless. Doesn'treadthewholefilebeforestarting. Supportsbackwardmovementinthefile(?command).Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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TaskControl



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FullControlOverTasksSincethebeginning,Unixsupportstruepreemptive multitasking. Abilitytorunmanytasksinparallel,andabortthemevenif theycorrupttheirownstateanddata. Abilitytochoosewhichprogramsyourun. Abilitytochoosewhichinputyourprogramstakes,and wheretheiroutputgoes.



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ProcessesEverythinginUnixisafile EverythinginUnixthatisnotafileisaprocess Processes Instancesofarunningprograms Severalinstancesofthesameprogramcanrunatthesametime Dataassociatedtoprocesses: Openfiles,allocatedmemory,processid,parent,priority,state...



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RunningJobsinBackgroundSameusagethroughoutalltheshells. Useful:Forcommandlinejobswhichoutputcanbeexaminedlater, especiallyfortimeconsumingones. Tostartgraphicalapplicationsfromthecommandlineand thencontinuewiththemouse.

Startingatask:add&attheendofyourline:find_prince_charmingcutecleverrich&



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BackgroundJobControljobs Returnsthelistofbackgroundjobsfromthesameshell[1]Running~/bin/find_meaning_of_lifewithoutgod& [2]+Runningmakemistakes&

fg fg% Putsthelast/nthbackgroundjobinforegroundmode Movingthecurrenttaskinbackgroundmode: [Ctrl]Z bg kill% Abortsthenthjob.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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JobControlExample>jobs [1]Running~/bin/find_meaning_of_lifewithoutgod& [2]+Runningmakemistakes& >fg makemistakes >[Ctrl]Z [2]+Stoppedmakemistakes >bg [2]+makemistakes& >kill%1 [1]+Terminated~/bin/find_meaning_of_lifewithoutgod



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ListingAllProcesses...whatevershell,scriptorprocesstheyarestartedfrom psux Listsalltheprocessesbelongingtothecurrentuser psaux(Note:psedfonSystemVsystems) Listsalltheprocessesrunningonthesystempsaux USERPID%CPU%MEMVSZRSSTTYSTATSTARTTIMECOMMAND bart30390.00.259161380pts/2S14:350:00/bin/bash bart31340.00.253881380pts/3S14:360:00/bin/bash bart31900.00.263681360pts/4S14:370:00/bin/bash bart34160.00.000pts/2R15:070:00[bash]... PID: VSZ: RSS: TTY: STAT: Processid Virtualprocesssize(code+data+stack) Processresidentsize:numberofKBcurrentlyinRAM Terminal Status:R(Runnable),S(Sleep),D(Uninterruptedsleep),Z(Zombie),T(Traced)Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license


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LiveProcessActivitytopDisplaysmostimportantprocesses,sortedbycpupercentagetop15:44:33up1:11,5users,loadaverage:0.98,0.61,0.59 Tasks:81total,5running,76sleeping,0stopped,0zombie Cpu(s):92.7%us,5.3%sy,0.0%ni,0.0%id,1.7%wa,0.3%hi,0.0%si Mem:515344ktotal,512384kused,2960kfree,20464kbuffers Swap:1044184ktotal,0kused,1044184kfree,277660kcached PIDUSERPRNIVIRTRESSHRS%CPU%MEMTIME+COMMAND 3809jdoe250625639321312R93.80.80:21.49bunzip2 2769root160157m80m90mR2.716.05:21.01X 3006jdoe1503092815m27mS0.33.00:22.40kdeinit 3008jdoe16056248924468S0.30.20:06.59autorun 3034jdoe1502676412m24mS0.32.50:12.68kscd 3810jdoe16028929161620R0.30.20:00.06top

Youcanchangethesortingorderbytyping M:Memoryusage,P:%CPU,T:Time. Youcankillataskbytypingkandtheprocessid.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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KillingProcesses(1)kill Sendsaterminationsignal(SIGTERM)tothegivenprocesses.Lets processessavedataandexitbythemselves.Shouldbeusedfirst. Example: kill3039313431903416 kill9 Sendsanimmediateterminationsignal(SIGKILL).Thesystem itselfterminatestheprocesses.Usefulwhenaprocessisreallystuck (doesn'tanswertokill1). kill91 Killsalltheprocessesofthecurrentuser.1:meansallprocesses.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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KillingProcesses(2)killall[] Killsallthejobsrunning.Example: killallbash



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SequentialCommandsCantypethenextcommandinyourterminalevenwhenthe currentoneisnotover. Canseparatecommandswiththe;symbol: echonIlovethee;sleep5;echonot Conditionals:use||(or)or&&(and): moreGod||echoSorry,Goddoesn'texist Runsechoonlyifthefirstcommandfails lsmydir&&catmydir/* Onlycatsthedirectorycontentsifthelscommandsucceeds (meansreadaccess).Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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SystemAdministrationBasics



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ShuttingDownshutdownh+5(h:halt) Shutsthesystemdownin5minutes. Usersgetawarningintheirconsoles. shutdownrnow(r:reboot) init0 Anotherwaytoshutdown(usedbyshutdown). init6 Anotherwaytoreboot(usedbyshutdown). [Ctrl][Alt][Del] AlsoworksonGNU/Linux(atleastonPCs!).Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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NetworkSetup(1)ifconfiga Printsdetailsaboutallthenetworkinterfacesavailable onyoursystem. ifconfigeth0 Listsdetailsabouttheeth0interface ifconfigeth0192.168.0.100 Assignsthe192.168.0.100IPaddress toeth0(1IPaddressperinterface). ifconfigeth0down Shutsdowntheeth0interface (freesitsIPaddress).Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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NetworkSetup(2)routeadddefaultgw192.168.0.1 Setsthedefaultrouteforpacketsoutsidethelocalnetwork. Thegateway(here192.168.0.1)isresponsiblefor sendingthemtothenextgateway,etc.,untilthefinal destination. route Liststheexistingroutes routedeldefault routedel Deletesthegivenroute Usefultoredefineanewroute.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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NetworkTestingpingfreshmeat.net ping192.168.1.1 Triestosendpacketstothegivenmachineandgetacknowledgment packetsinreturn.PING192.168.1.1(192.168.1.1)56(84)bytesofdata. 64bytesfrom192.168.1.1:icmp_seq=0ttl=150time=2.51ms 64bytesfrom192.168.1.1:icmp_seq=1ttl=150time=3.16ms 64bytesfrom192.168.1.1:icmp_seq=2ttl=150time=2.71ms 64bytesfrom192.168.1.1:icmp_seq=3ttl=150time=2.67ms

Whenyoucanpingyourgateway,yournetworkinterfaceworksfine. WhenyoucanpinganexternalIPaddress,yournetworksettingsare correct!Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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NetworkSetupSummaryOnlyforsimplecaseswithoneinterface,noDHCPserver... Connecttothenetwork(cable,wirelesscardordevice...) Identifyyournetworkinterface: ifconfiga AssignanIPaddresstoyourinterface(assumingeth0) ifconfigeth0192.168.0.100(example) Addaroutetoyourgateway(assuming192.168.0.1)for packetsoutsidethenetwork: routeadddefaultgw192.168.0.1Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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NameResolutionYourprogramsneedtoknowwhatIPaddresscorrespondstoa givenhostname(suchaskernel.org) DomainNameServers(DNS)takecareofthis. YoujusthavetospecifytheIPaddressof1ormoreDNS serversinyour/etc/resolv.conffile: nameserver217.19.192.132 nameserver212.27.32.177 Thechangestakeseffectimmediately!



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MountingDevices(1)Tomakefilesystemsonanydevice(internalorexternalstorage) visibleonyoursystem,youhavetomountthem. Thefirsttime,createamountpointinyoursystem: mkdir/mnt/usbdisk(example) Now,mountit: mounttvfat/dev/sda1/mnt/usbdisk /dev/sda1:physicaldevice t:specifiesthefilesystem(format)type (ext2,ext3,vfat,reiserfs,iso9660...)



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MountingDevices(2)Lotsofmountoptionsareavailable,inparticulartochoosepermissionsorthe fileownerandgroup...Seethemountmanualpagefordetails. Mountoptionsforeachdevicecanbestoredinthe/etc/fstabfile. Youcanalsomountafilesystemimagestoredinaregularfile(loopback devices) UsefultoaccessthecontentsofanISOcdromimagewithouthavingto burnit. UsefultocreateaLinuxpartitiononaharddiskwithonlyWindows partitions cp/dev/sda1usbkey.img mountolooptvfatusbkey.img/mnt/usbdiskCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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ListingMountedFilesystemsJustusethemountcommandwithnoargument:/dev/hda6on/typeext3(rw,noatime) noneon/proctypeproc(rw,noatime) noneon/systypesysfs(rw) noneon/dev/ptstypedevpts(rw,gid=5,mode=620) usbfson/proc/bus/usbtypeusbfs(rw) /dev/hda4on/datatypeext3(rw,noatime) noneon/dev/shmtypetmpfs(rw) /dev/hda1on/wintypevfat(rw,uid=501,gid=501) noneon/proc/sys/fs/binfmt_misctypebinfmt_misc(rw)

Ordisplaythe/etc/mtabfile (sameresult,updatedbymountandumounteachtimetheyarerun)Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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UnmountingDevicesumount/mnt/usbdisk Commitsallpendingwritesandunmountsthegivendevice, whichcanthenberemovedinasafeway. Tobeabletounmountadevice,youhavetocloseallthe openfilesinit:Closeapplicationsopeningdatainthemountedpartition Makesurethatnoneofyourshellshaveaworkingdirectoryin thismountpoint. Youcanrunthelsofcommand(listopenfiles)toview whichprocessesstillhaveopenfilesinthemountedpartition.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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CreatingFilesystemsExamples mkfs.ext2/dev/sda1 FormatsyourUSBkey(/dev/sda1:1stpartitionrawdata)inext2format. mkfs.ext2Fdisk.img Formatsadiskimagefileinext2format mkfs.vfatvF32/dev/sda1(v:verbose) FormatsyourUSBkeybacktoFAT32format. mkfs.vfatvF32disk.img FormatsadiskimagefileinFAT32format. Blankdiskimagescanbecreatedasinthebelowexample: ddif=/dev/zeroof=disk.imgbs=1024count=65536Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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Help



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CommandHelpSomeUnixcommandsandmostGNU/Linuxcommandsoffer atleastonehelpargument: h (ismostlyusedtointroduce1characteroptions) help (isalwaysusedtointroducethecorrespondinglong optionname,whichmakesscriptseasiertounderstand) Youalsooftengetashortsummaryofoptionswhenyouinput aninvalidargument.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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ManualPagesman[section] Displaysoneorseveralmanualpagesforfrom optional[section]. manfork Manpageofthefork()systemcall manfstab Manpageofthefstabconfigurationfile manprintf Manofprintf()shellcommand man3printf Manofprintf()libraryfunctionCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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CodingEmbeddedLinuxApplications

WritingApplications



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ASimpleMakefileMakefile variables

CC=/usr/local/arm/2.95.3/bin/armlinuxgcc CFLAGS=g LDFLAGS=lpthreadsTo build target all build target test

Always build the clean and all targets, if asked

.PHONY:cleanall all:test

These must be a TAB, not a SPACE! To link test from test.o, run the following commandCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

To compile test.o test.o:test.ctest.h from test.c and test.h, run the $(CC)$(CFLAGS)Ctest.c following command

test:test.o $(CC)$(LDFLAGS)test.ootest clean: @rmf*.o*~testForfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

To clean, run the following command

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ANotSoSimpleMakefile.PHONY:cleanall SRC:=$(shellls*.c) EXE:=test OBJ:=$(SRC:.c=.o) LDFLAGS:=lsqlite3 CFLAGS:=g all:$(EXE)

%.o:%.c $(COMPILE.c)MDo$@$< @cp$*.d$*.P;\ sede's/#.*//'e's/^[^:]*:*//'e's/*\\$$//'\ e'/^$$/d'e's/$$/:/'>$*.P;\ rmf$*.d $(EXE):$(OBJ) $(CC)$(LDFLAGS)o$@$(OBJ) clean: rmf$(EXE)$(OBJ)$(SRC:.c=.P)*~ include$(SRC:.c=.P) #Source: http://make.paulandlesley.org/autodep.html

This Makefile automatically compiles all files ending in .c in the current directory, computes dependency information for them and links them into an executable called test.



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HelloWorld!Linux Application API is ISO C and POSIX!#include intmain(intargc,char**argv) { inti=1; printf("HelloWorld!%d\n",i); return0; } Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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CreatingProcessesUsingfork()pid_tpid; if(pid=fork()){ intstatus; printf("I'mtheparent!\n"); wait(&status); if(WIFEXITED(status)) printf("Childexistwithstatusof%d\n",WEXITSTATUS(status)); }else{ printf("I'mthechild!\n"); exit(0);

}Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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Howfork()SeemstoWorkParent ProcessA complete copy is created of the parent process. Both parent and child continue execution from the same spot.

Copy Parent Process Child Process



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Howfork()ReallyWorksParent Process RW MemoryChild process gets a copy of stack and file descriptors. Copy On Write reference is used for the memory.

Parent Process

RO

Memory

RO

Child Process



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WhatHappensDuringWrite?When write is attempted, a single memory page is copied and references updated. This is called: breaking the CoW.

Original Memory

RO

RW

Child Process

Parent Process

RO



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CreatingProcesses(2)This call will replace the program memory (code and data) with the image from storage at the specified path. Open file descriptors, priority and other properties remains the same.

pid_tpid; if(pid=fork()){ intstatus; printf("I'mtheparent!\n"); wait(&status); }else{ printf("I'mthechild!\n"); execve(/bin/ls,argv,envp); }



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ExitingProcessesAprocessexistswheneitherofthefollowingoccurs:Returnfromthemainfunction. Callingtheexit()function. Exception(moreonthoselater).

Aprocessshouldreturnanexitstatustoit'sparentprocess Uponexit:Allexithandlersarecalled(useatexit()toregisterthem) Allmemory,filedescriptorsandotherresourcesarereleased bythesystem.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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LinuxProcessStackLinuxprocessstackisautoexpanding. Adefaultstack(8Mb)isallocatedatprocesscreation. Bydefault,useofadditionalstackwillautomaticallytrigger allocationofmorestackspace. Thisbehaviorcanbelimitedbysettingaresourcelimiton thestacksize.Seesetrlimit()



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LinuxPrioritiesNice level -20 -19 -18 Real Time priority 99 98 97

...16 17 18 19Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

...4 3 2 1 0

Real time processes SCHED_FIFO SCHED_RR

Non real-time processes SCHED_OTHER


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POSIXPriorities(2)SCHED_OTHER:DefaultLinuxtimesharingscheduling Priority0isreservedforit. Fair,nostarvation. Usethisforanynontimecriticaltasks. SCHED_FIFO:FirstInFirstOutscheduling Priorities199. Preemptive. SCHED_RR:RoundRobinscheduling LikeSCHED_FIFO+timesliceCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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ChangingRealTimePrioritiesintsched_setscheduler(pid_tpid,intpolicy,conststructsched_param*p); structsched_param{ intsched_priority }; sched_setscheduler()setsboththeschedulingpolicyandtheassociated parametersfortheprocessidentifiedbypid.Ifpidequalszero,the schedulerofthecallingprocesswillbeset.Theinterpretationofthe parameterpdependsontheselectedpolicy.Currently,thefollowingthree schedulingpoliciesaresupportedunderLinux:SCHED_FIFO, SCHED_RR,andSCHED_OTHER; Thereisalsoasched_getscheduler().Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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LockingMemoryintmlock(constvoid*addr,size_tlen);mlockdisablespagingforthememoryintherangestartingataddrwithlengthlen bytes.

intmlockall(intflags);mlockall()disablespagingforallpagesmappedintotheaddressspaceofthecalling process. MCL_CURRENTlocksallpageswhicharecurrentlymappedintotheaddress spaceoftheprocess. MCL_FUTURElocksallpageswhichwillbecomemappedintotheaddressspace oftheprocessinthefuture.Thesecouldbeforinstancenewpagesrequiredbya growingheapandstackaswellasnewmemorymappedfilesorsharedmemory regions. Mustlockmemorytoguaranteerealtimeresponses!Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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RealTimeResponsesTogetdeterministicrealtimeresponsefromaLinuxprocess, makesureto:Puttheprocessinarealtimeschedulingdomain. mlockall()processmemory. Prefaultstackpages Todothiscalladummyfunctionthatallocatesonstackan automaticvariablebigenoughforyourentirefuturestack usageandwritestoit.



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ThreadsandProcessesStack State Signal Mask Priority Stack State Signal Mask Priority Stack State Signal Mask Priority Stack State Signal Mask Priority Stack State Signal Mask Priority

Thread 1Process 123

Thread

Thread

Thread

Thread

1

2Process 124

3

4

File Signal Memory Descriptors Handlers

File Signal Memory Descriptors Handlers



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POSIXThreadsLinuxusesthePOSIXThreadsthreadingmechanism. LinuxthreadsareLightWeightProcesseseachthreadisatask scheduledbythekernel'sscheduler. Processcreationtimeisroughlydoublethanthatofathread's. ButLinuxprocesscreationtimeisrelativelylow... Tousethreadsinyourcode: #include InyourMakefile: Add-lpthreadtoCFLAGS.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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CreatingThreadsFunction:pthread_create() intpthread_create(pthread_t*thread,constpthread_attr_t*attr, void*(*start_routine)(void*),void*arg); Thepthread_create()routinecreatesanewthreadwithina process.Thenewthreadstartsinthestartroutinestart_routine whichhasastartargumentarg.Thenewthreadhasattributes specifiedwithattr,ordefaultattributesifattrisNULL. Ifthepthread_create()routinesucceedsitwillreturn0andputthe newthreadIDintothread,otherwiseanerrornumbershallbe returnedindicatingtheerror.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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CreatingThreadAttributesFunction:pthread_attr_init() intpthread_attr_init(pthread_attr_t*attr); Settingattributesforthreadsisachievedbyfillingathreadattribute objectattroftypepthread_attr_t,thenpassingitasasecondargument topthread_create(3).PassingNULLisequivalenttopassingathread attributeobjectwithallattributessettotheirdefaultvalues. pthread_attr_init()initializesthethreadattributeobjectattrandfillsit withdefaultvaluesfortheattributes. Eachattributeattrnamecanbeindividuallysetusingthefunction pthread_attr_setattrname()andretrievedusingthefunction pthread_attr_getattrname().Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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DestroyingThreadAttributesFunction:pthread_attr_destroy() intpthread_attr_destroy(pthread_attr_t*attr); pthread_attr_destroy()destroysathreadattributeobject,which mustnotbereuseduntilitisreinitialized.pthread_attr_destroy() doesnothingintheLinuxThreadsimplementation. Attributeobjectsareconsultedonlywhencreatinganewthread. Thesameattributeobjectcanbeusedforcreatingseveralthreads. Modifyinganattributeobjectafteracalltopthread_create()does notchangetheattributesofthethreadpreviouslycreated.



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DetachStateThreadAttribute:detachstate Controlwhetherthethreadiscreatedinthejoinablestateorinthedetached state.Thedefaultisjoinablestate. Inthejoinablestate,anotherthreadcansynchronizeonthethreadtermination andrecoveritsterminationcodeusingpthread_join(3),butsomeofthethread resourcesarekeptallocatedafterthethreadterminates,andreclaimedonly whenanotherthreadperformspthread_join(3)onthatthread. Inthedetachedstate,thethreadresourcesareimmediatelyfreedwhenit terminates,butpthread_join(3)cannotbeusedtosynchronizeonthethread termination. Athreadcreatedinthejoinablestatecanlaterbeputinthedetachedthread usingpthread_detach(3).Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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SchedPolicyThreadAttribute:schedpolicy Selecttheschedulingpolicyforthethread:oneof SCHED_OTHER(regular,nonrealtimescheduling), SCHED_RR(realtime,roundrobin)orSCHED_FIFO(realtime, firstinfirstout). Defaultvalue:SCHED_OTHER. TherealtimeschedulingpoliciesSCHED_RRandSCHED_FIFO areavailableonlytoprocesseswithsuperuserprivileges. Theschedulingpolicyofathreadcanbechangedaftercreation withpthread_setschedparam(3).Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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SchedParamThreadAttribute:schedparam Containtheschedulingparameters(essentially,thescheduling priority)forthethread. Defaultvalue:priorityis0. Thisattributeisnotsignificantiftheschedulingpolicyis SCHED_OTHER;itonlymattersfortherealtimepolicies SCHED_RRandSCHED_FIFO. Theschedulingpriorityofathreadcanbechangedaftercreation withpthread_setschedparam(3).Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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InheritSchedThreadAttribute:inheritsched Indicatewhethertheschedulingpolicyandschedulingparameters forthenewlycreatedthreadaredeterminedbythevaluesofthe schedpolicyandschedparamattributes (PTHREAD_EXPLICIT_SCHED)orareinheritedfromtheparent thread(valuePTHREAD_INHERIT_SCHED). Defaultvalue:PTHREAD_EXPLICIT_SCHED.



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DestroyingThreadsFunction:pthread_exit() voidpthread_exit(void*status); Thepthread_exit()routineterminatesthecurrentlyrunningthread andmakesstatusavailabletothethreadthatsuccessfullyjoins, pthread_join(),withtheterminatingthread.Inaddition, pthread_exit()executesanyremainingcleanuphandlersinthe reverseordertheywerepushed,pthread_cleanup_push(),after whichallappropriatethreadspecificdestructorsarecalled. Animplicitcalltopthread_exit()ismadeifanythread,otherthan thethreadinwhichmain()wasfirstcalled,returnsfromthestart routinespecifiedinpthread_create().Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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ThreadandProcessTerminationActionpthread_exit()

Main ThreadThread terminates All threads terminate All threads terminate

Other ThreadThread terminates All threads terminate Thread terminates

exit() Thread function returns



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WaitingForaThreadtoFinishFunction:pthread_join() intpthread_join(pthread_tthread,void**status); Ifthetargetthreadthreadisnotdetachedandtherearenootherthreads joinedwiththespecifiedthreadthenthepthread_join()function suspendsexecutionofthecurrentthreadandwaitsforthetargetthread threadtoterminate.Otherwisetheresultsareundefined. Onasuccessfulcallpthread_join()willreturn0,andifstatusisnon NULLthenstatuswillpointtothestatusargumentofpthread_exit(). Onfailurepthread_join()willreturnanerrornumberindicatingthe error. Alsoexistspthread_tryjoin_np()andpthread_timedjoin_np().Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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CancelingThreadsFunction:pthread_cancel() intpthread_cancel(pthread_tthread); Cancellationisthemechanismbywhichathreadcanterminatethe executionofanotherthread.Moreprecisely,athreadcansenda cancellationrequesttoanotherthread.Dependingonitssettings,the targetthreadcantheneitherignoretherequest,honoritimmediately,or deferittillitreachesacancellationpoint. Whenathreadeventuallyhonorsacancellationrequest,itperformsas ifpthread_exit(PTHREAD_CANCELED)hasbeencalledatthatpoint. pthread_cancel()sendsacancellationrequesttothethreaddenotedby thethreadargument.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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CancelStateFunction:pthread_setcancelstate() intpthread_setcancelstate(intstate,int*oldstate); pthread_setcancelstate()changesthecancellationstateforthecalling threadthatis,whethercancellationrequestsareignoredornot. Thestateargumentisthenewcancellationstate:either PTHREAD_CANCEL_ENABLEtoenablecancellation,or PTHREAD_CANCEL_DISABLEtodisablecancellation (cancellationrequestsareignored). IfoldstateisnotNULL,thepreviouscancellationstateisstoredinthe locationpointedtobyoldstate,andcanthusberestoredlaterby anothercalltopthread_setcancelstate().Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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CancelTypeFunction:pthread_setcanceltype() intpthread_setcanceltype(inttype,int*oldtype); pthread_setcanceltype()changesthetypeofresponsestocancellation requestsforthecallingthread:asynchronous(immediate)ordeferred. Thetypeargumentisthenewcancellationtype:either PTHREAD_CANCEL_ASYNCHRONOUStocancelthecallingthread assoonasthecancellationrequestisreceived,or PTHREAD_CANCEL_DEFERREDtokeepthecancellationrequest pendinguntilthenextcancellationpoint.IfoldtypeisnotNULL,the previouscancellationstateisstoredinthelocationpointedtobyoldtype, andcanthusberestoredlaterbyanothercallto pthread_setcanceltype().Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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CancellationPointDefaultsThreadsarealwayscreatedbypthread_create(3)withcancellation enabledanddeferred.Thatis,theinitialcancellationstateis PTHREAD_CANCEL_ENABLEandtheinitialtypeis PTHREAD_CANCEL_DEFERRED.



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CancellationPointsCancellationpointsarethosepointsintheprogramexecutionwherea testforpendingcancellationrequestsisperformedandcancellationis executedifpositive. ThefollowingPOSIXthreadsfunctionsarecancellationpoints: pthread_join(3),pthread_cond_wait(3),pthread_cond_timedwait(3), pthread_testcancel(3),sem_wait(3),sigwait(3). AllotherPOSIXthreadsfunctionsareguaranteednottobecancellation points.Thatis,theyneverperformcancellationindeferredcancellation mode. Anumberofsystemcalls(basically,allsystemcallsthatmayblock, suchasread(2),write(2),wait(2),etc.)andlibraryfunctionsthatmay callthesesystemcalls(e.g.fprintf(3))arecancellationpoints.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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TestCancelFunction:pthread_testcancel() voidpthread_testcancel(void); pthread_testcancel()doesnothingexcepttestingforpending cancellationandexecutingit.Itspurposeistointroduceexplicit checksforcancellationinlongsequencesofcodethatdonotcall cancellationpointfunctionsotherwise.



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LinuxIPCPOSIXIPC SysVIPC Pipes UnixDomainSockets Signals TCP/IPSocketsCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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CreatingaMutexFunction:pthread_mutex_init() intpthread_mutex_init(pthread_mutex_t*mutex,const pthread_mutex_attr*attr); Thepthread_mutex_init()routinecreatesanewmutex,with attributesspecifiedwithattr,ordefaultattributesifattrisNULL. Ifthepthread_mutex_init()routinesucceedsitwillreturn0and putthenewmutexIDintomutex,otherwiseanerrornumbershall bereturnedindicatingtheerror.



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DestroyingaMutexFunction:pthread_mutex_destroy() intpthread_mutex_destroy(pthread_mutex_t*mutex); Thepthread_mutex_destroy()routinedestroysthemutexspecified bymutex. Ifthepthread_mutex_destroy()routinesucceedsitwillreturn0, otherwiseanerrornumbershallbereturnedindicatingtheerror.



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LockingMutexesFunction:pthread_mutex_lock() intpthread_mutex_lock(pthread_mutex_t*mutex); Thepthread_mutex_lock()routineshalllockthemutexspecifiedbymutex.Ifthe mutexisalreadylocked,thecallingthreadblocksuntilthemutexbecomesavailable. Ifthepthread_mutex_lock()routinesucceedsitwillreturn0,otherwiseanerror numbershallbereturnedindicatingtheerror. Function:pthread_mutex_trylock() intpthread_mutex_trylock(pthread_mutex_t*mutex); Thepthread_mutex_trylock()routineshalllockthemutexspecifiedbymutexand return0,otherwiseanerrornumbershallbereturnedindicatingtheerror.Inallcases thepthread_mutex_trylock()routinewillnotblockthecurrentrunningthread.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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LockingMutexesFunction:pthread_mutex_timedlock() intpthread_mutex_timedlock(pthread_mutex_t*mutex,structtimespec*restrict abs_timeout); Thepthread_mutex_timedlock()functionshalllockthemutexobjectreferenced bymutex.Ifthemutexisalreadylocked,thecallingthreadshallblockuntilthe mutexbecomesavailableasinthepthread_mutex_lock()function.Ifthemutex cannotbelockedwithoutwaitingforanotherthreadtounlockthemutex,this waitshallbeterminatedwhenthespecifiedtimeoutexpires. Thetimeoutshallexpirewhentheabsolutetimespecifiedbyabs_timeout passes,asmeasuredbytheclockonwhichtimeoutsarebased(thatis,whenthe valueofthatclockequalsorexceedsabs_timeout),oriftheabsolutetime specifiedbyabs_timeouthasalreadybeenpassedatthetimeofthecall.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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UnlockingMutexesFunction:pthread_mutex_unlock() intpthread_mutex_unlock(pthread_mutex_t*mutex); Ifthecurrentthreadistheownerofthemutexspecifedbymutex, thenthepthread_mutex_unlock()routineshallunlockthemutex. Ifthereareanythreadsblockedwaitingforthemutex,the schedulerwilldeterminewhichthreadobtainsthelockonthe mutex,otherwisethemutexisavailabletothenextthreadthat callstheroutinepthread_mutex_lock(),or pthread_mutex_trylock(). Ifthepthread_mutex_unlock()routinesucceedsitwillreturn0, otherwiseanerrornumbershallbereturnedindicatingtheerror.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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PriorityInversion2. High Priority task preempts low priority task

99 Task Priority3. Hi Priority task block on mutex

50

4. Medium Priority task preempts low priority task and high priority task

31. Low Priority task takes mutex

TimeCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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PriorityInheritanceandCeilingsPriorityinheritanceandceilingsaremethodstoprotect againstpriorityinversions. Linuxonlygotsupportforthemin2.6.18. Patchestoaddsupportforolderversionexists.EmbeddedLinuxvendorsusuallyprovidepatchedkernels.

Ifthekernelversionyou'reusingisnotpatched,makesureto protectagainstthisscenarioindesignOnepossibleway:raisethepriorityofeachtaskstryingto grabamutextothemaximumpriorityofallpossible contenders.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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POSIXConditionVariablesintpthread_cond_wait(pthread_cond_t*cond, pthread_mutex_t*mutex); intpthread_cond_timedwait(pthread_cond_t*cond, pthread_mutex_t*mutex,conststructtimespec *abstime); intpthread_cond_signal(pthread_cond_t*cond); intpthread_cond_broadcast(pthread_cond_t*cond); Thefirsttwofunctioncallsareusedforwaitingontheconditionvar. Thelattertwofunctioncallsareusedtowakeupwaitingtasks: pthread_cond_signal()wakesuponesingletaskthatiswaitingforthe conditionvariable. pthread_cond_broadcast()wakesupallwaitingtasks.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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ConditionVariableUsageExamplevoidmy_wait_for_event(pthread_mutex_t*lock,pthread_cond_t*cond){ pthread_mutex_lock(lock); while(flag==0) pthread_cond_wait(cond,lock); flag=0; pthread_mutex_unlock(lock); } voidmy_post_event(pthread_mutex_t*lock,pthread_cond_t*cond){ pthread_mutex_lock(lock); flag=1; pthread_cond_signal(cond); pthread_mutex_unlock(lock); }

Theconditionvariablefunctioncallsareusedwithinanareaprotectedbythe mutexthatbelongtotheconditionvariable.Theoperatingsystemreleasesthe mutexeverytimeitblocksataskontheconditionvariable;andithaslocked themutexagainwhenitunblocksthecallingtaskfromthesignalingcall.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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POSIXSemaphoresPOSIXSemaphoresalsoavailable:#include intsem_init(sem_t*sem,intpshared,unsignedintvalue); intsem_wait(sem_t*sem); intsem_trywait(sem_t*sem); intsem_post(sem_t*sem); intsem_getvalue(sem_t*sem,int*sval); intsem_destroy(sem_t*sem);

ThepsharedflagismeaninglessinLinuxandmustbe0.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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SharedMemoryUsingmmap()void*mmap(void*start,size_tlength,intprot,intflags,intfd, off_toffset); Themmap()functionaskstomaplengthbytesstartingatoffset offsetfromthefile(ordevice)specifiedbythefiledescriptorfd intoprocessvirtualmemoryatakernelchosenaddress,but preferablystartwithprotectionofprot. Theactualplacewheretheobjectismappedisreturnedby mmap()(apointer). ThereturnvalueincaseofanerrorisMAP_FAILED(1)and NOT0!Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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MMAPFlagsMAP_SHARED:Sharethismappingwithallotherprocessesthatmapthis object.Storingtotheregionisequivalenttowritingtothefile.Thefilemay notactuallybeupdateduntilmsync(2)ormunmap(2)arecalled. MAP_FIXED:Donotselectadifferentaddressthantheonespecified.Ifthe specifiedaddresscannotbeused,mmap()willfail.IfMAP_FIXEDis specified,startmustbeamultipleofthepagesize. MAP_PRIVATE:Createaprivatecopyonwritemapping.Storestothe regiondonotaffecttheoriginalfile.Itisunspecifiedwhetherchangesmade tothefileafterthemmap()callarevisibleinthemappedregion. MAP_ANONYMOUS:Themappingisnotbackedbyanyfile;thefdand offsetargumentsareignored.



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POSIXSharedMemoryvoid*shm_open(constchar*name,intoflag,mode_tmode); shm_open()createsandopensanew,oropensanexisting,POSIXshared memoryobject.APOSIXsharedmemoryobjectisineffectahandlewhich canbeusedbyunrelatedprocessestommap(2)thesameregionofshared memory.Theshm_unlink()functionperformstheconverseoperation, removinganobjectpreviouslycreatedbyshm_open(). Theoperationofshm_open()isanalogoustothatofopen(2).namespecifies thesharedmemoryobjecttobecreatedoropened.Forportableuse,name shouldhaveaninitialslash(/)andcontainnoembeddedslashes. Tip:makesuretoreservethesizeofthesharedmemoryobjectusing ftruncate()!Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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SysVMailboxeskey_tftok(constchar*pathname,intproj_id); intmsgget(key_tkey,intmsgflg); intmsgsnd(intmsqid,structmsgbuf*msgp,size_tmsgsz,intmsgflg); ssize_tmsgrcv(intmsqid,structmsgbuf*msgp,size_tmsgsz,longmsgtyp, intmsgflg); ftok()generatesakeyfromthepathnameandprojectIDproj_id. msgget()createsamailboxfromthekey. msgsnd()sendsamessageandmsgrcv()retrievesit. Thedifferentflagscancontrolmessagestypes(priorities),ifreceivingisa blockingornonblockingoperationsandmailboxpermissions.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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SysVMailboxMessagesstructmsgbuf{ longmtype;/*messagetype,mustbe>0*/ charmtext[1];/*messagedata*/ }; Themtextfieldisanarray(orotherstructure)whosesizeis specifiedbymsgszparameter. Themtypefieldmusthaveapositiveintegervaluethatcan beusedbythereceivingprocessformessageselection.



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SysVMailboxMessageTypesFormsgrcv(),themsgtypargumentspecifiesthetypeof messagerequestedasfollows:Ifmsgtypis0,thenthefirstmessageinthequeueisread. Ifmsgtypisgreaterthan0,thenthefirstmessageonthequeue oftypemsgtypisread,unlessMSG_EXCEPTwasassertedin msgflg,inwhichcasethefirstmessageonthequeueoftype notequaltomsgtypwillberead. Ifmsgtypislessthan0,thenthefirstmessageonthequeue withthelowesttypelessthanorequaltotheabsolutevalueof msgtypwillberead.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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SysVMailboxesFlagsThemsgflgargumentassertsnone,oneormore(oringthem) ofthefollowingflags:IPC_NOWAITForimmediatereturnifnomessageofthe requestedtypeisonthequeue.Thesystemcallfailswitherrno settoENOMSG. MSG_EXCEPTUsedwithmsgtypgreaterthan0toreadthe firstmessageonthequeuewithmessagetypethatdiffersfrom msgtyp. MSG_NOERRORTotruncatethemessagetextiflonger thanmsgszbytes.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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POSIXPipesintpipe(intfiledes[2]); pipe()createsapairoffiledescriptors,pointingtoapipeinode, andplacestheminthearraypointedtobyfiledes.filedes[0]is forreading,filedes[1]isforwriting. FILE*popen(constchar*command,constchar*type); Thepopen()functionopensaprocessbycreatingapipe,forking, andinvokingtheshell.Sinceapipeisbydefinitionunidirectional, thetypeargumentmayspecifyonlyreadingorwriting,notboth; theresultingstreamiscorrespondinglyreadonlyorwriteonly. Anamedversionofpipes,calledfifoalsoexists.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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UNIXDomainSocketsFilesinthefilesystemthatactslikesockets. Allnormalsocketoperationsapplies. structsockaddr_unserver; intsock; sock=socket(AF_UNIX,SOCK_STREAM,0); server.sun_family=AF_UNIX; strcpy(server.sun_path,SOCKET_NAME); bind(sock,(structsockaddr*)&server,sizeof(structsockaddr_un)); UDScanbeusedtopassfiledescriptorsbetweenprocessesusinga specialsocketoptionoperation.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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SignalsSignalsareasynchronousnotificationssenttoaprocessby thekerneloranotherprocess Signalsinterruptwhatevertheprocesswasdoingatthetime tohandlethesignal. Eachsignalmayhaveasignalhandler,whichisafunction thatgetscalledwhentheprocessreceivesthatsignal. Twodefaultsignalhandlersalsoexist:SIG_IGN:Causestheprocesstoignorethespecifiedsignal. SIG_DFL:Causesthesystemtosetthedefaultsignalhandler forthegivensignal.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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RegularSignals

SIGHUP SIGINT SIGILL SIGFPE

Hangupdetectedoncontrollingterminal Interruptfromkeyboard IllegalInstruction Floatingpointexception

SIGBUS

Buserror(badmemoryaccess)

SIGQUIT Quitfromkeyboard SIGABRT Abortsignalfromabort(3) SIGKILL Killsignal SIGSEGV Invalidmemoryreference SIGPIPE Brokenpipe:writetopipewithnoreaders SIGALRM Timersignalfromalarm(2) SIGTERM Terminationsignal

SIGPOLL Pollableevent(SysV).SynonymofSIGIO SIGPROF Profilingtimerexpired SIGSYS SIGURG SIGIO Badargumenttoroutine(SVID) Urgentconditiononsocket(4.2BSD) I/Onowpossible(4.2BSD) SIGTRAP Trace/breakpointtrap

Seesignal(7)fordefaultbehaviors. Therearetwosignalhandlersyou cannotmodifyorignoreSIGKILL andSIGSTOP.

SIGUSR1 Userdefinedsignal1 SIGUSR2 Userdefinedsignal2SIGCHLD Childstoppedorterminate SIGCONT Continueifstopped SIGSTOP Stopprocess SIGTSTP Stoptypedattty SIGTTIN ttyinputforbackgroundprocess SIGTTOU ttyoutputforbackgroundprocess



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LimitationsofRegularSignalsToofewsignals Only2signalsavailableforuserdefinedpurposes:USR1,USR2.Alltheresthave predefinedmeaning. Noqueuing Ifthesamesignalissentmultipletimesuntilataskprocessesit,itisonlydeliveredonce. Nopriority Multiplesignalsdeliveredaccordingtoordersent. Portability SysVsignalhandlersarepoppedbeforeexecuted(sadly,alsoaracecondition). SignalMasking Installingthesamesignalhandlerformultiplesignalsmaycausereentrancy. NopassingofavaluewiththesignalCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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RealTimeSignalsAdditional32signalsfromSIGRTMINtoSIGRTMAX Nopredefinedmeaning...ButLinuxThreadslibmakesuseofthefirst3.

Multipleinstancesofthesamesignalsarequeued. Valuecanbesentwiththesignal. Priorityisguaranteed:Lowestnumberrealtimesignalsaredeliveredfirst.Same signalsaredeliveredaccordingtoordertheyweresent. Regularsignalshavehigherprioritythenrealtimesignals.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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SignalActionintsigaction(intsignum,conststructsigaction*act,structsigaction *oldact); Registerasignalhandler. signum:signalnumber. act:pointertonewstructsigaction. oldact:pointertobuffertobefilledwithcurrentsigaction(or NULL,ifnotinterested).



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SignalActioncont.Thesigactionstructureisdefinedassomethinglike:structsigaction{ void(*sa_handler)(int); void(*sa_sigaction)(int,siginfo_t*,void*); sigset_tsa_mask; intsa_flags; ... } sa_maskgivesamaskofsignalswhichshouldbeblocked duringtheexecutionofthesignalhandler.Thesignalwhichtriggeredthehandlerwillalsobeblocked, unlesstheSA_NODEFERorSA_NOMASKflagsareused.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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RealTimeSignalsFlagssa_flagscanbeusedtopassflagstochangebehavior:SA_ONESHOT:Restorethesignalactiontothedefaultstate oncethesignalhandlerhasbeencalled. SA_RESTART:Makeblockingsystemcallsrestart automaticallyaftyerasignalisreceived. SA_NODEFER:Donotpreventthesignalfrombeing receivedfromwithinitsownsignalhandler. SA_SIGINFO:Thesignalhandlertakes3arguments,notone. Inthiscase,sa_sigactionshouldbesetinsteadofsa_handler.Fordetailsaboutsiginfo_tstructure,seesigaction(2).Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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SendingSignalsintsigqueue(pid_tpid,intsig,constunionsigvalvalue); Queuesignaltoprocess. pidistheprocessIDtosendthesignalto. sigisthesignalnumber. sigvalis: unionsigval{ intsival_int; void*sival_ptr; }; Thesigvalisavailabletothehandlerviathesig_valuefieldof siginfo_t.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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SignalMaskingThesigprocmask()callisusedtochangethelistofcurrently blockedsignals.intsigprocmask(inthow,constsigset_t*set,sigset_t*oldset); Thebehaviorofthecallisdependentonthevalueofhow,as follows:SIG_BLOCK:Thesetofblockedsignalsistheunionofthe currentsetandthesetargument. SIG_UNBLOCK:Thesignalsinsetareremovedfromthe currentsetofblockedsignals. SIG_SETMASK:Thesetofblockedsignalsissettothe argumentset.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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SignalSetsThesefunctionsallowthemanipulationofPOSIXsignalsets:intsigemptyset(sigset_t*set);Initializesthesignalsetgivenbysettoempty,withallsignals excludedfromtheset.

intsigfillset(sigset_t*set);Initializessettofull,includingallsignals.

intsigaddset(sigset_t*set,intsignum); intsigdelset(sigset_t*set,intsignum);Addanddeleterespectivelysignalsignumfromset.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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Signals&ThreadsSignalmasksareperthread. Signalhandlersareperprocess. Exceptionsignals(SIGSEGV,SIGBUS...)willbecaughtby threaddoingtheexception. Othersignalswillbecaughtbyanythreadintheprocess whosemaskdoesnotblockthesignaluse pthread_sigmask()tomodifythethread'ssignalmask. Tip:Useasignalhandlerthreadthatdoessigwait(3)to makethreadcatchinglessrandom!Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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ProcessesTimers#include intgetitimer(intwhich,structitimerval *value); intsetitimer(intwhich,conststructitimerval *value,structitimerval*ovalue);

Thesystemprovideseachprocesswiththreeintervaltimers, eachdecrementinginadistincttimedomain. Whenanytimerexpires,asignalissenttotheprocess,and thetimer(potentially)restarts.



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TimersExpiryThetimerintervaliscontrolledbythefollowingstructures:structitimerval{ structtimevalit_interval;/*nextvalue*/ structtimevalit_value;/*currentvalue*/ }; structtimeval{ longtv_sec;/*seconds*/ longtv_usec;/*microseconds*/ };

Timersdecrementfromit_valuetozero,generateasignal,and resettoit_interval. Atimerwhichissettozero(it_valueiszeroorthetimerexpires andit_intervaliszero)stops.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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TimeDomainsITIMER_REALDecrementsinrealtime,anddeliversSIGALRMupon expiration.

ITIMER_VIRTUALDecrementsonlywhentheprocessisexecuting,anddelivers SIGVTALRMuponexpiration.

ITIMER_PROFDecrementsbothwhentheprocessexecutesandwhenthe systemisexecutingonbehalfoftheprocess. SIGPROFisdelivereduponexpiration.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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MoreonTimersTimerswillneverexpirebeforetherequestedtime,instead expiringsomeshort,constanttimeafterwards,dependenton thesystemtimerresolution. Ifthetimerexpireswhiletheprocessisactive(alwaystrue forITIMER_VIRT),thesignalwillbedeliveredimmediately whengenerated. Otherwise,thedeliverywillbeoffsetbyasmalltime dependentonthesystemload. Analternateinterfacecalledtimer_create()allows specifyingwhichsignalwillbesent(utilizerealtimesignals) andspawningofathreadontimerexpiry.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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DebuggingGDB:TheGNUDebugger GDBrunsonthehost EitherstandaloneorviaagraphicalfrontendlikeEclipse/CDT. GDBserverrunsonthetarget GDBservercanattachtoanalreadyrunningprocessesorstartnew processesunderthedebugger. GDBtalkstoGDBserverviaTCPorUART Needtohavetheexecutablewithdebuginformationonthehost. Also,systemandapplicationdynamiclibraries,ifused. Noneedtohavedebugsymbolsinexecutableontarget.Seehttp://www.codefidence.com/sourcedrop/course/gdb_with_embedded_linux_cheat_sheet.pdfCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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EmbeddedLinuxDriverDevelopment

KernelOverviewLinuxFeatures



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LinuxKernelDevelopmentTimeline



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LinuxStableReleasesMajorversions 1majorversionevery2or3years Examples:1.0,2.0,2.4,2.6 Stablereleases 1stablereleaseevery1or2months Examples:2.0.40,2.2.26,2.4.27,2.6.7... Stablereleaseupdates(sinceMarch2005) Updatestostablereleasesuptoseveraltimesaweek Addressonlycriticalissuesinthelateststablerelease Examples:2.6.11.1to2.6.11.7Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

Evennumber

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LinuxDevelopmentandTestingReleasesTestingreleases Severaltestingreleasespermonth,beforethenextstableone. Youcancontributetomakingkernelreleasesmorestableby testingthem! Example:2.6.12rc1 Developmentversions Unstableversionsusedbykerneldevelopers beforemakinganewstablemajorrelease Examples:2.3.42,2.5.74

Oddnumber



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ContinuedDevelopmentinLinux2.6Since2.6.0,kerneldevelopershavebeenabletointroducelots ofnewfeaturesonebyoneonasteadypace,withouthavingto makemajorchangesinexistingsubsystems. OpeninganewLinux2.7(or2.9)developmentbranchwillbe requiredonlywhenLinux2.6isnolongerabletoaccommodate keyfeatureswithoutundergoingtraumaticchanges. Thankstothis,morefeaturesarereleasedtousersatafasterpace. However,theinternalkernelAPIcanundergochangesbetween two2.6.xreleases.Amodulecompiledforagivenversionmay nolongercompileorworkonamorerecentone.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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2.4vs.2.6Linux2.4Mature Butdevelopmentsstopped;very fewdeveloperswillingtohelp. Nowobsoleteandlacksrecent features. Stillfineifyougetyour sources,toolsandsupportfrom commercialLinuxvendors.

Linux2.63.5yearsoldstableLinuxrelease! SupportfromtheLinux developmentcommunityandall commercialvendors. Nowmatureandmoreexhaustive. Mostdriversupgraded. Cuttingedgefeaturesand increasedperformance.



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LinuxKernelKeyFeaturesPortabilityandhardwaresupport Runsonmostarchitectures. Scalability Canrunonsupercomputersas wellasontinydevices (4MBofRAMisenough). Compliancetostandardsand interoperability. Exhaustivenetworkingsupport. Security Itcan'thideitsflaws.Itscodeis reviewedbymanyexperts. Stabilityandreliability. Modularity Canincludeonlywhatasystem needsevenatruntime. Easytoprogram Youcanlearnfromexistingcode. Manyusefulresourcesonthenet.



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SupportedHardwareArchitecturesSeethearch/directoryinthekernelsources Minimum:32bitprocessors,withorwithoutMMU 32bitarchitectures(arch/subdirectories) alpha,arm,cris,frv,h8300,i386,m32r,m68k,m68knommu, mips,parisc,ppc,s390,sh,sparc,um,v850,xtensa 64bitarchitectures: ia64,mips64,ppc64,sh64,sparc64,x86_64 Seearch//Kconfig,arch//README,or Documentation//fordetails



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KernelOverviewKernelCode



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LinuxSourcesStructure(1)arch/ arch//mach COPYING CREDITS crypto/ Documentation/ drivers/ fs/ include/ include/asm include/linux init/ ipc/Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

Architecturespecificcode Machine/boardspecificcode Linuxcopyingconditions(GNUGPL) Linuxmaincontributors Cryptographiclibraries Kerneldocumentation.Don'tmissit! Alldevicedrivers(drivers/usb/,etc.) Filesystems(fs/ext3/,etc.) Kernelheaders Architectureandmachinedependentheaders Linuxkernelcoreheaders Linuxinitialization(includingmain.c) Codeusedforprocesscommunication


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LinuxSourcesStructure(2)kernel/ lib/ MAINTAINERS Makefile mm/ net/ README REPORTINGBUGS scripts/ security/ sound/ usr/Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

Linuxkernelcore(verysmall!) Misclibraryroutines(zlib,crc32...) Maintainersofeachkernelpart.Veryuseful! TopLinuxmakefile(setsarchandversion) Memorymanagementcode(smalltoo!) Networksupportcode(notdrivers) Overviewandbuildinginstructions Bugreportinstructions Scriptsforinternalorexternaluse Securitymodelimplementations(SELinux...) Soundsupportcodeanddrivers Earlyuserspacecode(initramfs)Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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LXR:LinuxCrossReferencehttp://sourceforge.net/projects/lxr Genericsourceindexingtoolandcode browser Webserverbased Veryeasyandfasttouse Identifierortextsearchavailable Veryeasytofindthedeclaration, implementationorusagesofsymbols SupportsCandC++ Supportshugecodeprojectssuchasthe Linuxkernel(260MinApr.2006)Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

Takesalittlebitoftimeandpatiencetosetup (configuration,indexing,server configuration). Initialindexingquiteslow: Linux2.6.11:1h40minonP4M 1.6GHz,2MBcache Youdon'tneedtosetupLXRbyyourself. Useourhttp://lxr.freeelectrons.comserver! OtherserversavailableontheInternet:http://freeelectrons.com/community/kernel/lxr/

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ImplementedinCImplementedinClikeallUnixsystems. (CwascreatedtoimplementthefirstUnixsystems) AlittleAssemblyisusedtoo: CPUandmachineinitialization,criticallibraryroutines. Seehttp://www.tux.org/lkml/#s153 forreasonsfornotusingC++ (mainreason:thekernelrequiresefficientcode).



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CompiledwithGNUCNeedGNUCextensionstocompilethekernel. So,youcannotuseanyANSICcompiler! SomeGNUCextensionsusedinthekernel:InlineCfunctions Inlineassembly Structurememberinitialization inanyorder(alsoinANSIC99) Branchannotation(seenextpage)



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HelpgccOptimizeYourCode!Usethelikelyandunlikelystatements (include/linux/compiler.h) Example: if(unlikely(err)){ ... } TheGNUCcompilerwillmakeyourcodefaster forthemostlikelycase. Usedinmanyplacesinkernelcode! Don'tforgettousethesestatements!Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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NoClibraryThekernelhastobestandaloneandcan'tuseuserspacecode. Userspaceisimplementedontopofkernelservices,nottheopposite. Kernelcodehastosupplyitsownlibraryimplementations (stringutilities,cryptography,uncompression...) So,youcan'tusestandardClibraryfunctionsinkernelcode. (printf(),memset(),malloc()...). YoucanalsousekernelCheaders. Fortunately,thekernelprovidessimilarCfunctionsforyour convenience,likeprintk(),memset(),kmalloc()...



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KernelStackVerysmallandfixedstack.2pagestack(8k),pertask. Or1pagestack,pertaskandoneforinterrupts. 2.6 Choseninbuildtimeviamenu. Notforallarchitectures

Forsomearchitectures,thekernelprovidesdebugfacilityto detectstackoverruns.



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ManagingEndianessLinuxsupportsbothlittleandbigendianarchitectures Eacharchitecturedefines__BIG_ENDIANor__LITTLE_ENDIAN in Canbeconfiguredinsomeplatformssupportingboth. Tomakeyourcodeportable,thekerneloffersconversionmacros (thatdonothingwhennoconversionisneeded).Mostusefulones: u32cpu_to_be32(u32); //CPUbyteordertobigendian u32cpu_to_le32(u32); //CPUbyteordertolittleendian u32be32_to_cpu(u32); //LittleendiantoCPUbyteorder u32le32_to_cpu(u32); //BigendiantoCPUbyteorderCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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KernelCodingGuidelinesNeverusefloatingpointnumbersinkernelcode.Yourcode mayberunonaprocessorwithoutafloatingpointunit(likeon arm).Floatingpointcanbeemulatedbythekernel,butthisis veryslow. Defineallsymbolsasstatic,exceptexportedones(avoidname spacepollution) Allsystemcallsreturnnegativenumbers(errorcodes)for errors:#include

SeeDocumentation/CodingStyleformoreguidelinesCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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KernelLogPrintingtothekernellogisdoneviatheprintk()function. Thekernelkeepsthemessagesinacircularbuffer (sothatdoesn'tconsumemorememorywithmanymessages). Kernellogmessagescanbeaccessedfromuserspacethroughsystem calls,orthrough/proc/kmsg Kernellogmessagesarealsodisplayedinthesystemconsole.



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printk()Theprintkfunction: Similartostdlib'sprintf(3) Nofloatingpointformat. Logmessageareprefixedwitha,wherethenumber denotesseverity,from0(mostsevere)to7. Macrosaredefinedtobeusedforseveritylevels: KERN_EMERG,KERN_ALERT,KERT_CRIT, KERN_ERR,KERN_WARNING,KERN_NOTICE, KERN_INFO,KERN_DEBUG. Usageexample:printk(KERN_DEBUGHelloWorldnumber%d\n,num);Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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AccessingtheKernelLogManywaysareavailable!Watchthesystemconsole syslogd/klogd Daemongatheringkernelmessages in/var/log/messages Followchangesbyrunning: tailf/var/log/messages Caution:thisfilegrows! Uselogrotatetocontrolthis dmesg Foundinallsystems DisplaysthekernellogbufferCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

logread Same.Oftenfoundinsmall embeddedsystemswithno /var/log/messagesorno dmesg.ImplementedbyBusybox. cat/proc/kmsg Waitsforkernelmessagesand displaysthem. Usefulwhennoneoftheabove userspaceprogramsareavailable (tinysystem)


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LinkedListsManyconstructsusedoublylinkedlists. Listdefinitionandinitialization:structlist_headmylist=LIST_HEAD_INIT(mylist);

orLIST_HEAD(mylist);

orINIT_LIST_HEAD(&mylist);



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ListManipulationListdefinitionandinitialization:voidlist_add(structlist_head*new,struct list_head*head); voidlist_add_tail(structlist_head*new,struct list_head*head); voidlist_del(structlist_head*entry); voidlist_del_init(structlist_head*entry); voidlist_move(structlist_head*list,struct list_head*head);



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ListManipulation(cont.)Listsplicingandquery:voidlist_splice(structlist_head*list,struct list_head*head); voidlist_add_splice_init(structlist_head*list, structlist_head*head); voidlist_empty(structlist_head*head);

In2.6,therearevariantsoftheseAPI'sforRCUprotected lists(seesectionaboutLocksahead).



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ListIterationListsalsohaveiteratormacrosdefined:list_for_each(pos,head); list_for_each_prev(pos,head); list_for_each_safe(pos,n,head); list_for_each_entry(pos,head,member);

Example:structmydata*pos; list_for_each_entry(pos,head,dev_list){ pos>some_data=0777; }



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KernelOverviewBootSequence



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LinuxBootProcessBIOSand/orbootloaderinitializeshardware. Bootloaderloadskernelimageintomemory.Bootloadercangetkernelimagefromflash,HD,network. PossiblyalsoloadsafilesystemtoRAMintheformofinitrd orinitramfsarchives.

Bootloaderorkerneldecompresscompressedkernel. Kernelperformsinternal(hashtable,listsetc.)andhardware (devicedriver)setup. Kernelfindsandmountstherootfilesystem. Kernelexecutesthe/sbin/initapplication.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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BootSequencesx86 PCInitCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

ArmKernel

PowerPC

Setup and BIOS and Decompression Boot loader

bzImage Real Mode trampoline zImage

[ U-boot decompresses the kernel image on it's own ]

GRUB RedBoot BIOSForfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

U-Boot

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RootFilesystemOptionsExternal FSKernel Version Storage format Provided by Stored in Run time Location

Initrd Style 2.4 and 2.6 Filesystem image

Initramfs style 2.6 only CPIO archive

2.4 and 2.6 Filesystem on storage device or network Passed to kernel as parameter (root=) On device Copied to page cache

Loaded and Like initrd or location provided statically linked by boot loader into kernel image Dynamically Fixed allocation allocated RAM disk page cache Copied to page cache Already in page cache



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inittheFirstProcessThefirst(andonly)processthekernelstartsisinit.Bydefaultitissearchedin/sbin/init Canbeoverriddenbythekernelparameterinit=.

inithasthreeroles:Tosetupthesystemconfigurationandstartapplications. Toshutdownthesystemapplications. Theserveastheparentprocessofallchildprocesseswhose parenthasexited.

Thedefaultinitimplementationreadsitsinstructionsfrom thefile/etc/inittabCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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inittabThis is an Busybox style inittab (for an example for Sys V inittab see the Appendix.) Format: id: runlevel : action : command id: To which device should std input/output go (empty means the console) runlevel: ignored. For compatibility with Sys V init. action: one of sysinit, respawn, askfirst, wait, shutdown and once command: shell command to executeCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

#Startupthesystem ::sysinit:/bin/mountoremount,rw/ ::sysinit:/bin/mounttprocproc/proc ::sysinit:/bin/mounta ::sysinit:/sbin/ifconfiglo127.0.0.1up #Putagettyontheserialport ::respawn:/sbin/gettyLttyS1115200vt100 #Startsystemloggers null::respawn:/sbin/syslogdnm0 null::respawn:/sbin/klogdn #Stufftodobeforerebooting null::shutdown:/usr/bin/killallklogd null::shutdown:/usr/bin/killallsyslogd null::shutdown:/bin/umountar


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KernelOverviewKernelSubsystems



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KernelArchitectureApp1 App2 Clibrary Systemcallinterface Process management Memory management Filesystem support Filesystem types CPUsupport code CPU/MMU supportcode Storage drivers Character devicedrivers Network devicedrivers Hardware CPU RAM Storage Device control Networking ... User space

Kernel space



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KernelModevs.UserModeAllmodernCPUssupportadualmodeofoperation:Usermode,forregulartasks. Supervisor(orprivileged)mode,forthekernel.

ThemodetheCPUisindetermineswhichinstructionsthe CPUiswillingtoexecute:SensitiveinstructionswillnotbeexecutedwhentheCPUis inusermode.

TheCPUmodeisdeterminedbyoneoftheCPUregisters, whichstoresthecurrentRingLevel0forsupervisormode,3forusermode,12unusedbyLinux.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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TheSystemCallInterfaceWhenauserspacetasksneedstouseakernelservice,itwillmakea SystemCall. TheClibraryplacesparametersandnumberofsystemcallinregisters andthenissuesaspecialtrapinstruction. Thetrapatomicallychangestheringleveltosupervisormodeandthe setstheinstructionpointertothekernel. Thekernelwillfindtherequiredsystemcalledviathesystemcalltable andexecuteit. Returningfromthesystemcalldoesnotrequireaspecialinstruction, sinceinsupervisormodetheringlevelcanbechangeddirectly.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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LinuxSystemCallPathdo_name() sys_name() entry.S Function call Trap

Kernel

Task

Glibc Task



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DriverDevelopmentLoadableKernelModules



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LoadableKernelModules(1)Modules:addagivenfunctionalitytothekernel(drivers, filesystemsupport,andmanyothers). Canbeloadedandunloadedatanytime,onlywhentheir functionalityisneed.Onceloaded,havefullaccesstothe wholekernel.Noparticularprote

programming embedded linux

Documents

ddif dev zeroof disk

struct list head head

running bin find meaning of lifewithoutgod

real sleep time

data len

child process

page cache

pthread attr t attr