Control Flow CPU Sean Barker 1 Physical Control Flow Physical control flow <startup> inst1 inst Time 2 inst3 … instn <shutdown> Sean Barker 2 Operating System User-level Applications Operating System Hardware Resources Sean Barker 3 Processes Memory Memory Memory Stack Stack Stack Heap Heap Heap Data Data … Data Code Code Code CPU CPU CPU Registers Registers Registers Sean Barker 4 Control Flow Abstraction Process A Process B Process C Time Sean Barker 5 Control Flow Time-Sharing Process A Process B Process C Time Sean Barker 6 Context Switching Process A Process B user code kernel code context switch Time user code kernel code context switch user code 11/12/15 Sean Barker 7 Exceptions:'hardware/support/for/OS/ Interrupt'Vector stored/in/memory Exceptions special/register/holds/base/address transfer' control' to'OS'in'response' to'event What'code'should'the'OS'run'to'handle'the'event? code/for// Unique'ID'per'type'of'event exception/handler/ 0 Exception Ta bl e code/for/ ID'='index'into'exception'table' User-Process OS exception/handler/ 1 0 (a.k.a./interrupt/vector) 1 code/for exception 2 event' ... exception/handler/ 2 Handler'i is'called'each'time' exception'processing n&1 exception'i occurs by/exception'handler ... return'or'abort code/for/ exception/handler/ n<1 Exception/ numbers a/ jump/table/ for/exceptions… 9 10 Sean Barker 8 Open'a'file (trap/system/ call) Segmentation'Fault User/process/calls:/open(filename, options) int a[1000]; open executes/system/call/instruction/int void bad () { Write/to/invalid/memory/location. a[5000] = 13; 0804d070 <__li b c _o p e n>: } . 804d082: cd 80 int $0x80 80483b7: c7 05 60 e3 04 08 0d movl $0xd,0x804e360 804d084: 5b pop %eb x . User-Process OS User-Process OS movl exception:'page'fault detect'invalid'address int exception pop signal'process open-file returns aborts/process/with/SIGSEGV/ signal 12 13 3 Example: Segmentation Fault int a[1000]; main () { a[5000] = 13; } 80483b7: c7 05 60 e3 04 08 0d movl $0xd,0x804e360 User code Kernel code Excep&on: page fault movl Detect invalid address Signal process Sean Barker 9 Exception Table Excep&on numbers Code for excep&on handler 0 Excep&on Code for Table excep&on handler 1 0 1 Code for 2 ... excep&on handler 2 n-1 ... Code for excep&on handler n-1 Sean Barker 10 Process Management Memory Memory Memory Stack Stack Stack Heap Heap Heap Data Data … Data Code Code Code CPU CPU CPU Registers Registers Registers Sean Barker 11 11/12/15 Fork/Exec When.you.run.the.command.ls forkGexec ExecGing1a1new1program in.a.shell: forkGexec1model: Stack fork() clone.current.process Code/state.of.shell.process. execv() replace.process.code.and.context.(registers,.memory) with.a.fresh.program. 1 See.man131execv,.man.2.execv e Heap Replaced.by.code/state.of.ls. Data Copy.of.code/state. // Example arguments: path="/usr/bin/ls”, Code:./usr/bin/bash of.shell.process. // argv[0]="/usr/bin/ls”, argv[1]="-ahl", argv[2]=NULL fork(): void fork_exec(char* path, char* argv[]) { parent child child pid_t pid = fork(); Stack if (pid != 0) { Stack Stack printf("Parent: created a child %d\n”, pid); } else { exec(): printf("Child: exec-ing new program now\n"); 2 2 3 execv(path, argv); Heap Heap } Data Data Data printf("This line printed by parent only!\n"); Code:./usr/bin/bash Code:./usr/bin/bash Code:./usr/bin/ls } 6 Code/state.of.shell.process. 7 Sean Barker 12 execv: load/start1program exit:1end1a1process Stack-bottom NullYterminated int execv(char* filename, env var strings void exit(int status) End1process with.status:.0.=.normal,.nonzero.=.error. char* argv[]) NullYterminated argument.strings atexit() registers.functions.to.be.executed.upon.exit loads/starts1 program1 in1current1process: unused Executable.filename envp[n].==.NULL With.argument.list.argv envp[nY1] … overwrites1code,1data,1 and1stack envp[0] Keeps.pid,.open.files,.a.few.other.items. argv[argc]. ==. NULL does-not-return argv[argcY1] … unless.error argv[0] Linker.vars envp argv Also.sets.up environment...See.also:.execve. argc Stack. frame.for. main Stack-top 8 9 2 Zombies! Sean Barker 13 11/12/15 Reaping: waitpid Zombies! wait for.child.processes.to.terminate Terminated1 process1still1consumes1system1resources pid_t waitpid(pid_t pid, int* stat, int ops) Various.tables.maintained.by.OS Suspend.current.process.(i.e..parent).until.child.with.pid ends. A.living.corpse,.half.alive.and.half.dead On.success: Reaping1with wait/waitpid waitReturn. setpid when.child.terminates. Parent.waits to.reap.child.once.child.terminates Reap.child. Parent.receives.child.exit.status. Kernel.discards.process. If.stat != NULL,. waitpid saves.termination. reason. where. it. points. What1if1parent1doesn’t1reap? If.any.parent.terminates.without.reaping.a.child,.then.child.will.be.reaped. See. also:.man&3&waitpid by.init process.(pid.==.1) But.in.longYrunning.processes.we.need.explicit reaping e.g.,.shells.and.servers Sean Barker 14 10 11 waitpid example HCBye ErrorGchecking CTBye Check1return1results1of1system1calls1for1errors!1(No.exceptions.) void fork_wait() { Read.documentation. for.return.values. int child_status; pid_t child_pid == fork(); Use.perror to.report.error,. then.exit. if (child_pid == 0) { printf("HC: hello from child\n"); void perror(char* message) } else { Print."message:. reason.that.last.system.call.failed." if (-1 == waitpid(child_pid, &child_status, 0) { perror("waitpid"); exit(1); } printf("CT: child %d has terminated\n”, child_pid); } printf("Bye\n"); exit(0); } 12 3 11/12/15 Status Macros Zombies! wait for.child.processes.to.terminate Terminated1 process1still1consumes1system1resources pid_t waitpid(pid_t pid, int* stat, int ops) Various.tables.maintained.by.OS Suspend.current.process.(i.e..parent).until.child.with.pid ends. A.living.corpse,.half.alive.and.half.dead On.success: Reaping1with wait/waitpid WEXITSTATUS(stat) child exit code Return.pid when.child.terminates. Parent.waits to.reap.child.once.child.terminates Reap.child. Parent.receives.child.exit.status. true if terminated normally WIFEXITED(stat) (called exit or returned from main) Kernel.discards.process. If.stat != NULL,. waitpid saves.termination. reason. where. it. points. What1if1parent1doesn’t1reap? If.any.parent.terminates.without.reaping.a.child,.then.child.will.be.reaped. See.WIFSIGNALED(stat) also:.man&3&waitpid true if terminated by signal by.init process.(pid.==.1) But.in.longYrunning.processes.we.need.explicit reaping WIFSTOPPED(stat) true if paused by signal e.g.,.shells.and.servers 11/12/15 Sean Barker 15 10 11 Option Macros Zombies! wait for.child.processes.to.terminate HCBye waitpidTerminated1 process1examplestill1consumes1system1resources Errorpid_t Gwaitpidchecking(pid_t pid, int* stat, int ops) Various.tables.maintained.by.OS Suspend.current.process.(i.e..parent).until.child.with.pid ends. A.living.corpse,.half.alive.and.half.dead CTBye Check1return1results1of1system1calls1for1errors!1(No.exceptions.) On.success: return immediately if child not voidReaping1 fork_wait()with wait/ waitpid{ Read.documentation.WNOHANG for.return.values. already terminated int child_status; Return.pid when.child.terminates. Parent.waits to.reap.child.once.child.terminates pid_t child_pid == fork(); Use.perrorReap.child.to.report.error,. then.exit. Parent.receives.child.exit.status. If.stat != NULL,. waitpid saves.termination. reason. ifKernel.discards. (child_pidprocess.== 0) { WUNTRACED also wait for paused (stopped) children voidwhere. perror it. points.(char* message) What1if1printf("HC:parent1doesn’t1reap hello from? child\n"); } else { If.any.parent.terminates.without.reaping.a.child,.then.child.will.be.reaped. See.Print."message:. also:.man&3&waitpid reason.that.last.system.call.failed." if (-1 == waitpid(child_pid, &child_status, 0) { by.init process.(pid.==.1) perror("waitpid"); WCONTINUED also wait for resumed children But.in.longexit(1);Yrunning.processes.we.need.explicit reaping } e.g.,.shells.and.servers printf("CT: child %d has terminated\n”, child_pid); Sean Barker 16 } printf("Bye\n"); 10 11 exit(0); } 12 waitpid example HCBye ErrorGchecking 3 CTBye Check1return1results1of1system1calls1for1errors!1(No.exceptions.) void fork_wait() { Read.documentation. for.return.values. int child_status; pid_t child_pid == fork(); Use.perror to.report.error,. then.exit. if (child_pid == 0) { printf("HC: hello from child\n"); void perror(char* message) } else { Print."message:. reason.that.last.system.call.failed." if (-1 == waitpid(child_pid, &child_status, 0) { perror("waitpid"); exit(1); } printf("CT: child %d has terminated\n”, child_pid); } printf("Bye\n"); exit(0); } 12 3 shell:(program(that(runs(other(programs System Call Error Handling Building(a(Shell Always check return values! if ((pid = fork()) < 0) {! fprintf(stderr, "fork error: %s\n", strerror(errno));! exit(0);! } global var 1 5 Sean Barker 17 Unix/Linux( Process(Hierarchy shell program(that(runs(other(programs(on(behalf(of(the(user Basicsh ShellOriginal( DesignUnix(shell((Stephen(Bourne,(AT&T(Bell(Labs,(1977) [0] bash “Bourne?Again”(Shell,(widely(used default(on(most(Unix/Linux/Mac(OS(X(systems others.. init [1] while (true) { Print command prompt. Daemon Login shell Read command line from user. e.g. httpd Parse command line. If command is built-in, do it. Child Child Child Else fork process to execute command. in child: Execute requested command with execv. (never returns) Grandchild Grandchild in parent: Wait for child to complete. } 6 7 Sean Barker 18 1 terminal ≠(shell Background(vs.(Foreground User(interface( to(shell(and( other(programs. Users(generally( run(one(command( at(a(time Graphical((GUI)( vs.(command?line( (CLI) Type(command,(read(output,(type(another(command Command@line( terminal( (emulator): Input((keyboard) Some(programs( run(“for(a(long(time” Output((screen) $ emacs fizz. txt # shell stuck until ema cs exi ts. A( “background”( job(is(a(process(we(don't(want(to(wait(for $ emacs boom. txt & # em acs ru ns in backg round [1] 9073 # wh ile sh ell i s... $ gdb ./ umbre lla # im mediat ely r eady f or nex t com mand don't(do(this(with(emacs un l es s( u si n g( X( wi nd o ws( vers i o n 8 9 Managing(Background(Jobs Signals SignalsSignal:(small(message(notifying(a(process(of(event(in(system Shell(waits(for(and(reaps(foreground(jobs.