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Ch.7 Process Contro

S.N.P.I.T & R.C,UM

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PROCESS CONTROL Control of Process Context

fork: create a new process

exit : terminate process execution

wait : allow parent process to synchronize its execution witchild process

exec : invoke a new program

brk: allocate more memory dynamically

signal : inform asynchronous event

major loop of the shell andof init

fork

dupreg

attachregdetachregallocregattachreggrowregloadregmapreg

exec brk

growreg

exit

detachreg

wait signal kill se

System Calls Dealing

with Memory Management

System Calls Dealing

with Synchronization

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Process Creation

The only way for a user to create a new process in the unixsystem is to invoke the fork system call.

The process that invokes fork is called the parent process, newly created process is called child process.

The syntax for the fork system call is

Pid = fork();

On return from the fork system call, the two processes havecopies of their user-level context except for the return valu

In the parent process ,pid is the child process ID; in the chil,pid is 0.

The process 0,created internally by the kernel when the sysbooted, is the only process not created via fork.

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The kernel does the following sequence of operafork.

1. It allocates a slot in the process table for the new

2. It assigns a unique ID number to the child proces

3. It makes a logical copy of the context for the parprocess.

4. It increments file and inode table counters for file

associated with the process.5. It returns the ID number of the child to the parent

and a 0 value to the child process.

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Algorithm for forkinput : none

output : to parent process, child PID number

to child process, 0

{

check for available kernel resources;

get free process table slot, unique PID number;

check that user not running too many process;

mark child state "being created";

copy data from parent process table to new child slot;

increment counts on current directory inode and changed root(if applicable);

increment open file counts in file table;

make copy of parent context(u area, text, data, stack) in memory;

push dummy system level context layer onto child system level context;

dummy context contains data allowing child process

to recognize itself, and start from here

when scheduled;

if ( executing process is parent )

{

change child state to "ready to run";

return(child ID); /* from system to user */

}

else /* executing process is the child process */

{

initialize u area timing fields;

return(0); /* to user */

}

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Fork Creating a New Process Co

Parent

DataPer Process

Region Table Open FilesCurrent Directory

Changed Root.....Kernel Stack

U Area

Per Process

Region Table Open FilesCurrent Directory

Changed Root.....Kernel Stack

U Area

Parent Process

Child Process

Parent

User

Stack

Shared

Text

Child

Data

Child

User

Stack

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Example of Sharing File Ac

#include

int fdrd, fdwt;

char c;

main( argc,argv )

int argc;

char *argv[];

{

if ( argc != 3 )

exit(1);

if ((fdrd=open(argv[1],O_RDONLY))==-1)

exit(1);

if ((fdwt=creat(argv[2],0666))==-1)

exit(1);

fork();

/*both process execute same code*/

rdwt();

exit(0);

}

rdwt(){

for(;;){if (read(fdrd,&c,1)!=-1)

return;write(fdwt,&c,1);

}}

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SIGNALS Signals inform processes of the occurrence asynchrono

Processes may send each signals with the kill system cmay send signals internally.

1. Signal having to do with the termination of a process, sprocess exits or when a process invokes the signal systdeath of child parameter.

2. Signals having to do with process induced exception suprocess accesses an address outside its virtual addressattempts to write memory that is read-only ,or when it eprivileged instruction or for various hardware errors.

3. Signal having to do with the unrecoverable conditions call , such as running out of system resources during exoriginal address space has been released.

4. Signal caused by an unexpected error condition duringsuch as making a nonexistent system call writing pipe reader processes , or using an illegal reference value

system call.

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5. Signal originating from a process in user mode, such as process wishes to receive an alarm signal after a periodwhen process send arbitrary signals to each other with tcall.

6. Signals related to terminal interaction such as when a ua terminal ,or when a user process the break or deletterminal keyboard.

7. Signals for tracing execution of a process.

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Handling signals There are three cases for handling signals:

1. The process exist on receipt of the signal.

2. It ignores the signals.

3. It executes a particular function on receipt of the signal.

The default action is to call exit in kernel mode but a process cspecial action to take on receipt of certain signals with the sig

The syntax for the signal system call is

Oldfunction = system (signum , function); where signum = signal number the process is specifying the ac

Function= address of the function the process wants to invokethe signal.

Oldfunction = value of function in the most recently specified signum.

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The process can pass the value 1 or 0 instead

address :

The process will ignore future occurrences of t

the parameter value is 1 and exit in the kerneof the signal if its value is 1 .

And exit in the kernel on receipt of the signal

0.

The u area contains an array of signal-handle

for each signal defined in the system .

The kernel stores the address of the user funct

field that corresponds to the signal number.

Specification to handle signals of one type ha

on handling signals of other types.

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Process Groups Processes on a Unix system are identified by a unique ID nu

must sometimes identify processes by group.

For instance ,processes with a common ancestor process th

are generally related, and therefore all such processes recea user hits the delete or break key or when the terminal

The setpgrp system call initializes the process group numberand sets it equal to the value of its process ID.

The syntax for the system call is

grp = setgrp()

Where , grp = new process group number

A child retain the process group number of its parent during

Setpgrp also has important ramification for setting up the coa process.

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Sending signal from processes Processes use the kill system call to send signals..

The syntax for the system call is

kill (pid,signum)

Where pid = set of processes to receive the signal

Signum = signal number being sent.

The following list shows the correspondence between values oprocesses.

1. If pid is a positive integer ,the kernel sends the signal to the pr

Id pid.

2. If pid is 0, the kernel sends the signal to all processes in the segroup.

3. If pid is -1 the kernel sends the signal to all processes whose rthe effective user ID of the sender.

4. If pid is a negative integer but not -1 the kernel sends the signin the process group equal to the absolute value of pid.

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Process Termination Processes on a UNIX system terminate by executing

system call.

An exiting process enters the zombie state ,relinquisresources and dismantles its context except for its slprocess table.

The syntax for call is

Exit (status);

Where the value of status is returned to the parent foexamination .

a gor m ex

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a gor m exinput : return code for parent process

output : none

{

ignore all signals;

if ( process group leader with associated control termina

{

send hangup signal to all members of process group;

reset process group for all members to 0;

}

close all open files(internal version of algorithm close)

release current directory(algorithm iput);

release current(changed) root, if exists (algorithm iput);

free regions, memory associated with process(algorithmwrite accounting record;

make process state zombie;

assign parent process ID of all child processes to be init

if any children were zombie, send death of child signal to

send death of child signal to parent process;

context switch;

}

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Awaiting Process Termination

wait system call

synchronize its execution with the terminatio

process

pid = wait(stat_addr);

pid : process id of the zombie child process

stat_addr : address of an integer to contain the exi

child

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Algorithm for Awaiting Process Term

1. searches the zombie child process

2. If no children, return error

3. if finds zombie children, extracts PID number and ex

4. adds accumulated time the child process executeuser and kernel mode to the fields in u area

5. Release process table slot

Al ith f W it

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Algorithm for Waitalgorithm wait

input : address of variables to store status of exiting process

output : child ID, child exit code

{

if (waiting process has no child process)

return(error);

for(;;)

{

if (waiting process has zombie child)

{

pick arbitrary zombie child;

add child CPU usage to parent;

free child process table entry;

return(childID,child exit code);

}

if (process has no child process)

return(error);

sleep at interruptible priority(event child process exits);

}

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Invoking Other Programs exec system call

invokes another program, overlaying the memory spacprocess with a copy of an executable file

execve(filename, argv, envp)

filename : the name of executable file being invoked

argv : a pointer to an array of character pointers that arethe executable program

envp : a pointer array of character pointers that are envirexecuted program

several library functions that calls exec system call

execl, execv, execle...

Algorithm for Exec

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Algorithm for Execalgorithm exec

input : (1) file name

(2) parameter list

(3) environment variables list

output : none

{

get file inode(algorithm namei)

verify file executable, user has permission toexecute;

read file headers, check that it is a load module;

copy exec parameters from old address space tosystem space;

for(every region attached to process)

detach all old regions(algorithm detach);

for(every region specified in load module)

{

allocate new regions(algorithm allocreg);

attach the regions(algorithm attachreg);

load region into memory if appropriate(algorithmloadreg);

}

copy exec parameters into nspecial processing for setuiinitialize user register save arelease inode of file(iput);}

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THE USER ID OF A PROCESS

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THE USER ID OF A PROCESS

The kernel associates two user IDs with a process, independenID the real user ID and the effective user ID or setuid (set user I

The real user ID identifies the user who is responsible for the run

The effective user ID is used to assign ownership of newly creatcheck file access permissions, and to check permission to senprocesses via the kill system call.

The kernel allows a process to change its effective user ID whesetuid program or when it invokes the setuid system call explic

The syntax for the setuid system call is

Setuid (uid)

Where uid = new user ID

New user ID ,its result depends on the current value of the effec

If the effective user ID of the calling process is super user the kreal and effective user ID fields in the process table and u area

CHANGING THE SIZE OF A PROCESS

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CHANGING THE SIZE OF A PROCESS

A process may increase or decrease the size of its data region bsystem call.

The syntax for the brk system call is brk(ends);

where ends becomes the value of the highest virtual address of t

of the process (called its break value)

Alternatively a user can call

Oldendds = sbrk (increment);

Where increment changes the current break value by the specifie

bytes and oldendds is the break value before the call.

Sbrk is a C library routine that call brk.

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THE SHELL