Processes, Exceptional Control Flow

Transcription

1 CIS330, Week 9 Prcesses, Exceptinal Cntrl Flw CSAPPe2, Chapter 8

2 Plan fr Tday Exceptinal Cntrl Flw Exceptins Prcess cntext switches Creating and destrying prcesses

3 Cntrl Flw Cmputers d Only One Thing Frm startup t shutdwn, a CPU simply reads and executes (interprets) a sequence f instructins, ne at a time. This sequence is the system s physical cntrl flw (r flw f cntrl). Time Physical cntrl flw <startup> inst 1 inst 2 inst 3 inst n <shutdwn>

4 Altering the Cntrl Flw Up t Nw: tw mechanisms fr changing cntrl flw: Jumps and branches Call and return using the stack discipline. Bth react t changes in prgram state. Insufficient fr a useful system Difficult fr the CPU t react t changes in system state. data arrives frm a disk r a netwrk adapter. Instructin divides by zer User hits Ctrl-c at the keybard System timer expires System needs mechanisms fr exceptinal cntrl flw

5 Exceptinal Cntrl Flw Mechanisms fr exceptinal cntrl flw exists at all levels f a cmputer system. Lw level Mechanism exceptins change in cntrl flw in respnse t a system event (i.e., change in system state) Cmbinatin f hardware and OS sftware Higher Level Mechanisms Prcess cntext switch Signals Nnlcal jumps (setjmp/lngjmp) Implemented by either: Ø OS sftware (cntext switch and signals). Ø C language runtime library: nnlcal jumps.

6 System cntext fr exceptins USB Prts Keybard Muse Mdem Printer Prcessr Interrupt cntrller Timer Serial prt cntrllers Parallel prt cntrller I/O Chip Lcal/IO Bus Memry IDE disk cntrller SATA cntrller Vide adapter Netwrk adapter disk SATA bus Display Netwrk disk CDROM

7 Exceptins An exceptin is a transfer f cntrl t the OS in respnse t sme event (i.e., change in prcessr state) User Prcess OS event current next exceptin exceptin prcessing by exceptin handler exceptin return (ptinal)

8 Interrupt Vectrs n-1 Exceptin numbers interrupt vectr... cde fr exceptin handler 0 cde fr exceptin handler 1 cde fr exceptin handler 2... cde fr exceptin handler n-1 Each type f event has a unique exceptin number k Index int jump table (a.k.a., interrupt vectr) Jump table entry k pints t a functin (exceptin handler). Handler k is called each time exceptin k ccurs.

9 Asynchrnus Exceptins (Interrupts) Caused by events external t the prcessr Indicated by setting the prcessr s interrupt pin handler returns t next instructin. Examples: I/O interrupts hitting ctl-c at the keybard arrival f a packet frm a netwrk arrival f a data sectr frm a disk Hard reset interrupt hitting the reset buttn

10 Synchrnus Exceptins Caused by events that ccur as a result f executing an instructin: Traps intentinal, e.g., system calls, breakpints, special instructins; returns cntrl t next instructin. Faults unintentinal but pssibly recverable, e.g., page faults (recverable), prtectin faults (unrecverable), flatingpint exceptins; re-executes faulting ( current ) instructin r abrts. Abrts: unintentinal and unrecverable, e.g., parity errr, machine check; abrts current prgram

11 Precise vs. Imprecise Faults Precise Faults: the exceptin handler knws exactly which instructin caused the fault. All prir instructins have cmpleted and n subsequent instructins had any effect. Imprecise Faults: the CPU was wrking n multiple instructins cncurrently and an ambiguity may exists as t which instructin caused the Fault. Fr example, multiple FP instructins were in the pipe and ne caused an exceptin.

12 Trap Example Opening a File User calls pen(filename, ptins) 0804d070 < libc_pen>: d082: cd 80 int $0x80 804d084:... 5b pp %ebx Functin pen executes system call instructin int OS must find r create file, get it ready fr reading r writing Returns integer file descriptr User Prcess int pp return exceptin OS Open file

13 Fault Example #1 Memry Reference User writes t memry lcatin That prtin (page) f user s memry is currently n disk Page handler must lad page int physical memry Returns t faulting instructin Successful n secnd try int a[1000]; main () { a[500] = 13; 80483b7: c d d mvl $0xd,0x8049d10 User Prcess OS event mvl return page fault Create page and lad int memry

14 Fault Example #2 Memry Reference with TLB miss User writes t memry lcatin User Prcess int a[1000]; main () { a[500] = 13; That prtin (page) f user s memry is currently in physical memry, but the prcessr has frgtten hw t translate the this virtual address t the physical address TLB must be reladed with current translatin Returns t faulting instructin Successful n secnd try OS r Hardware event mvl return TLB miss Lk up address translatin and stre it in a TLB entry

15 Fault Example Memry Reference User writes t memry lcatin Address is nt valid int a[1000]; main () { a[5000] = 13; 80483b7: c e d mvl $0xd,0x804e360 Page handler detects invalid address Sends SIGSEG signal t user prcess User prcess exits with segmentatin fault User Prcess OS event mvl page fault Detect invalid address Signal prcess

16 Prcesses Definitin: A prcess is an instance f a running prgram. One f the mst prfund ideas in cmputer science Nt the same as prgram r prcessr Prcess prvides each prgram with tw key abstractins: Lgical cntrl flw Each prgram seems t have exclusive use f the CPU Private address space Each prgram seems t have exclusive use f main memry Hw are these Illusins maintained? Prcess executins interleaved (multitasking) Address spaces managed by virtual memry system

17 Lgical Cntrl Flws Each prcess has its wn lgical cntrl flw Prcess A Prcess B Prcess C Time

18 Cncurrent Prcesses Tw prcesses run cncurrently (are cncurrent) if their flws verlap in time. Otherwise, they are sequential. Examples: Cncurrent: A & B, A & C Sequential: B & C Prcess A Prcess B Prcess C Time

19 User View f Cncurrent Prcesses Cntrl flws fr cncurrent prcesses are disjint in time. Hwever, we can think f cncurrent prcesses are running in parallel with each ther. Time Prcess A Prcess B Prcess C

20 Cntext Switching Prcesses are managed by a shared chunk f OS cde called the kernel Imprtant: the kernel is nt a separate prcess, but rather runs as part f sme user prcess Cntrl flw passes frm ne prcess t anther via a cntext switch. Time Prcess A cde Prcess B cde user cde kernel cde user cde kernel cde user cde cntext switch cntext switch

21 Private Address Spaces Each prcess has its wn private address space. 0xffffffff 0xc x kernel virtual memry (cde, data, heap, stack) user stack (created at runtime) memry mapped regin fr shared libraries Memry invisible t user cde %esp (stack pinter) 0x run-time heap (managed by mallc) read/write segment (.data,.bss) read-nly segment (.init,.text,.rdata) unused brk laded frm the executable file

22 execve: Lading and Running Prgrams int execve( char *filename, char *argv[], char *envp ) Lads and runs Executable filename With argument list argv And envirnment variable list envp Des nt return (unless errr) Overwrites prcess, keeps pid Envirnment variables: name=value strings 0xbfffffff Null-terminated envirnment variable strings Stack Null-terminated cmmandline arg strings unused envp[n] = NULL envp[n-1] envp[0] argv[argc] = NULL argv[argc-1] argv[0] Linker vars envp argv argc

23 execve: Example envp[n] = NULL envp[n-1] envp[0] PWD=/hmes/iws/luisceze PRINTER=ps581 USER=luisceze argv[argc] = NULL argv[argc-1] argv[0] /usr/include -l ls

24 Virtual Machines All current general purpse cmputers supprt multiple, cncurrent user-level prcesses. Is it pssible t run multiple kernels n the same machine? Yes: Virtual Machines (VM) were supprted by IBM mainframes fr ver 30 years Intel s IA32 instructin set architecture is nt virtualizable (neither are the Sparc, Mips, and PPC ISAs) With a lt f clever hacking, Vmware managed t virtualize the IA32 ISA in sftware User Mde Linux

25 frk: Creating new prcesses int frk(vid) creates a new prcess (child prcess) that is identical t the calling prcess (parent prcess) returns 0 t the child prcess returns child s pid t the parent prcess if (frk() == 0) { printf("hell frm child\n"); else { printf("hell frm parent\n"); Frk is interesting (and ften cnfusing) because it is called nce but returns twice

26 Frk Example #1 Parent and child bth run same cde Distinguish parent frm child by return value frm frk Start with same state, but each has private cpy Including shared utput file descriptr Relative rdering f their print statements undefined vid frk1() { int x = 1; pid_t pid = frk(); if (pid == 0) { printf("child has x = %d\n", ++x); else { printf("parent has x = %d\n", --x); printf("bye frm prcess %d with x = %d\n", getpid(), x);

27 Frk Example #2 Bth parent and child can cntinue frking vid frk2() { printf("l0\n"); frk(); printf("l1\n"); frk(); printf("bye\n"); L1 L0 L1 Bye Bye Bye Bye

28 Frk Example #3 Bth parent and child can cntinue frking vid frk3() { printf("l0\n"); frk(); printf("l1\n"); frk(); printf("l2\n"); frk(); printf("bye\n"); L0 L2 L1 L2 L2 L1 L2 Bye Bye Bye Bye Bye Bye Bye Bye

29 Frk Example #4 Bth parent and child can cntinue frking vid frk4() { printf("l0\n"); if (frk()!= 0) { printf("l1\n"); if (frk()!= 0) { printf("l2\n"); frk(); printf("bye\n"); child L0 L1 L2 parent Bye Bye Bye Bye

30 Frk Example #5 vid frk5() { printf("l0\n"); if (frk() == 0) { printf("l1\n"); if (frk() == 0) { printf("l2\n"); frk(); printf("bye\n"); L1 L0 Bye L2 Bye Bye Bye

31 exit: Destrying Prcess vid exit(int status) exits a prcess Nrmally return with status 0 atexit() registers functins t be executed upn exit vid cleanup(vid) { printf("cleaning up\n"); vid frk6() { atexit(cleanup); frk(); exit(0);

32 Zmbies Idea When prcess terminates, still cnsumes system resurces Varius tables maintained by OS Called a zmbie Reaping Perfrmed by parent n terminated child Parent is given exit status infrmatin Kernel discards prcess What if Parent Desn t Reap? If any parent terminates withut reaping a child, then child will be reaped by init prcess Only need explicit reaping fr lng-running prcesses E.g., shells and servers

33 Zmbie Example linux>./frks 7 & [1] 6639 Running Parent, PID = 6639 Terminating Child, PID = 6640 linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6639 ttyp9 00:00:03 frks 6640 ttyp9 00:00:00 frks <defunct> 6641 ttyp9 00:00:00 ps linux> kill 6639 [1] Terminated linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6642 ttyp9 00:00:00 ps ps shws child prcess as defunct Killing parent allws child t be reaped vid frk7() { if (frk() == 0) { /* Child */ printf("terminating Child, PID = %d\n", getpid()); exit(0); else { printf("running Parent, PID = %d\n", getpid()); while (1) ; /* Infinite lp */

34 Nnterminating Child Example Child prcess still active even thugh parent has terminated Must kill explicitly, r else will keep running indefinitely linux>./frks 8 Terminating Parent, PID = 6675 Running Child, PID = 6676 linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6676 ttyp9 00:00:06 frks 6677 ttyp9 00:00:00 ps linux> kill 6676 linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6678 ttyp9 00:00:00 ps vid frk8() { if (frk() == 0) { /* Child */ printf("running Child, PID = %d\n", getpid()); while (1) ; /* Infinite lp */ else { printf("terminating Parent, PID = %d\n", getpid()); exit(0);

35 wait: Synchrnizing with children int wait(int *child_status) suspends current prcess until ne f its children terminates return value is the pid f the child prcess that terminated if child_status!= NULL, then the bject it pints t will be set t a status indicating why the child prcess terminated

36 wait: Synchrnizing with children vid frk9() { int child_status; if (frk() == 0) { printf("hc: hell frm child\n"); else { printf("hp: hell frm parent\n"); wait(&child_status); printf("ct: child has terminated\n"); printf("bye\n"); exit(); HC Bye HP CT Bye

37 Wait() Example If multiple children cmpleted, will take in arbitrary rder Can use macrs WIFEXITED and WEXITSTATUS t get infrmatin abut exit status vid frk10() { pid_t pid[n]; int i; int child_status; fr (i = 0; i < N; i++) if ((pid[i] = frk()) == 0) exit(100+i); /* Child */ fr (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 terminate abnrmally\n", wpid);

38 Waitpid() waitpid(pid, &status, ptins) Can wait fr specific prcess Varius ptins vid frk11() { pid_t pid[n]; int i; int child_status; fr (i = 0; i < N; i++) if ((pid[i] = frk()) == 0) exit(100+i); /* Child */ fr (i = 0; i < N; i++) { pid_t wpid = waitpid(pid[i], &child_status, 0); if (WIFEXITED(child_status)) printf("child %d terminated with exit status %d\n", wpid, WEXITSTATUS(child_status)); else printf("child %d terminated abnrmally\n", wpid);

39 Wait/Waitpid Example Outputs Using wait (frk10) Child 3565 terminated with exit status 103 Child 3564 terminated with exit status 102 Child 3563 terminated with exit status 101 Child 3562 terminated with exit status 100 Child 3566 terminated with exit status 104 Using waitpid (frk11) Child 3568 terminated with exit status 100 Child 3569 terminated with exit status 101 Child 3570 terminated with exit status 102 Child 3571 terminated with exit status 103 Child 3572 terminated with exit status 104

40 exec: Running new prgrams int execl(char *path, char *arg0, char *arg1,, 0) lads and runs executable at path with args arg0, arg1, path is the cmplete path f an executable arg0 becmes the name f the prcess typically arg0 is either identical t path, r else it cntains nly the executable filename frm path real arguments t the executable start with arg1, etc. list f args is terminated by a (char *)0 argument returns -1 if errr, therwise desn t return! main() { if (frk() == 0) execl("/usr/bin/cp", "cp", "f", "bar", 0); wait(null); printf("cpy cmpleted\n"); exit();

41 Summary Exceptins Events that require nn-standard cntrl flw Generated externally (interrupts) r internally (traps and faults) Prcesses At any given time, system has multiple active prcesses Only ne can execute at a time, hwever, Each prcess appears t have ttal cntrl f the prcessr + has a private memry space

42 Summary (cnt d) Spawning prcesses Call t frk One call, tw returns Prcess cmpletin Call exit One call, n return Reaping and waiting fr Prcesses Call wait r waitpid Lading and running Prgrams Call execl (r variant) One call, (nrmally) n return