Signals and Inter-Process Communicaonporter/courses/cse506/s16/slides/ipc.pdf · CSE 506: Operang...

CSE506:Opera.ngSystems

SignalsandInter-ProcessCommunica.on

DonPorter

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Housekeeping•  Paperreadingassignedfornextclass

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LogicalDiagram

MemoryManagement

CPUScheduler

User

Kernel

Hardware

BinaryFormats

Consistency

SystemCalls

Interrupts Disk Net

RCU FileSystem

DeviceDrivers

Networking Sync

MemoryAllocators Threads

Today’sLectureProcess

CoordinaKon

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LastKme…•  We’vediscussedhowtheOSschedulestheCPU–  Andhowtoblockaprocessonaresource(disk,network)

•  Today:–  Howdoprocessesblockoneachother?–  Andmoregenerallycommunicate?

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Outline•  Signals–  OverviewandAPIs–  Handlers–  Kernel-leveldelivery–  Interruptedsystemcalls

•  InterprocessCommunicaKon(IPC)–  PipesandFIFOs–  SystemVIPC– WindowsAnalogs

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Whatisasignal?•  Likeaninterrupt,butforapplicaKons–  <64numberswithspecificmeanings–  Aprocesscanraiseasignaltoanotherprocessorthread–  AprocessorthreadregistersahandlerfuncKon

•  ForbothIPCanddeliveryofhardwareexcepKons–  ApplicaKon-levelhandlers:divzero,segfaults,etc.

•  No“message”beyondthesignalwasraised–  Andmaybealialemetadata

•  PIDofsender,faulKngaddress,etc.•  Butplaborm-specific(non-portable)

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Example

Pid300

int main() { ... signal(SIGUSR1, &usr_handler); ...

}

Registerusr_handler()tohandleSIGUSR1 7


Example

Pid300

kill(300, SIGUSR1);

SendsignaltoPID300

Pid400

int main() { ...

} int usr_handler() { …

PC

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BasicModel•  ApplicaKonregistershandlerswithsignalorsigacKon•  Sendsignalswithkillandfriends–  OrraisedbyhardwareexcepKonhandlersinkernel

•  Signaldeliveryjumpstosignalhandler–  Irregularcontrolflow,similartoaninterrupt

APInamesareadmiaedlyconfusing 9


SignalTypes•  Seeman7signalforthefulllist:(variesbysys/arch)SIGTSTP–1–Stoptypedatterminal(Ctrl+Z)SIGKILL–9–Killaprocess,forrealziesSIGSEGV–11–SegmentaKonfaultSIGPIPE–13–Brokenpipe(writewithnoreaders)SIGALRM–14–TimerSIGUSR1–10–User-definedsignal1SIGCHLD–17–ChildstoppedorterminatedSIGSTOP–19–StopaprocessSIGCONT–18–ConKnueifstopped

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LanguageExcepKons•  SignalsaretheunderlyingmechanismforExcepKonsandcatchblocks

•  JVMorotherrunKmesystemsetssignalhandlers–  SignalhandlercausesexecuKontojumptothecatchblock

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SignalHandlerControlFlow

This is the Title of the Book, eMatter EditionCopyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

Delivering a Signal | 443

starts executing the signal handler, because the handler’s starting address was forcedinto the program counter. When that function terminates, the return code placed onthe User Mode stack by the setup_frame( ) or setup_rt_frame() function is exe-cuted. This code invokes the sigreturn( ) or the rt_sigreturn() system call; the cor-responding service routines copy the hardware context of the normal program to theKernel Mode stack and restore the User Mode stack back to its original state (byinvoking restore_sigcontext( )). When the system call terminates, the normal pro-gram can thus resume its execution.

Let’s now examine in detail how this scheme is carried out.

Setting up the frame

To properly set the User Mode stack of the process, the handle_signal( ) functioninvokes either setup_frame( ) (for signals that do not require a siginfo_t table; seethe section “System Calls Related to Signal Handling” later in this chapter) or setup_rt_frame( ) (for signals that do require a siginfo_t table). To choose among thesetwo functions, the kernel checks the value of the SA_SIGINFO flag in the sa_flags fieldof the sigaction table associated with the signal.

The setup_frame( ) function receives four parameters, which have the followingmeanings:

sigSignal number

kaAddress of the k_sigaction table associated with the signal

oldsetAddress of a bit mask array of blocked signals

Figure 11-2. Catching a signal

Normalprogram

flow

Signalhandler

Return codeon the stack

do_signal()

handle_signal()

setup_frame()

system_call()

sys_sigreturn()

restore_sigcontext()

User Mode Kernel Mode

FromUnderstandingtheLinuxKernel 12


AlternateStacks•  SignalhandlersexecuteonadifferentstackthanprogramexecuKon.– Why?

•  Safety:Appcanensurestackisactuallymapped–  AndavoidassumpKonsaboutapplicaKonnotusingspacebelowrsp

–  Setwithsigaltstack()systemcall

•  Likeaninterrupthandler,kernelpushesregisterstateoninterruptstack–  Returntokernelwithsigreturn()systemcall–  Appcanchangeitsownon-stackregisterstate!

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NestedSignals•  Whathappenswhenyougetasignalinthesignalhandler?

•  Andwhyshouldyoucare?

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TheProblemwithNesKngint main() {

/* ... */ signal(SIGINT, &handler); signal(SIGTERM, &handler); /* ... */

} int handler() {

free(buf1); free(buf2);

}

SIGINT

SIGTERM

SignalStack

PC Callsmunmap()

Anothersignaldeliveredon

returnDoublefree!

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NestedSignals•  Theoriginalsignal()specificaKonwasatotalmess!–  Nowdeprecated---donotuse!

•  NewsigacKon()APIletsyouspecifythisindetail– Whatsignalsareblocked(anddeliveredonsigreturn)–  Similartodisablinghardwareinterrupts

•  Asyoumightguess,blockingsystemcallsinsideofasignalhandlerareonlysafewithcarefuluseofsigacKon()

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ApplicaKonvs.Kernel•  App:signalsappeartobedeliveredroughlyimmediately

•  Kernel(lazy):–  Sendasignal==markapendingsignalinthetask

•  AndmakerunnableifblockedwithTASK_INTERRUPTIBLEflag

–  Checkpendingsignalsonreturnfrominterruptorsyscall•  Deliverifpending

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Example

Pid300RUNNING

kill(300, SIGUSR1);

SendsignaltoPID300

Pid400

int main() { read();

} int usr_handler() { …

PC

…

…10Pid300

INTERRUPTIBLE

Blockondisk

read!

Markpendingsignal,unblock

Whathappenstoread?

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InterruptedSystemCalls•  IfasystemcallblocksintheINTERRUPTIBLEstate,asignalwakesitup

•  Yetsignalsaredeliveredonreturnfromasystemcall•  Howisthisresolved?•  Thesystemcallfailswithaspecialerrorcode–  EINTRandfriends– Manysystemcallstransparentlyretryaversigreturn–  Somedonot–checkforEINTRinyourapplicaKons!

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Defaulthandlers•  Signalshavedefaulthandlers:–  Ignore,kill,suspend,conKnue,dumpcore–  Theseexecuteinsidethekernel

•  Installingahandlerwithsignal/sigacKonoverridesthedefault

•  Afew(SIGKILL)cannotbeoverridden

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RTSignals•  Defaultsignalsareonlyin2states:signaledornot–  IfIsend2SIGUSR1’stoaprocess,onlyonemaybedelivered

–  IfsystemisslowandIfuriouslyhitCtrl+Coverandover,onlyoneSIGINTdelivered

•  RealKme(RT)signalskeepacount–  Deliveronesignalforeachonesent

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SignalSummary•  AbstracKonlikehardwareinterrupts–  Somecaremustbetakentoblockotherinterrupts–  EasytowritebuggyhandlersandmissEINTR

•  UnderstandcontrolflowfromapplicaKonandkernelperspecKve

•  UnderstandbasicAPIs

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OtherIPC•  Pipes,Sockets,andFIFOs•  SystemVIPC•  Windowscomparison

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Pipes•  Streamofbytesbetweentwoprocesses•  Readandwritelikeafilehandle–  Butnotanywhereinthehierarchicalfilesystem–  Andnotpersistent–  Andnocursororseek()-ing–  Actually,2handles:areadhandleandawritehandle

•  Primarilyusedforparent/childcommunicaKon–  Parentcreatesapipe,childinheritsit

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Exampleint pipe_fd[2]; int rv = pipe(pipe_fd); int pid = fork(); if (pid == 0) {

close(pipe_fd[1]); //Close unused write end dup2(pipe_fd[0], 0); // Make the read end stdin exec(“grep”, “quack”);

} else { close (pipe_fd[0]); // Close unused read end …

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FIFOs(akaNamedPipes)•  ExisKngpipescan’tbeopened---onlyinherited–  OrpassedoveraUnixDomainSocket(beyondtoday’slec)

•  FIFOs,orNamedPipes,addaninterfaceforopeningexisKngpipes

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Sockets•  Similartopipes,exceptfornetworkconnecKons•  SetupandconnecKonmanagementisabittrickier–  Atopicforanotherday(orclass)

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Select•  WhatifIwanttoblockunKloneofseveralhandleshasdatareadytoread?

•  Readwillblockononehandle,butperhapsmissdataonasecond…

•  SelectwillblockaprocessunKlahandlehasdataavailable–  UsefulforapplicaKonsthatusepipes,sockets,etc.

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SynthesisExample:TheShell•  Almostall‘commands’arereallybinaries–  /bin/ls

•  KeyabstracKon:RedirecKonoverpipes–  ‘>’,‘<‘,and‘|’implementedbytheshellitself

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ShellExample•  Ex:ls | grep foo •  ImplementaKonsketch:–  ShellparsestheenKrestring–  Setsupchainofpipes–  Forksandexec’s‘ls’and‘grep’separately– Waitonoutputfrom‘grep’,printtoconsole

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Jobcontrolinashell•  Shellkeepsitsown“scheduler”forbackgroundprocesses•  Howto:–  Putaprocessinthebackground?

•  SIGTSTPhandlercatchesCtrl-Z•  SendSIGSTOPtocurrentforegroundchild

–  ResumeexecuKon(fg)?•  SendSIGCONTtopausedchild,usewaitpid()toblockunKlfinished

–  Executeinbackground(bg)?•  SendSIGCONTtopausedchild,butblockonterminalinput

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Otherhints•  Splice(),tee(),andsimilarcallsareusefulforconnecKngpipestogether–  Avoidscopyingdataintoandout-ofapplicaKon

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SystemVIPC•  Semaphores–Lock•  MessageQueues–Likeamailbox,“small”messages•  SharedMemory–parKcularlyuseful–  Aregionofnon-COWanonymousmemory– Mapatagivenaddressusingshmat()

•  CanpersistlongerthananapplicaKon– Mustbeexplicitlydeleted–  Canleakatsystemlevel–  Butclearedaverareboot

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SystemVKeysandIDs•  Programmerspickarbitrary32-bitkeys–  UsethesekeystonamesharedabstracKons

•  Findakeyusingshmget(),msgget(),etc.–  Kernelinternallymapskeytoa32-bitID

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WindowsComparison•  HardwareexcepKonsaretreatedseparatelyfromIPC–  Upcallstontdll.dll(libcequivalent),tocallhandlers

•  AllIPCtypescanberepresentedashandles–  ProcessterminaKon/suspend/resumesignaledwithprocesshandles

–  SignalscanbeanEventhandle–  SemaphoresandMutexeshavehandles–  Sharedmemoryequallycomplicated(butsKllhandles)

•  Singleselect()-likeAPItowaitonahandletobesignaled

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Summary•  Understandsignals•  Understandhigh-levelproperKesofpipesandotherUnixIPCabstracKons–  High-levelcomparisonwithWindows

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