Fork creation

8/19/2019 Fork creation

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Unix system calls

fork( )

wait( )exit( )


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Processes are the primitive units for allocation of system

resources. Each process has its own address space and (usually) one thread

of control.

A process executes a program; you can have multiple processes

executing the same program !ut each process has its own copy ofthe program within its own address space and executes it

independently of the other copies.

Processes are organi"ed hierarchically. Each process has a

parent process which explicitly arranged to create it. #he processes created !y a given parent are called its child

processes.

A child inherits many of its attri!utes from the parent process.

Process $reation


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• A process ID number names each process.

• A uni%ue process &' is allocated to each process when it iscreated.

• #he lifetime of a process ends when its termination is reported to

its parent process; at that time all of the process resources

including its process &' are freed.• Processes are created with the fork() system call (so the operation

of creating a new process is sometimes called forking a process).

• #he child process created !y fork is a copy of the original parent

process except that it has its own process &'.

Process $reation


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ow #o $reate ew Processes*

Underlying mechanism

-A process runs fork to create a child process

- Parent and children execute concurrently

- $hild process is a duplicate of the parent process

After a fork !oth parent and child keep running and each can fork

off other processes.

parent

child

fork()


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+ootstrapping

,hen a computer is switched on or reset there must !e an

initial program that gets the system running

#his is the !ootstrap program

- &nitiali"e $PU registers device controllers memory

--oad the / into memory

- /tart the / running

/ starts the first process (such as 0init1)

/ waits for some event to occur

- ardware interrupts or software interrupts (traps)


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2ork /ystem $all

$urrent process split into 3 processes4 parent child

#ext

'ata

/tack

#ext

'ata

/tack

fork() 5eturns 67 if unsuccessful

5eturns 8 in the child

5eturns the child9sidentifier in the parent


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2ork /ystem $all

#he child process inherits from parent

-identical copy of memory

- $PU registers

- all files that have !een opened !y the parent

Execution proceeds concurrently with the instruction following

the fork system call

#he execution context (P$+) for the child process is a copy of the parent9s context at the time of the call


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Process 'escription4 Process $ontrol +lock (P$+)

• &nformation associated with each process

– Process state

– Program counter

– $PU registers

– $PU scheduling information

– :emory6management information

– Accounting information

– & status information

•

A process is named using its process &' (P&')• 'ata is stored in a process control !lock (P$+)


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ow fork ,orks (7)

TextStack

Data

File

Resources

pid = !

ret = fork();

switch(ret)

{

case -1:

perror(“fork”);

exit(1);

case 0: // I am the child

code for child !

exit(0);

defa"lt: // I am pare#t $$$

code for parent !

wait(0);

%

"#$

%&I'


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ow fork ,orks (3)

Text

"#$

Stack

Data

File

Resources

pid = !

ret = fork();

switch(ret)

{

case -1:

perror(“fork”);

exit(1);

case 0: // I am the child code for child !

exit(0);


code for parent !

wait(0);

%

%&I'

ret =

Text

"#$

Stack

Data

pid =

ret = fork();

switch(ret)

{

case -1:

perror(“fork”);

exit(1);


exit(0);


code for parent !

wait(0);

%

ret =


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ow fork ,orks (


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ow fork ,orks (=)

Text

"#$

Stack

Data

File

Resources

pid = !

Text

"#$

Stack

Data

pid =

ret = fork();

switch(ret)

{

case -1:

perror(“fork”);

exit(1);


exit(0);


code for parent !

wait(0);

%

ret = fork();

switch(ret)

{

case -1:

perror(“fork”);

exit(1);


exit(0);


code for parent !

wait(0);

%

%&I'

ret = ret =


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ret = fork();

switch(ret)

{

case -1:

perror(“fork”);

exit(1);


exit(0);


code for parent !

wait(0);

%

ow fork ,orks (>)

Text

"#$

Stack

Data

File

Resources

pid = !

Text

"#$

Stack

Data

pid =

ret = fork();

switch(ret)

{

case -1:

perror(“fork”);

exit(1);


exit(0);


code for parent !

wait(0);

%

%&I'

ret = ret =


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ow fork ,orks (?)

Text

"rocess Status

Stack

Data

File

Resources

pid = !

ret = fork();

switch(ret)

{

case -1:

perror(“fork”);

exit(1);


exit(0);


code for parent !

wait(0);

& !

ret =

%&I'


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@includestdio.hB

@includesystypes.hB

@includestdli!.hB

void main()

C int pidn; pid*t pid+ printf(DEnter the num!ernD);

scanf(DFdDGn);

pidHfork();

if(pid8)C printf(0Error1);

exit(7);

I

else if(pidHH8)

C printf(Dchild process4nD);Ielse

C wait(7);

printf(Dparent process4nD);

I

I


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rderly #ermination4 exit()

#o finish execution a child may call exit(number )

#his system call4

- /aves result H argument of exit

- $loses all open files connections

-'eallocates memory

- $hecks if parent is alive

- &f parent is alive holds the result value until the parent re%uests

it (with wait); in this case the child process does not really die

!ut it enters a "om!iedefunct state- &f parent is not alive the child terminates (dies)


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,aiting for the $hild to 2inish

Parent may want to wait for children to finish

-Example4 a shell waiting for operations to complete

,aiting for any some child to terminate4 wait()

- +locks until some child terminates

- 5eturns the process &' of the child process

- r returns 67 if no children exist (i.e. already exited)

#include #include

pid_t wait(int *status);

pid_t waitpid(pid_t pid, int *status, int options);


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ther useful system calls4 getpid, getppid

getpid returns the identifier of the calling process. Example

call (pid is an integer)4

pid getpid();

getppid returns the identifier of the parent.

i l d di h


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@includestdio.hB

@includesystypes.hB

@includestdli!.hB

void main() C int pidn; pidJt pid;

printf(DEnter the num!ernD);

scanf(DFdDGn);

pidHfork();

printf(Dout pid4FdnDpid);

if(pid8)

C printf(DE5rD);

exit(7);

I

3

$


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$ont..

if(pidHH8)

C printf(Dchild process4 Fd nDKKn);

printf(Dch pid HFdnDpid); printf(Dch4fn pid FdnDgetpid());

printf(Dch4fn ppidFdnDgetppid());

exit(8);

I

else C wait(7);

printf(Dpar4 pidFdnDgetppid());

printf(Dpar4 pid varFdnDpid); printf(Dparent process4Fd nDn);

printf(Dpar4 pidFdnDgetpid());

I

3


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,ait()- .xit() / Sleep()

,ait()

- for a child process to terminate or stop and determine its status.

- will force a parent process to wait for a child process to stop or

terminate.

- return the pid of the child or 67 for an error.

.xit()

- exit() terminates the process which calls this function and returns

the exit status value.

- +y convention a status of 8 means normal termination.

- Any other value indicates an error or unusual occurrence.

Sleep ()

- A process may suspend for a period of time using the sleep

command3


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/ummary

2ork

-$reates a duplicate of the calling process

- #he result is two processes4 parent and child

- +oth continue executing from the same point on

Exit- rderly program termination

- Un!locks waiting parent

,ait- Used !y parent

- ,aits for child to finish execution

Documents

Fork creation