Process Management Compatibility Mode

7/26/2019 Process Management Compatibility Mode

1/23

Process & Process Management

Prachi Pandey

[email protected]

Hybrid Computing Group (SSDH)CDAC, Bangalore
mailto:[email protected]:[email protected]


2/23

Outline

Process concept

Process address space

PCB Process Creation and Management

Fork, exec and other system calls

Process life cycle


3/23

Process

The process is the OSs abstraction for

execution

A Program in ExecutionAn active instance of a program

Process is often called a job, or task


4/23

Program and Process

Program

Hello.c Compiler a.out

Resides in secondary memory

Process

is created by OS to execute a program

OS puts it in the main memory

Creates a data structure


5/23

Process Address Space


6/23

Process Control Block (PCB)

Information associated with each process

Process Id

Process state

Program counter

CPU registers

Priority

Memory-management info List of open files

Protection


7/23

Process Management

Complete activity required for managing a

process throughout its lifecycle

Allocate resources to processesEnable processes to share and exchange

information

Protect the resources of each process from other

processesEnable synchronization among processes.

Maintain a data structure for each process

Execution and Control of processes


8/23

Process Management System Calls

Fork()

Execv()

Wait() Getpid()


9/23

How To Create New Processes?

Underlying mechanism

A process runs fork to create a

child process

Child process is a duplicate of theparent process

Both parent and child execute next

line

Parent and children executeconcurrently

parent

child

fork()


10/23

Fork

Child and parent are identical

child returns a 0

parent returns nonzero (child PID) PID and PPID differs

Synopsis

pid_t fork(void);


11/23

Fork()

Splits into . Process The child process

inherits from parent

Identical copy of

memoryCPU registers

all files that have beenopened by the parent

PCB of child is copy ofparents context attime of call

Text

Data

Stack

Text

Data

Stack

ret = xxx

ret = 0

fork()


12/23

Example: Process Creation in Unix

int pid;

pid = fork()if (pid > 0){

/* parent */..

}else if (pid == 0)

{/* child */..exit(status);

}

The fork syscall returns twice: itreturns a zero to the child and thechild process ID (pid) to the

parent.


13/23

Fork Example 1: What Output?

int main()

{

pid_t pid;

int x = 1;

pid = fork();

if (pid != 0) {

printf(parent: x = %d\n, --x);

exit(0);

} else {printf(child: x = %d\n, ++x);

exit(0);

}

}


14/23

Fork Example 2

Both parent and child can continue forking

void fork2(){

printf("L0\n");

fork();

printf("L1\n");

fork();

printf("Bye\n");

}

L0 L1

L1

Bye

Bye

Bye

Bye


15/23

exit()

Closes all open files, connections and

deallocates memory

Checks if parent is alive If parent is alive, holds the result value until the

parent requests it (with wait); in this case, the

child process does not really die, but it enters a

zombie/defunct state


16/23

Waiting for the Child to Finish

Waiting for some child to terminate: wait()

Blocks until some child terminates

Returns the process ID of the child process

Or returns -1 if no children exist (i.e., alreadyexited)

Waiting for a specific child to terminate:waitpid()

Blocks till a child with particular process IDterminates

#include

#include

pid_t wait(int *status);

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


17/23

Unixs execv

The system call execv executes a file, transforming thecalling process into a new process.

After a successful execv, there is no return to the calling

process.

execv(const char * path, char * const argv[])

- path is the full path for the file to be executed

- argv is the array of arguments for the program to

execute

- each argument is a null-terminated string

- the first argument is the name of the program (argv[0])

- the last entry in argv is NULL


18/23

Example: A Simple Shell

Shell is the parentprocess

E.g., bash

Creates a new process

fork

Child executes a newprocess (ls)

execv

Parent waits for Childprocess

Wait

fork

ls

waitexecvp

bash


19/23

execv Example#include

#include

char * argv[] = {/bin/ls, -l, 0};

int main()

{

int pid, status;

if ( (pid = fork() ) < 0 ){

printf(Fork error \n);

exit(1);

}

if(pid == 0) { /* Child executes here */

execv(argv[0], argv);

printf(Exec error \n);exit(1);

} else /* Parent executes here */

wait(&status);

printf("Hello there! \n);

return 0;

}

Note the NULL string

at the end


20/23

Variations of execv()

execvProgram arguments passed as an array of strings

execvp

Searches for the program name in the PATHenvironment

execl

Program arguments passed directly as a list

execlpSearches for the program name in the PATH

environment


21/23

Other useful system calls: getpid, getppid

getpid returns the identifier of the calling

process. Example call (pid is an integer):

pid = getpid();

getppid returns the identifier of the parent.

ppid = getppid();


22/23

Process State

As a process executes, it changes state

new: The process is being created

ready: The process is waiting to be assigned to aprocessor

running: Instructions are being executed

waiting: The process is waiting for some event to

occurterminated: The process has finished execution


23/23

Diagram of Process State

Process Management Compatibility Mode

Documents