class15 processes

7/18/2019 Class15 Processes

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Processes

TopicsTopics Process context switches

Creating and destroying processes

cs105

CS 105Tour of the Black Holes of Computing!


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ProcessesDef: ADef: Aprocessprocessis an instance of a running program.is an instance of a running program.

One of the most profound ideas in computer science. Not the same as program or processor

Process provides each program with two keyProcess provides each program with two keyabstractions:abstractions:

Logical control flowEach program seems to have exclusive use of the CPU.

Private address spaceEach program seems to have exclusive use of main memory.

How are these illusions maintained?How are these illusions maintained? Process executions interleaved (multitasking)

Address spaces managed by virtual memory system


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Logical Control Flows

Time

Process A Process B Process C

Each process has its own logical control flow


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Concurrent Processes

Two processesTwo processesrun concurrentlyrun concurrently((are concurrent)are concurrent)if theirif theirflows overlap in time.flows overlap in time.

Otherwise, they areOtherwise, they aresequential.sequential.

Examples:Examples: Concurrent: A & B, A & C

Sequential: B & C

Time



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User View: Concurrent Processes

Control flows for concurrent processes are physicallyControl flows for concurrent processes are physicallydisjoint in time. (Except on multi-CPU machines.)disjoint in time. (Except on multi-CPU machines.)

However, we can think of concurrent processes asHowever, we can think of concurrent processes asrunning in parallel with each other.running in parallel with each other.

Time



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Context Switching

Processes are managed by a shared chunk of OS codeProcesses are managed by a shared chunk of OS code

called thecalled thekernelkernel Important: the kernel is not a separate process, but ratherruns as part of (or on behalf of) some user process

Control flow passes from one process to another via aControl flow passes from one process to another via a

context switchcontext switchProcess Acode

Process Bcode

user code

kernel code

user code

kernel code

user code

Time

context switch

context switch


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Private Address Spaces

Each process has its own private address space.Each process has its own private address space.

kernel virtual memory(code, data, heap, stack)

memory mapped region forshared libraries

run-time heap(managed by malloc)

user stack(created at runtime)

unused0

%esp (stack pointer)

memoryinvisible touser code

brk

0xc0000000

0x08048000

0x40000000

read/write segment(.data, .bss)

read-only segment(.init, .text, .rodata)

loaded from theexecutable file

0xffffffff


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fork: Creating New Processes

int fork(void)int fork(void) Creates a new process (child process) that is identical to the

calling process (parent process)

Returns 0 to the child process

Returns childspidto the parent process

if (fork() == 0) { printf("hello from child\n");} else {printf("hello from parent\n");

}

Fork is interesting(and often confusing)because it is called

oncebut returnstwice

Huh? Run thatby me again!


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Fork Example #1

void fork1(){ int x = 1; // arent and child !ill et private copies# pid$t pid = fork(); if (pid == 0) {

printf("%hild has x = &d\n"' x); } else {

printf("arent has x = &d\n"' x); } printf("*+e from process &d !ith x = &d\n"' etpid()' x);}

Key PointsKey Points

Parent and child both run same codeDistinguish parent from child by return value fromfork

Start with same state, but each has private copyIncluding shared input and output file descriptors

Relative ordering of their print statementsundefined


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Fork Example #2

void fork,(){ printf("-0\n"); fork(); printf("-1\n");

fork(); printf("*+e\n");}

Key PointsKey Points Both parent and child can continue forking

-0 -1

-1

*+e*+e

*+e

*+e


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Fork Example #3

void fork.(){ printf("-0\n");

fork(); printf("-1\n");

fork(); printf("-,\n");

fork(); printf("*+e\n");

}


-1 -,

-,

*+e*+e

*+e

*+e

-1 -,

-,

*+e

*+e*+e

*+e

-0


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Fork Example #4

void fork(){ printf("-0\n");

if (fork() #= 0) {printf("-1\n");if (fork() #= 0) { printf("-,\n"); fork();}

} printf("*+e\n");}


-0 -1

*+e

-,

*+e

*+e

*+e


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Fork Example #5

void fork5(){ printf("-0\n");

if (fork() == 0) {printf("-1\n");if (fork() == 0) { printf("-,\n"); fork();}

} printf("*+e\n");}


-0 *+e

-1

*+e

*+e

*+e

-,


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exit: Destroying Process

void exit(int stats)void exit(int stats) Exits a process

Normally return with status 0 (success)

atexit()registers functions to be executed upon exit

void cleanp(void) { printf("cleanin p\n");}

void fork() { atexit(cleanp);

fork(); exit(0);}


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Zombies

IdeaIdea

When process terminates, still consumes system resourcesVarious tables maintained by OS (to store exit status)

Called a zombieLiving corpse, half alive and half dead

ReapingReaping Performed by parent on terminated child

Parent is given exit status information

Kernel discards process

What If Parent Doesnt Reap?What If Parent Doesnt Reap? If any parent terminates without reaping a child, then child

will be reaped byinitprocess

Only need explicit reaping for long-running processesE.g., shells and servers


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linx2 ./forks 7 &314 .6nnin arent' 78 = .9erminatin %hild' 78 = 0linx2ps 78 99: 97< %855 tt+p 00>00>00 tcsh. tt+p 00>00>0. forks0 tt+p 00>00>00 forks ?defnct2

1 tt+p 00>00>00 pslinx2kill 6639314 9erminatedlinx2ps 78 99: 97< %855 tt+p 00>00>00 tcsh, tt+p 00>00>00 ps

Zombie Example

psshows childprocess as defunct

Killing parent allowschild to be reaped

void fork@(){

if (fork() == 0) {/A %hild A/printf("9erminatin %hild' 78 = &d\n"' etpid());exit(0);

} else {printf("6nnin arent' 78 = &d\n"' etpid());

!hile (1) ; /A 7nfinite loop A/

}}


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linx2 ./forks 89erminatin arent' 78 = @56nnin %hild' 78 = @linx2ps 78 99: 97< %855 tt+p 00>00>00 tcsh@ tt+p 00>00>0 forks@@ tt+p 00>00>00 ps

linux>kill @linux>ps 78 99: 97< %855 tt+p 00>00>00 tcsh@ tt+p 00>00>00 ps

NonterminatingChild

Example

Child process still active

even though parent has

terminated

Must kill explicitly, or elsewill keep running

indefinitely

void fork(){ if (fork() == 0) {

/A %hild A/printf("6nnin %hild' 78 = &d\n"'

etpid());!hile (1) ; /A 7nfinite loop A/

} else {printf("9erminatin arent' 78 = &d\n"' etpid());exit(0);

}}


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!ait: Synchronizing With Children

int !ait(int Achild$stats)int !ait(int Achild$stats)

Suspends current process until one of its childrenterminates

Return value ispidof child process that terminated

Ifchild$stats#= BC--, then integer it points to will beset to indicate why child terminated


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!ait: Synchronizing WithChildrenvoid fork() {

int child$stats;

if (fork() == 0) { printf("D%> hello from child\n"); } else { printf("D> hello from parent\n"); !ait(Echild$stats); printf("%9> child has terminated\n"); } printf("*+e\n");

exit(0);}D

D% *+e

%9 *+e


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Wait Example If multiple children completed, will take in arbitrary order

Can use macros WIFEXITED and WEXITSTATUS to getinformation about exit status

void fork10(){ pid$t pid3B4; int i;

int child$stats; for (i = 0; i ? B; i)

if ((pid3i4 = fork()) == 0) exit(100i); /A %hild A/

for (i = 0; i ? B; i) {pid$t !pid = !ait(Echild$stats);

if (F7G


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Waitpid!aitpid(pid' Estats' options)

Can wait for specific process

Various options available (see man page)

void fork11(){ pid$t pid3B4; int i;

int child$stats; for (i = 0; i ? B; i)

if ((pid3i4 = fork()) == 0) exit(100i); /A %hild A/

for (i = 0; i ? B; i) {pid$t !pid = !aitpid(pid3i4' Echild$stats' 0);

if (F7G


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Wait/Waitpid Example Outputs

%hild .55 terminated !ith exit stats 10.%hild .5 terminated !ith exit stats 10,%hild .5. terminated !ith exit stats 101%hild .5, terminated !ith exit stats 100%hild .5 terminated !ith exit stats 10

%hild .5 terminated !ith exit stats 100

%hild .5 terminated !ith exit stats 101%hild .5@0 terminated !ith exit stats 10,%hild .5@1 terminated !ith exit stats 10.%hild .5@, terminated !ith exit stats 10

Using!ait (fork10)

Using!aitpid (fork11)


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exec: Running New Programsint execl(char Apath' char Aar0' char Aar1' L' 0)int execl(char Apath' char Aar0' char Aar1' L' 0)

Loads and runs executable atpathwith argsar0,ar1, pathis the complete path of an executablear0becomes the name of the process

Typicallyar0is either identical topath, or else it contains only theexecutable filename frompath

Real arguments to the executable start withar1, etc.

List of args is terminated by a(char A)0argument

Returns1if error, otherwise doesnt return!

main() { if (fork() == 0) {

execl("/sr/Kin/cp"' "cp"' "foo"' "Kar"' BC--); } !ait(BC--); printf("cop+ completed\n"); exit(0);}


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Summarizing

ProcessesProcesses At any given time, system has multiple active processes

But only one (per CPU core) can execute at a time

Each process appears to have total control of processor +

private memory space


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Summarizing (cont.)

Spawning ProcessesSpawning Processes

Call toforkOne call, two returns

Terminating ProcessesTerminating Processes Callexit

One call, no return

Reaping ProcessesReaping Processes Call!aitor!aitpid

Replacing Program Executed by ProcessReplacing Program Executed by Process Callexecl (or variant)

One call, (normally) no return

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