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3ff@338; ʚ;J%5ʚ;g4dddd  0@ppp@ <4dddd@k 0t\8 <4BdBd@l 0tg4<d<d̐ 0p p %0___PPT10 ___PPT9nnIt>"ՌPNG  IHDR +tsRGB PLTEfffm cmPPJCmp0712 7tRNS@fIDATWc`  SP1H $XLIENDB`? -O  =5K=aExceptional Control Flow IISToday Process Hierarchy Shells Signals Nonlocal jumps Next time Virtual memoryP4 4 +!>b$ECF exists at all levels of a systemExceptions Hardware and operating system kernel software Concurrent processes Hardware timer and kernel software Signals Kernel software Non-local jumps Application code /# /# ?cThe world of multitaskingSystem runs many processes concurrently Process: executing program State consists of memory image + register values + program counter Continually switches from one process to another Suspend process when it needs I/O resource or timer event occurs Resume process when I/O available or given scheduling priority Appears to user(s) as if all processes executing simultaneously Except possibly with lower performance Even though most systems can only execute one at a time (C1@`(C1@ `B@dDProgrammer s model of multitaskingBasic functions fork() spawns new process Called once, returns twice exit() terminates own process Called once, never returns Puts it into  zombie status wait() and waitpid() wait for and reap terminated children execl() and execve() run a new program in an existing process Called once, (normally) never returns Programming challenge Understanding the nonstandard semantics of the functions Avoiding improper use of system resources E.g.  Fork bombs can disable a system.fUZaZ8ayZ&aUcZ(a8      '        *  &c(>)AeUnix process hierarchyBfUnix startup: Step 1CgUnix startup: Step 2DhUnix startup: Step 3EiUnix startup: Step 4FjShell programsjA shell is an application program that runs programs on behalf of the user. sh - Original Unix Bourne Shell csh - BSD Unix C Shell tcsh  Enhanced C Shell bash  Bourne-Again Shell Lj3E>L/GkSimple shell eval function$    Hl!Problem with simple shell example_Shell correctly waits for and reaps foreground jobs. But what about background jobs? Will become zombies when they terminate. Will never be reaped because shell (typically) will not terminate. Creates a memory leak that will eventually crash the kernel when it runs out of memory. Solution: Reaping background jobs requires a mechanism called a signal\UGU@3ImSignalsA signal is a small message that notifies a process that an event of some type has occurred in the system. Kernel abstraction for exceptions and interrupts. Sent from the kernel (sometimes at the request of another process) to a process. Different signals are identified by small integer ID s The only information in a signal is its ID and the fact that it arrived.HkUZ3cJnSignal concepts Sending a signal Kernel sends (delivers) a signal to a destination process by updating some state in the context of the destination process. Kernel sends a signal for one of the following reasons: Kernel has detected a system event such as divide-by-zero (SIGFPE) or the termination of a child process (SIGCHLD) Another process has invoked the kill system call to explicitly request the kernel to send a signal to the destination process. 33{[ KoSignal concepts (cont)Receiving a signal A destination process receives a signal when it is forced by the kernel to react in some way to the delivery of the signal. Three possible ways to react: Ignore the signal (do nothing) Terminate the process. Catch the signal by executing a user-level function called a signal handler. Akin to a hardware exception handler being called in response to an asynchronous interrupt.\3|6373\ LpSignal concepts (cont)A signal is pending if it has been sent but not yet received. There can be at most one pending signal of any type. Important: Signals are not queued If a process has a pending signal of type k, then subsequent signals of type k that are sent to that process are discarded. A process can block the receipt of certain signals. Blocked signals can be delivered, but will not be received until the signal is unblocked. A pending signal is received at most once. >W|4Z, 3+W|    3!  Z,MqSignal concepts Kernel maintains pending and blocked bit vectors in the context of each process. pending  represents the set of pending signals Kernel sets bit k in pending whenever a signal of type k is delivered. Kernel clears bit k in pending whenever a signal of type k is received blocked  represents the set of blocked signals Can be set and cleared by the application using the sigprocmask function.Q00J-)B*)4   t  NrProcess groups2Every process belongs to exactly one process group33Os!Sending signals with kill program$" |kill program sends arbitrary signal to a process or process group Examples kill  9 24818 Send SIGKILL to process 24818 kill  9  24817 Send SIGKILL to every process in process group 24817. K9< 7    Pt!Sending signals from the keyboardTyping ctrl-c (ctrl-z) sends a SIGTERM (SIGTSTP) to every job in the foreground process group. SIGTERM  default action is to terminate each process SIGTSTP  default action is to stop (suspend) each process._UrZ_rQuExample of ctrl-c and ctrl-z. Rv"Sending signals with kill function$# SwReceiving signalsSuppose kernel is returning from exception handler and is ready to pass control to process p. Kernel computes pnb = pending & ~blocked The set of pending nonblocked signals for process p If (pnb == 0) Pass control to next instruction in the logical flow for p. Else Choose least nonzero bit k in pnb and force process p to receive signal k. The receipt of the signal triggers some action by p Repeat for all nonzero k in pnb. Pass control to next instruction in logical flow for p. 5<\29      3(35bo) gz;TxDefault actionsEach signal type has a predefined default action, which is one of: The process terminates The process terminates and dumps core. The process stops until restarted by a SIGCONT signal. The process ignores the signal. ZC"3UyInstalling signal handlers4The signal function modifies the default action associated with the receipt of signal signum: handler_t *signal(int signum, handler_t *handler) Different values for handler: SIG_IGN: ignore signals of type signum SIG_DFL: revert to the default action on receipt of signals of type signum. Otherwise, handler is the address of a signal handler Called when process receives signal of type signum Referred to as  installing the handler. Executing handler is called  catching or  handling the signal. When the handler executes its return statement, control passes back to instruction in the control flow of the process that was interrupted by receipt of the signal.^ZZ2ZZZZBZL3       D)3, 3,33V   JEdVzSignal handling exampleW{Signal handler funkinessPending signals are not queued For each signal type, just have single bit indicating whether or not signal is pending Even if multiple processes have sent this signal&X|Living with nonqueuing signals  ;Must check for all terminated jobs Typically loop with wait&##Y}External event handling=A program that reacts to externally generated events (ctrl-c)Z~Internal event handling[Nonlocal jumps: setjmp/longjmp$Powerful (but dangerous) user-level mechanism for transferring control to an arbitrary location. Controlled to way to break the procedure call/return discipline Useful for error recovery and signal handling int setjmp(jmp_buf j) Must be called before longjmp Identifies a return site for a subsequent longjmp. Called once, returns one or more times Implementation: Remember where you are by storing the current register context, stack pointer, and PC value in jmp_buf. Return 0aUnUxUsanx  sb+ \setjmp/longjmp (cont)void longjmp(jmp_buf j, int i) Meaning: return from the setjmp remembered by jump buffer j again... & this time returning i instead of 0 Called after setjmp Called once, but never returns longjmp Implementation: Restore register context from jump buffer j Set %eax (the return value) to i Jump to the location indicated by the PC stored in jump buf j.  a3      *<X BQ]setjmp/longjmp example.  ^Putting it all togetherXA program that restarts itself when ctrl-c d)_Limitations of nonlocal jumpsvWorks within stack discipline Can only long jump to environment of function that has been called but not yet completed*YY`!Limitations of long jumps (cont.)vWorks within stack discipline Can only long jump to environment of function that has been called but not yet completed*YYaSummarySignals provide process-level exception handling Can generate from user programs Can define effect by declaring signal handler Some caveats Very high overhead >10,000 clock cycles Only use for exceptional conditions Don t have queues Just one bit for each pending signal type Nonlocal jumps provide exceptional control flow within process Within constraints of stack discipline 1N 9*A(1N   9  *A(a/ bcdef g h i j klmnopqrstuvwxyz{|} ~!"#$%&'()O   0` .T3f` T3f3f` 999MMM` lff3f3޲` eoHff33Ҷ` ff!` T3f3fffq` T3f3fff[>?" dd@ ?nPd@ d " @ ` n?" dd@   @@``PR   @ ` `$p>>  C; ,(  , , s *ཉ "`0  T Click to edit Master title style! ! , c $ "@0  RClick to edit Master text styles Second level Third level Fourth level Fifth level!     S  , c $Pʼn ":   AEECS 213 Introduction to Computer Systems Northwestern University2B0(2* 3 3 , Nff??"` , Nff??"` F   ," , Nff??" , Nff??"  , Nff??"  , Nff??"`  , c $Ή "ce  L*"0(2Z , B޽h))? ? T3f3fff[ 0___PPT10.Zj&___PPT92p22 1_aqualab01R  0  0@(  0~ 0 Hff??"A 0 0䥤 "    A Click to edit  0 < "   T Click to edit Master title style! !T   0 "  0 T䬤ff??"  4 0 0 Thff??"  4 0~  0 Hff??"A~  0 Hff??"A  0 RAk B?nwu-seal-gray"@P%  0 c $ȴ "`0   !Fabin E. Bustamante, Spring 2007H"0(2 3& 3& 3Z 0 B޽h))? ? T3f3fff[80___PPT10.Zj&( 0 d8(  d  d Nyy .   v* a11aa d Nyy 2 .  x* a11aad d c $ ?  4 d Npyy 9 3  RClick to edit Master text styles Second level Third level Fourth level Fifth level!     S d Tpyy q   v* a11aa d Tyy q2   x* a11aaH d 0ηo~ ? 3380___PPT10.eۡ ((    N< yy .    \* a11aa  NDyy 2 .   ^* a11aa  T yy q    \* a11aa  T yy q2    ^* a11aaH  0ηo~ ? 3380___PPT10.dz͓ 0L0  $(   r  S 0   r  S ෤0    H  0޽h ? GGGy___PPT10Y+D='  = @B +  0L0   T(   ~  s *H,`0   ~  s *I,@0     f jJ11?@P`   f jJ11?   3 r jJ11?` NPrevious Lecture"0 Z  3 r% jJ11? p  J This Lecture" 0 Z H  0޽h ? f33f3y___PPT10Y+D='  = @B +  0 0$ <(  $ ~ $ s *X[,`0   ~ $ s *0\,@0  H $ 0޽h ? GGGy___PPT10Y+D='  = @B +  0 @( <(  ( ~ ( s *lo,`0   ~ ( s *w,@0  H ( 0޽h ? GGGy___PPT10Y+D='  = @B +7    0 ^ V P,  (  , r , S ĕ,`0   2 , B8c?p  G Login shell 0  2 , Bț8c? 00  AChild 0 2 , B8c? 0  AChild 0 2 , BT8c? 0  AChild 0 2 , B(8c?  F Grandchild 0  2 , B 8c? 0P  F Grandchild 0  dB  , @ <D8c? P  dB  , <D8c? `  2  , B8c?  ?[0] 0 dB  , @ <D8c?  dB  , @ <D8c?P  pdB , @ <D8c?   dB , <D8c?0 p dB , @ <D8c?0 p dB , @ <D8c? P 2 , H8c?p  {Daemon e.g. httpd,0   2 , B8c? P Dinit [1] 0  H , 0޽h ? GGGy___PPT10Y+D='  = @B +    0 ` 0 j(  0 r  0 S x,`0   2 0 B(é8c?  Dinit [1] 0  2 0 Blǩ8c?0  ?[0] 0 dB 0 @ <D8c?6   0 HD˩8c?  _%Process 0: handcrafted kernel process&0 &  0 HЩ8c?    Child process 1 execs /sbin/init6!0  ? 0 H ֩8c?pxe ;1. Pushing reset button loads the PC with the address of a small bootstrap program. 2. Bootstrap program loads the boot block (disk block 0). 3. Boot block program loads kernel binary (e.g., /boot/vmlinux) 4. Boot block program passes control to kernel. 5. Kernel handcrafts the data structures for process 0. N<0 " kk  0 # l jJ11?f   0 3 r jJ11?{ jQ  ]Process 0 forks child process 1" 0 Z H 0 0޽h ? GGGy___PPT10Y+D='  = @B +Q    0 x p p 4  (  4 ~ 4 s *,`0   2 4 Bĩ8c?Z  Dinit [1] 0  2 4 B88c? : ?[0] 0 dB 4 @ <D8c?: Z2 4 B|8c?   [getty 0 dB 4 @ <D8c? dB 4 @ <D8c?zm2  4 H`8c?  Daemons e.g. ftpd, httpd80   ,   4 HDg ?dM h /etc/inittab 0  jB  4 BD8c?   4  fl jJ11?   binit forks and execs daemons per /etc/inittab, and forks and execs a getty program for the console\c0 Z ,&  4 # l jJ11? ` H 4 0޽h ? GGGy___PPT10Y+D='  = @B +>   0 e]8 (  8 ~ 8 s *,`0   2 8 Bx8c?Z  Dinit [1] 0  2 8 BP8c? : ?[0] 0 dB 8 @ <D8c?: Z- 8 <8c??   'The getty process execs a login program\(0 2 8 B'8c?   Alogin 0 dB 8 @ <D8c? H 8 0޽h ? GGGy___PPT10Y+D='  = @B +   0  < E(  < r  < S +,`0   2 < B48c?Z  Dinit [1] 0  2 < B78c? : ?[0] 0 dB < @ <D8c?: Zp < <h;8c? p 0`  Xlogin reads login and passwd. if OK, it execs a shell. if not OK, it execs another getty\Y0 +,72 < BHE8c?   Ztcsh 0 dB < @ <D8c? H < 0޽h ? GGGy___PPT10Y+D='  = @B +  0L0 @ b(  @ ~ @ s *P,`0   ~ @ s *Q,@0  4 @ # lRjJ11? a int main() { char cmdline[MAXLINE]; while (1) { /* read */ printf("> "); Fgets(cmdline, MAXLINE, stdin); if (feof(stdin)) exit(0); /* evaluate */ eval(cmdline); } }"0 Z~"   #   @ T\oaoa ? `  ^.Execution is a sequence of read/evaluate steps//H @ 0޽h ? f33f3y___PPT10Y+D='  = @B +U   0L0 d \ D (  D ~ D s *Tk,`0   6 D # l|ujJ11?p Tvoid eval(char *cmdline) { char *argv[MAXARGS]; /* argv for execve() */ int bg; /* should the job run in bg or fg? */ pid_t pid; /* process id */ bg = parseline(cmdline, argv); if (!builtin_command(argv)) { if ((pid = Fork()) == 0) { /* child runs user job */ if (execve(argv[0], argv, environ) < 0) { printf("%s: Command not found.\n", argv[0]); exit(0); } } if (!bg) { /* parent waits for fg job to terminate */ int status; if (waitpid(pid, &status, 0) < 0) unix_error("waitfg: waitpid error"); } else /* otherwise, don t wait for bg job */ printf("%d %s", pid, cmdline); } }"0 Z   ( "  H ?  8  H D 0޽h ? f33f3y___PPT10Y+D='  = @B +  0L0 H <(  H ~ H s *蓭,`0   ~ H s *,@0  H H 0޽h ? f33f3y___PPT10Y+D='  = @B +_  0 ~(+L (  L ~ L s *d,`0   ~ L s *̠,@0  x ` 5 +L  #"* 0 L  <@8c?  5  ^ Timer signal  4YYii L  <08c?p  [ Terminate  4YYii L  <@í8c?p p  [SIGALRM4YYii L  <˭8c?` p  V144YYii  L  <hέ8c? H 5  hSegmentation violation4YYii  L  <4߭8c?pH   bTerminate & Dump4YYii  L  < 8c?pH p  [SIGSEGV4YYii  L  <\8c?`H p  V114YYii  L  <8c?` p V174YYii L  <8c?` pH  U94YYii L  <8c?` p  U24YYii L  <8c?` p  TID4YYii L  <8c? 5 mChild stopped or terminated4YYii L  <(8c?p  XIgnore4YYii L  <18c?p p [SIGCHLD4YYii L  <:8c? 5H  z(Kill program (cannot override or ignore)))4YYii L  <C8c?p H  [ Terminate  4YYii L  <M8c?p pH  [SIGKILL4YYii L  <>8c? 5  Interrupt from keyboard (ctl-c), 4YYii L  <a8c?p  [ Terminate  4YYii L  <Dk8c?p p  ZSIGINT4YYii L  <|t8c? 5  eCorresponding Event4YYii L  < ~8c?p  `Default Action4YYii L  <Px8c?p p  VName4YYii`B L  0o ?` 5 ZB L  s *1 ?` 5 ZB L  s *1 ?` 5 ZB  L  s *1 ?`H 5H `B !L  0o ?`5`B "L  0o ?` `ZB #L  s *1 ?p pZB $L  s *1 ? `B %L  0o ?5 5ZB &L  s *1 ?p pZB 'L  s *1 ?` 5 ZB (L  s *1 ?` 5 H L 0޽h ? GGGy___PPT10Y+D='  = @B +  0L0 P <(  P ~ P s *E,`0   ~ P s * ,@0  H P 0޽h ? f33f3y___PPT10Y+D='  = @B +  0L0 T <(  T ~ T s *,`0   ~ T s *x,@0  H T 0޽h ? f33f3y___PPT10Y+D='  = @B +  0L0 X <(  X ~ X s *Hů,`0   ~ X s *ů,@0  H X 0޽h ? f33f3y___PPT10Y+D='  = @B +  0L0 \ <(  \ ~ \ s *ٯ,`0   ~ \ s *گ,@0  H \ 0޽h ? f33f3y___PPT10Y+D='  = @B +5  0L0 D< ` (  ` ~ ` s *,`0   ~ ` s *,@0  2 ` N<1?0  LFore- ground job 0 2 ` Nï1? ~ "  OBack- ground job #1 0 2 ` N1?`0  OBack- ground job #2 0 2 ` N1?  AShell 0 2 ` Nx1? L  AChild 0 2  ` ND1? }  AChild 0 |B  ` @ TD1? $ |B  ` TD1?  B  ` ZD1?N N |B  ` @ TD1?fu _|B ` TD1?M: Z ` T1?(' H xpid=10 pgid=10"0  $v ` N1?C  ` Th 1?` i WForeground process group 20 0 v ` N1? k  ` T1?g   WBackground process group 32 0  ` T1?k O  WBackground process group 40 0 v ` N1?k  ` Z41?Xx xpid=20 pgid=20"0  $ ` Z$1?x  vpid=32 pgid=32 0  $ ` ZL 1? vpid=40 pgid=40 0  $ ` Z$&1? q{  xpid=21 pgid=20"0  $ ` Z+1? AK  xpid=22 pgid=20"0  $ ` T0oaoa ?p _ _ Lgetpgrp()  Return process group of current process setpgid()  Change process group of a processBb * !$-'H ` 0޽h ? f33f3y___PPT10Y+D='  = @B +  0L0 0d (  d ~ d s *4G,`0   ~ d s *G,@0  O d BMg ?4  ylinux> ./forks 16 linux> Child1: pid=24818 pgrp=24817 Child2: pid=24819 pgrp=24817 linux> ps PID TTY TIME CMD 24788 pts/2 00:00:00 tcsh 24818 pts/2 00:00:02 forks 24819 pts/2 00:00:02 forks 24820 pts/2 00:00:00 ps linux> kill -9 -24817 linux> ps PID TTY TIME CMD 24788 pts/2 00:00:00 tcsh 24823 pts/2 00:00:00 ps linux>  z0 z    7X8H d 0޽h ? f33f3y___PPT10Y+D='  = @B +e  0L0 tl@h (  h ~ h s *|m,`0   ~ h s *Tn,@0  | h T1?f 2 h Np1?s[  LFore- ground job 0 2 h Nt1? O  OBack- ground job #1 0 2 h Ny1?[  OBack- ground job #2 0 2 h N,}1?  AShell 0 2  h N 1? jZz  AChild 0 2  h N1?  z  AChild 0 |B  h @ TD1?;  |B  h TD1?9 c B  h ZD1? |B h @ TD1?R:% |B h TD1?> X h T1?7 W xpid=10 pgid=10"0  $ h Tj1?   WForeground process group 20 0 v h N1?f  h TX1? =  XBackground process group 32 0  h Tؖ1? d  XBackground process group 40 0 v h N1?f2  h Z(1?  xpid=20 pgid=20"0  $ h Z1?   vpid=32 pgid=32 0  $ h Z1?#  vpid=40 pgid=40 0  $ h Zܪ1?l )  xpid=21 pgid=20"0  $ h Z̯1?q `j  xpid=22 pgid=20"0  $H h 0޽h ? f33f3y___PPT10Y+D='  = @B +  0L0 Pl V(  l ~ l s *Ű,`0    l Bǰg ?6  &linux> ./forks 17 Child: pid=24868 pgrp=24867 Parent: pid=24867 pgrp=24867 Suspended linux> ps a PID TTY STAT TIME COMMAND 24788 pts/2 S 0:00 -usr/local/bin/tcsh -i 24867 pts/2 T 0:01 ./forks 17 24868 pts/2 T 0:01 ./forks 17 24869 pts/2 R 0:00 ps a bass> fg ./forks 17 linux> ps a PID TTY STAT TIME COMMAND 24788 pts/2 S 0:00 -usr/local/bin/tcsh -i 24870 pts/2 R 0:00 ps a  0 #Dn D H l 0޽h ? f33f3y___PPT10Y+D='  = @B +  0L0 +#`p (  p ~ p s *d,`0    p 6Pg ?   avoid fork12() { pid_t pid[N]; int i, child_status; for (i = 0; i < N; i++) if ((pid[i] = fork()) == 0) while(1); /* Child infinite loop */ /* Parent terminates the child processes */ for (i = 0; i < N; i++) { printf("Killing process %d\n", pid[i]); kill(pid[i], SIGINT); } /* Parent reaps terminated children */ for (i = 0; i < N; i++) { pid_t wpid = wait(&child_status); if (WIFEXITED(child_status)) printf("Child %d terminated with exit status %d\n", wpid, WEXITSTATUS(child_status)); else printf("Child %d terminated abnormally\n", wpid); } } b0 b 4 ]; ,)  H p 0޽h ? f33f3y___PPT10Y+D='  = @B +  0L0 pt <(  t ~ t s *$,`0   ~ t s *,@0  H t 0޽h ? f33f3y___PPT10Y+D='  = @B +  0L0 x <(  x ~ x s *,`0   ~ x s *,@0  H x 0޽h ? f33f3y___PPT10Y+D='  = @B +  0L0 | <(  | ~ | s *,`0   ~ | s *%,@0  H | 0޽h ? f33f3y___PPT10Y+D='  = @B +  0 @8 (   ~  s *:,`0   3  6 <g ?p i  void int_handler(int sig) { printf("Process %d received signal %d\n", getpid(), sig); exit(0); } void fork13() { pid_t pid[N]; int i, child_status; signal(SIGINT, int_handler); . . . } 0  ('%   6Gg ?` p q linux> ./forks 13 Killing process 24973 Killing process 24974 Killing process 24975 Killing process 24976 Killing process 24977 Process 24977 received signal 2 Child 24977 terminated with exit status 0 Process 24976 received signal 2 Child 24976 terminated with exit status 0 Process 24975 received signal 2 Child 24975 terminated with exit status 0 Process 24974 received signal 2 Child 24974 terminated with exit status 0 Process 24973 received signal 2 Child 24973 terminated with exit status 0 linux>  0   $H  0޽h ? GGGy___PPT10Y+D='  = @B +*  0 QI (   r  S  ,`0   r  S  ,@ 0    6`g ?`  Oint ccount = 0; void child_handler(int sig) { int child_status; pid_t pid = wait(&child_status); ccount--; printf("Received signal %d from process %d\n", sig, pid); } void fork14() { pid_t pid[N]; int i, child_status; ccount = N; signal(SIGCHLD, child_handler); for (i = 0; i < N; i++) if ((pid[i] = fork()) == 0) { /* Child: Exit */ exit(0); } while (ccount > 0) pause();/* Suspend until signal occurs */ } 0  2 +LG3H  0޽h ? GGGy___PPT10Y+D='  = @B +   0  (   r  S | ,`0   r  S  ,@0   _  6g ?   void child_handler2(int sig) { int child_status; pid_t pid; while ((pid = wait(&child_status)) > 0) { ccount--; printf("Received signal %d from process %d\n", sig, pid); } } void fork15() { . . . signal(SIGCHLD, child_handler2); . . . } 0   WH  0޽h ? GGGy___PPT10Y+D='  = @B +: ! 0 aY (   ~  s *8,`0   x  c $,@0    B̟g ?0 AS#include #include #include void handler(int sig) { printf("You think hitting ctrl-c will stop the bomb?\n"); sleep(2); printf("Well..."); fflush(stdout); sleep(1); printf("OK\n"); exit(0); } main() { signal(SIGINT, handler); /* installs ctl-c handler */ while(1) { } }  T0 T    ?   G"H  0޽h ? GGGy___PPT10Y+D='  = @B + " 0  :(   r  S x,`0   .  Bĸg ?nf #include #include int beeps = 0; /* SIGALRM handler */ void handler(int sig) { printf("BEEP\n"); fflush(stdout); if (++beeps < 5) alarm(1); else { printf("BOOM!\n"); exit(0); } }  0  *  9H  Bg ?p 9  main() { signal(SIGALRM, handler); alarm(1); /* send SIGALRM in 1 second */ while (1) { /* handler returns here */ } }  0   BƲg ?V P` :linux> a.out BEEP BEEP BEEP BEEP BEEP BOOM! bass>  ;0 ;6H  0޽h ? GGGy___PPT10Y+D='  = @B + # 0  <(   ~  s *Ӳ,`0   ~  s *pӲ,@0  H  0޽h ? GGGy___PPT10Y+D='  = @B + $ 0  <(   ~  s *T,`0   ~  s *x,@0  H  0޽h ? GGGy___PPT10Y+D='  = @B +  % 0 0( (   ~  s *,`0     HD8c? 4 0 N  6g ?`   #include jmp_buf buf; main() { if (setjmp(buf) != 0) { printf("back in main due to an error\n"); else printf("first time through\n"); p1(); /* p1 calls p2, which calls p3 */ } ... p3() { if (error) longjmp(buf, 1) }  0    , y  H  0޽h ? GGGy___PPT10Y+D='  = @B +  & 0 0 (    (   r  S ,`0   r  S l,@0    Bg ?. #include #include #include sigjmp_buf buf; void handler(int sig) { siglongjmp(buf, 1); } main() { signal(SIGINT, handler); if (!sigsetjmp(buf, 1)) printf("starting\n"); else printf("restarting\n");  0  "  <     B!g ? s Bwhile(1) { sleep(1); printf("processing...\n"); } }  C0 CS  B'8c?P =\ bass> a.out starting processing... processing... restarting processing... processing... processing... restarting processing... restarting processing... processing... 0   H.8c?| P  BCtrl-c 0 jB  BD8c? `@   H18c?   BCtrl-c 0 jB  BD8c?V `@V   H58c?   BCtrl-c 0 jB  BD8c?v `@v H  0޽h ? GGGy___PPT10Y+D='  = @B +  ' 0   0 X (   r  S x>,`0   r  S E,@0    6Fg ? H ,jmp_buf env; P1() { if (setjmp(env)) { /* Long Jump to here */ } else { P2(); } } P2() { . . . P2(); . . . P3(); } P3() { longjmp(env, 1); } 0 H i   B@:8c? DP1&0   BC8c?p DP2&0   BT8c?p  DP2&0   BW8c?   DP2&0   B[8c?   DP3&0 dB  <D8c?   N_8c? `  Yenv 0   Bd8c? DP1&0   Hg8c? U t nBefore longjmp"0   Hm8c?{0 m After longjmp"0 H  0޽h ? GGGy___PPT10Y+D='  = @B +: ( 0 aY@ (   r  S ,`0   r  S \,@0  s  6g ?0 x jmp_buf env; P1() { P2(); P3(); } P2() { if (setjmp(env)) { /* Long Jump to here */ } } P3() { longjmp(env, 1); } 0 H) 1 F     p P0    N8c?  Yenv 0 GN P p       B`8c? p DP1&0    B8c?p DP2&0 lB   <D8c?P    H8c?O i At setjmp" 0  F          B8c? P  DP1&0    B8c?P  DP3&0 lB   <D8c? @@   NT8c? P $ Yenv 0    H8c?0 j At longjmp" 0     H8c? y =X 0 F     L P    B8c?P DP1&0    B8c?P  DP2&0 lB   <D8c?`@@   Hp8c?0 y  H P2 returns" 0     N|8c?P[$ Yenv 0    Ht8c?Y =X 0 H  0޽h ? GGGy___PPT10Y+D='  = @B +m ) 0 P $(   r  S մ,`0   r  S pִ,@0  H  0޽h ? GGGy___PPT10Y+D='  = @B + a 0 ` (   X  C d     S 8/d9 3    H  0ηo~ ? 3380___PPT10.3L b 0 p (   X  C d     S xUd9 3    H  0ηo~ ? 3380___PPT10.3L c 0  (   X  C d     S Rfz.BVj~ 2FZan1Oh+'0`U px$8 Xd    EECS 213Fabian E. BustamantepBased on the slides provided with "Computer Systems: A programmer's perspective", R. Bryant and D. O'Hallaron aqualab01Fabian E. Bustamante99Microsoft PowerPoint@ @QZ@|ÉGSg  )'    """)))UUUMMMBBB999|PP3f333f3333f3ffffff3f̙3ff333f333333333f33333333f33f3ff3f3f3f3333f33̙33333f333333f3333f3ffffff3f33ff3f3f3f3fff3ffffffffff3ffff̙fff3fffff3fff333f3f3ff3ff33f̙̙3̙ff̙̙̙3f̙3f333f3333f3ffffff3f̙3f3f3f333f3333f3ffffff3f̙3f3ffffffffff!___wwwThin4'A x(xKʦ """)))UUUMMMBBB999|PP3f3333f333ff3fffff3f3f̙f3333f3333333333f3333333f3f33ff3f3f3f3333f3333333f3̙33333f333ff3ffffff3f33f3ff3f3f3ffff3fffffffff3fffffff3f̙ffff3ff333f3ff33fff33f3ff̙3f3f3333f333ff3fffff̙̙3̙f̙̙̙3f̙3f3f3333f333ff3fffff3f3f̙3ffffffffff!___wwweeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeϮϮϮϮϮϮϮϮϮϮϮϮϮϮϮϮϮϮϮϮ՜.+,0P   $ ,4<D L Lecture notesOn-screen Show$CompSci - Northwestern University%% .ArialTahomaTimes New Roman WingdingsVerdana HelveticaTimes Courier New 1_aqualab01Exceptional Control Flow II%ECF exists at all levels of a systemThe world of multitasking#Programmers model of multitaskingUnix process hierarchyUnix startup: Step 1Unix startup: Step 2Unix startup: Step 3Unix startup: Step 4Shell programsSimple shell eval function"Problem with simple shell exampleSignalsSignal concepts Signal concepts (cont)Signal concepts (cont)Signal concepts Process groups"Sending signals with kill program"Sending signals from the keyboardExample of ctrl-c and ctrl-z#Sending signals with kill functionReceiving signalsDefault actionsInstalling signal handlersSignal handling exampleSignal handler funkinessLiving with nonqueuing signalsExternal event handlingInternal event handlingNonlocal jumps: setjmp/longjmpsetjmp/longjmp (cont)setjmp/longjmp examplePutting it all togetherLimitations of nonlocal jumps"Limitations of long jumps (cont.)Summary  Fonts UsedDesign Template Slide Titles%,_0Fabian E. BustamanteFabian E. Bustamante  !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~      !"#$%&'()*+,-./0123456789:;<=>?@ABCEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnpqrstuvxyz{|}~Root EntrydO)Pictures2Current UserwSummaryInformation(DUPowerPoint Document(MDocumentSummaryInformation8o