unix process states

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Understanding Linux Process StatesAuthor: Yogesh BabarTechnical Reviewer: Chris NegusEditor: Allison Pranger08/!/"0!"

OVERVIEW

Aprocessis an instance o# a co$%uter %rogra$ that is currentl& being e'ecuted( Associated with a %rocess

is a variet& o# attributes )ownershi%* nice value* and +E,inu' conte't* to na$e a #ew- that e'tend or li$it its

abilit& to access resources on the co$%uter( .uring the li#e o# a %rocess* it can go through di##erent states(

To get started with the various states o# a %rocess* co$%are a ,inu' %rocess to a hu$an being( Ever&

hu$an being has di##erent stages o# li#e( The li#e c&cle begins with the %arents giving birth to an o##s%ring)s&non&$ous to a %rocess being forkedb& its %arent %rocess-(

A#ter birth* hu$ans start living their lives in their surroundings and start using available resources #or their

survival )s&non&$ous to a %rocess being in a Runningstate-( At so$e %oint in the li#e c&cle* hu$ans need to

wait #or so$ething that the& $ust have be#ore the& can continue to the ne't ste% in their lives )this is

s&non&$ous to a %rocess being in the Sleepstate-( And ust as ever& hu$an li#e $ust co$e to an end*

ever& %rocess $ust also die at so$e %oint in ti$e(

This tech brie# will hel% &ou understand the di##erent states in which a %rocess can be(

UNDERSTANDING PROCESS TYPES

There are di##erent t&%es o# %rocesses in a ,inu' s&ste$( These t&%es include user %rocesses* dae$on

%rocesses* and ernel %rocesses(

User Processes

1ost %rocesses in the s&ste$ are user %rocesses( A user %rocess is one that is initiated b& a regular user

account and runs in user s%ace( 2nless it is run in a wa& that gives the %rocess s%ecial %er$issions* an

ordinar& user %rocess has no s%ecial access to the %rocessor or to #iles on the s&ste$ that don3t belong to

the user who launched the %rocess(

Daeon Process

A dae$on %rocess is an a%%lication that is designed to run in the bacground* t&%icall& $anaging so$e ind

o# ongoing service( A dae$on %rocess $ight listen #or an inco$ing re4uest #or access to a service( 5or

e'a$%le* the httpddae$on listens #or re4uests to view web %ages( 6r a dae$on $ight be intended toinitiate activities itsel# over ti$e( 5or e'a$%le* the cronddae$on is designed to launch cron obs at %reset

ti$es(

Although dae$on %rocesses are t&%icall& $anaged as services b& the root user* dae$on %rocesses o#ten

run as non7root users b& a user account that is dedicated to the service( B& running dae$ons under di##erent

user accounts* a s&ste$ is better %rotected in the event o# an attac( 5or e'a$%le* i# an attacer were to

tae over the httpddae$on )web server-* which runs as the A%ache user* it would give the attacer no

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s%ecial access to #iles owned b& other users )including root- or other dae$on %rocesses(+&ste$s o#ten start dae$ons at boot ti$e and have the$ run continuousl& until the s&ste$ is shut down(

.ae$ons can also be started or sto%%ed on de$and* set to run at %articular s&ste$ run levels* and* in so$e

cases* signaled to reload con#iguration in#or$ation on the #l&(

!erne" Processes

ernel %rocesses e'ecute onl& in ernel s%ace( The& are si$ilar to dae$on %rocesses( The %ri$ar&

di##erence is that ernel %rocesses have #ull access to ernel data structures* which $aes the$ $ore

%ower#ul than dae$on %rocesses that run in user s%ace(

ernel %rocesses also are not as #le'ible as dae$on %rocesses( You can change the behavior o# a dae$on

%rocess b& changing con#iguration #iles and reloading the service( Changing ernel %rocesses* however*

$a& re4uire reco$%iling the ernel(

INTRODUCING SYSTE# STATES

9hen a %rocess is created* the s&ste$ assigns it a state( The state #ield o# the %rocess descri%tor describes

the current state o# the %rocess( The value o# the state #ield is usuall& set with a si$%le assign$ent* as

shown in this e'a$%le:

p->state = TASK_RUNNING

ere* pstands #or Process* stateis the #lag* and TASK_RUNNINGindicates that %rocess is currentl&

running or read& to run(

1ost %rocesses are in one o# the #ollowing two states:

A %rocess that is on the CP2 )a running %rocess-

A %rocess that is o## the CP2 )a not7running %rocess-

6nl& one %rocess can run at a ti$e on a single CP2( All other %rocesses have to wait or be in so$e other

state( This is wh& a %rocess that is not running a%%ears in a di##erent state( +tates include the #ollowing:

Runnable state

+lee%ing state

2ninterru%table slee% state

.e#unct or ;o$bie state


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$igure %& Processes t'roug' di((erent states )et*een )irt' and terination

Starting a Process +,orn or $or-ed.As %er the fork(2)$an %age* forkcreates a new %rocess b& du%licating the calling %rocess( The new

%rocess* re#erred to as the child* is an e'act du%licate o# the calling %rocess )re#erred to as the %arent-*

e'ce%t #or the #ollowing %oints:

The child has a uni4ue %rocess


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Running StateThe $ost %recious resource in the s&ste$ is the CP2( The %rocess that is e'ecuting and using the CP2 at a

%articular $o$ent is called a running%rocess( You can run the psand topco$$ands to see the state o#

each %rocess( th #ield in the "proc""stat#ile )where re%resents the %rocess


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top 2*2+*25 up 2*5, users, load average* %3+, 3%-, 3%$asks* . total, 2 running, .2 sleeping, / stopped, / 0ombie!pu(s)* %&'$#s, '*$s+, /%/1ni, %1id, /%/1wa, /%1hi, /%1si, /%/1st

Runna)"e State

9hen a %rocess is in a Runnable state* it $eans it has all the resources it needs to run* e'ce%t that the CP2

is not available( The Runnable state o# this %rocess is shown as R in psout%ut(

Consider a e'a$%le( A %rocess is dealing with


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is bus& with so$e other %rocess(


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SU##ARY5ro$ the ti$e a %rocess is born to when it is ter$inated* the %rocess %roceeds through various states(

Those states can include Runnable* Running* +lee%ing )in $e$or& and on dis-* and ;o$bie states( The

out%ut o# the ps ,elco$$and lets &ou see the various states associated with each %rocess on &our

s&ste$(

Understanding Linux Process States | Yogesh Babar 7

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