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Exceptions and Processes

Slides adapted from:Gregory Kesden and Markus Pschel of Carnegie Mellon University

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Control Flow

inst1

inst2inst3

instn

Processors do only one thing:

From startup to shutdown, a CPU simply reads and executes

(interprets) a sequence of instructions, one at a time

This sequence is the CPUs control flow(orflow of control)

Physical control flow

Time

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Altering the Control Flow

Up to now: two mechanisms for changing control flow: Jumps and branches

Call and return

Both react to changes inprogram state

Insufficient for a useful system:

Difficult to react to changes in system state

data arrives from a disk or a network adapter

instruction divides by zero

user hits Ctrl-C at the keyboard

System timer expires

System needs mechanisms for exceptional control flow

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Exceptional Control Flow

Exists at all levels of a computer system Low level mechanisms

Exceptions

change in control flow in response to a system event

(i.e., change in system state)

Combination of hardware and OS software

Higher level mechanisms

Process context switch

Signals

Nonlocal jumps: setjmp()/longjmp()

Implemented by either:

OS software (context switch and signals)

C language runtime library (nonlocal jumps)

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Exceptions

An exceptionis a transfer of control to the OS in response tosome event (i.e., change in processor state)

Examples:

div by 0, arithmetic overflow, page fault, I/O request completes, Ctrl-C

User Process OS

exception

exception processing

by exception handler

return to I_current

return to I_next

abort

event I_currentI_next

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01

2...

n-1

Interrupt Vectors

Each type of event has a

unique exception number k

k = index into exception table

(a.k.a. interrupt vector)

Handler k is called each time

exception k occurs

ExceptionTable

code for

exception handler 0

code forexception handler 1

code for

exception handler 2

code for

exception handler n-1

...

Exception

numbers

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Asynchronous Exceptions (Interrupts)

Caused by events external to the processor Indicated by setting the processors interrupt pin

Handler returns to next instruction

Examples: I/O interrupts

hitting Ctrl-C at the keyboard

arrival of a packet from a network

arrival of data from a disk

Hard reset interrupt

hitting the reset button

Soft reset interrupt

hitting Ctrl-Alt-Delete on a PC

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Synchronous Exceptions

Caused by events that occur as a result of executing an

instruction:

Traps

Intentional

Examples: system calls, breakpoint traps, special instructions

Returns control to next instruction

Faults

Unintentional but possibly recoverable

Examples: page faults (recoverable), protection faults

(unrecoverable), floating point exceptions Either re-executes faulting (current) instruction or aborts

Aborts

unintentional and unrecoverable

Examples: parity error, machine check

Aborts current program

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Trap Example: Opening File

User calls: open(filename, options)

Function openexecutes system call instruction int

OS must find or create file, get it ready for reading or writing

Returns integer file descriptor

0804d070 :. . .804d082: cd 80 int $0x80804d084: 5b pop %ebx

. . .

User Process OS

exception

open file

returns

intpop

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Fault Example: Page Fault

User writes to memory location That portion (page) of users memory

is currently on disk

Page handler must load page into physical memory

Returns to faulting instruction

Successful on second try

int a[1000];main ()

{a[500] = 13;

}

80483b7: c7 05 10 9d 04 08 0d movl $0xd,0x8049d10

User Process OS

exception: page fault

Create page and

load into memory

returns

movl

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Fault Example: Invalid Memory Reference

Page handler detects invalid address

Sends SIGSEGVsignal to user process

User process exits with segmentation fault

int a[1000];

main (){

a[5000] = 13;}

80483b7: c7 05 60 e3 04 08 0d movl $0xd,0x804e360

User Process OS

exception: page fault

detect invalid address

movl

signal process

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User mode vs. Kernel mode

Priviliged instructions

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Exception handlers

Return adress Depends on the class of exception

Current instruction (page fault)

Next instruction

Push some additional processor state Necessary to restart the interrupted program when the handler

returns

If the control is being transferred from user to kernel

All these items are pushed on the kernel stack

Exception handlers run in kernel mode

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Exception Table IA32 (Excerpt)

Exception Number Description Exception Class

0 Divide error Fault

13 General protection fault Fault

14 Page fault Fault

18 Machine check Abort

32-127 OS-defined Interrupt or trap

128 (0x80) System call Trap

129-255 OS-defined Interrupt or trap

Check pp. 183:

http://download.intel.com/design/processor/manuals/253665.pdf

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http://download.intel.com/design/processor/manuals/253665.pdfhttp://download.intel.com/design/processor/manuals/253665.pdfhttp://download.intel.com/design/processor/manuals/253665.pdfhttp://download.intel.com/design/processor/manuals/253665.pdf

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# write our string to stdout

movl $len,%edx # third argument: message lengthmovl $msg,%ecx # second argument: message to writemovl $1,%ebx # first argument: file handle (stdout)

movl $4,%eax # system call number (sys_write)int $0x80 # call kernel

# and exit

movl $0,%ebx # first argument: exit codemovl $1,%eax # system call number (sys_exit)int $0x80 # call kernel

.data # section declaration

msg:.ascii "Hello, world!\n" # our dear stringlen = . - msg # length of our dear string

int main(){

write(1, hello, world\n, 13);exit(0);

}

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Today

Exceptional Control Flow Processes

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Processes

Definition: Aprocessis 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 key abstractions:

Logical control flow

Each program seems to have exclusive use of the CPU

Private virtual address space

Each program seems to have exclusive use of main memory

How are these Illusions maintained?

Process executions interleaved (multitasking)

Address spaces managed by virtual memory system

well talk about this in a couple of weeks

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What is a process?

A process is the OS's abstraction for execution A process represents a single running application on the system

Process has three main components:

1) Address space

The memory that the process can access Consists of various pieces: the program code, static variables,

heap, stack, etc.

2) Processor state

The CPU registers associated with the running process

Includes general purpose registers, program counter, stack pointer,

etc.

3) OS resources

Various OS state associated with the process

Examples: open files, network sockets, etc.

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Process Address Space

The range ofvirtual memoryaddresses that theprocess can access

Includes the codeof the running

program The data of the

running program(static variablesand heap)

An execution stack

Local variablesand savedregisters foreachprocedure call

Stack

Heap

Initialized vars(data segment)

Code(text segment)

Address space

0x00000000

0xFFFFFFFF

Stack pointer

Program counte

Uninitialized vars(BSS segment)

(Reserved for OS)

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Process Address Space

Note!!! This is theprocess's own viewof the address space---

physicalmemory may not belaid out this way at

all. In fact, on systems

that supportmultiple runningprocesses, it's prettymuchguaranteed to lookdifferent.

The virtual memorysystem provides thisillusion to eachprocess.

Stack

Heap

Initialized vars(data segment)Code

(text segment)

Address space

0x00000000

0xFFFFFFFF

Stack pointer

Program counter


(Reserved for OS)

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Execution State of a Process

Each process has an execution state Indicates what the process is currently doing

Running:

Process is currently using the CPU

Ready:

Currently waiting to be assigned to a CPU That is, the process could be running, but another process is using the

CPU

Waiting (or sleeping):

Process is waiting for an event

Such as completion of an I/O, a timer to go off, etc. Why is this different than ready ?

As the process executes, it moves between these states

What state is the process in most of the time?

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Process State Transitions

What causes schedule and unschedule transitions?

New

Terminated

Ready

Running

Waiting

create

kill orexit I/O, page fault,

etc.

I/O done

scheduleunschedule

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g

Process Control Block

OS maintains a Process Control Block (PCB) for each process The PCB is a big data structure with many fields:

Process ID

User ID

Execution state

ready, running, or waiting Saved CPU state

CPU registers saved the last time the process was suspended.

OS resources

Open files, network sockets, etc.

Memory management info Scheduling priority

Give some processes higher priority than others

Accounting information

Total CPU time, memory usage, etc.

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g

Context Switching

Processes are managed by a shared chunk of OS code

called the kernel Important: the kernel is not a separate process, but rather runs as part

of some user process

Control flow passes from one process to another via a

context switch

Process A Process B

user code

kernel code

user code

kernel code

user code

context switch

context switch

Time

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g

Context Switching The act of swapping a process state on or off the CPU is a context

switch

PC

Registers

PC

Registers

PID 1342State: Running

PC

Registers

PID 4277State: Ready

PC

Registers


Save current CPU state

Currently running process

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g

Context Switching

The act of swapping a process state on or off the CPU is a contextswitch

PC

Registers

PC

Registers


PC

Registers


PC

Registers


Suspend process

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Context Switching

The act of swapping a process state on or off the CPU is a contextswitch

PC

Registers

PC

Registers


PC

Registers

PID 4277State: Running

PC

Registers


Restore CPU state of new process

Pick next process

PC

Registers

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Context Switch Overhead

Context switches are not cheap Generally have a lot of CPU state to save and restore

Also must update various flags in the PCB

Picking the next process to runschedulingis also expensive

Context switch overhead in Linux 2.4.21 About 5.4 usec on a 2.4 GHz Pentium 4

This is equivalent to about 13,200 CPU cycles!

Not quite that many instructions since CPI > 1

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Context Switching in Linux

Process A

time

Process A is happ i ly ru nn ing a long. ..

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Process A

time

Timer interrupthandler

1) Timer interrupt f ires

2) PC saved on stac kUser

Kernel

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Process A


time


2) PC saved on stack

Scheduler

4) Call sch edule() rou t ine

3) Rest o f CPU state

saved in PCB

User

Kernel

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Process A


time


2) PC saved on stack

Scheduler5) Decide n extp rocess to run

4) Call sch edule() rou t ine

3) Rest o f CPU state

saved in PCBTimer interrupt

handler

6) Resume Proc ess B(suspended wi th in

t imer in terrupt hand ler !)

User

Kernel

Process B

7) Return from in terrupt

hand lerproc ess CPU

state restored

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State Queues

The OS maintains a set of state queues for each process state Separate queues for ready and waiting states

Generally separate queues for each kind of waiting process

e.g., One queue for processes waiting for disk I/O

Another queue for processes waiting for network I/O, etc.

PC

Registers


PC

Registers

PID 4110State: Waiting

PC

Registers


PC

Registers


PC

Registers


Ready queue

Disk I/O queue

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State Queue Transitions

PCBs move between these queues as their state changes When scheduling a process, pop the head off of the ready queue

When I/O has completed, move PCB from waiting queue to ready

queue

PC

Registers

PID 4277

State: Ready

PC

Registers

PID 4110

State: Waiting

PC

Registers

PID 4002

State: Waiting

PC

Registers

PID 4391

State: Ready

PC

Registers

PID 4923

State: Waiting

Ready queue

Disk I/O queue

PC

Registers

PID 4923

State: Ready

Disk I/O completes

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Process Creation

One process can create, or fork, another process The original process is the parent

The new process is the child

What creates the first process in the system, and when?

Parent process defines resources and access rights ofchildren

Just like real life ...

e.g., child process inherits parent's user ID

% pstree -p

init(1)-+-apmd(687)|-atd(847)|-crond(793)|-rxvt(2700)---bash(2702)---ooffice(2853)`-rxvt(2752)---bash(2754)

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UNIX fork mechanism

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PC

Registers

PID 4110

State: Ready

UNIX fork mechanism In UNIX, use the fork() system call to create a new process

This creates an exact duplicateof the parent process!!

Creates and initializes a new PCB

Creates a new address space Copies entire contents of parent's address space into the child

Initializes CPU and OS resources to a copy of the parent's

Places new PCB on ready queue

PC

Registers

PID 4109

State: Running

PC

Registers


PC

Registers


Ready queue

Process calls fork()PC

Registers

PID 4110

State: ReadyCopy state

PC

Registers

PID 4110

State: Ready

Add to end ofready queue

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fork: Creating New Processes

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

Fork is interesting (and often confusing) because

it is called oncebut returns twice

pid_t pid = fork();if (pid == 0) {printf("hello from child\n");

} else {printf("hello from parent\n");

}

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Understanding fork



}

Process n



}

Child Process m



}

pid = m



}

pid = 0



}



}

hello from parent hello from childWhich one is first?

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Fork Example #1

void fork1(){

int x = 1;pid_t pid = fork();if (pid == 0) {

printf("Child has x = %d\n", ++x);} else {

printf("Parent has x = %d\n", --x);}printf("Bye from process %d with x = %d\n", getpid(), x);

}

Parent and child both run same code

Distinguish parent from child by return value from fork

Start with same state, but each has private copy

Including shared output file descriptor

Relative ordering of their print statements undefined

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Fork Example #2

void fork2(){

printf("L0\n");fork();


printf("Bye\n");}

Both parent and child can continue forking

L0 L1

L1

Bye

Bye

Bye

Bye

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Fork Example #3


void fork3(){




printf("Bye\n");} L1 L2

L2

Bye

Bye

Bye

Bye

L1 L2

L2

Bye

Bye

Bye

Bye

L0

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Fork Example #4


void fork4(){

printf("L0\n");if (fork() != 0) {printf("L1\n");if (fork() != 0) {


}}

printf("Bye\n");}

L0 L1

Bye

L2

Bye

Bye

Bye

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Fork Example #4


void fork5(){

printf("L0\n");if (fork() == 0) {printf("L1\n");if (fork() == 0) {


}}

printf("Bye\n");}

L0 Bye

L1

Bye

Bye

Bye

L2

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Why have fork() at all?

Why make a copy of the parent process? Don't you usually want to start a new program instead?

Where might cloning the parent be useful?

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Why have fork() at all?

Why make a copy of the parent process? Don't you usually want to start a new program instead?

Where might cloning the parent be useful?

Web servermake a copy for each incoming connection

Parallel processingset up initial state, fork off multiple copies todo work

UNIX philosophy: System calls should be minimal.

Don't overload system calls with extra functionality if it is not

always needed.

Better to provide a flexible set of simple primitives and let

programmers

combine them in useful ways.

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Memory concerns

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Memory concerns So fork makes a copy of a process. What about memory usage?

Stack

Heap


Code(text segment)


(Reserved for OS)Parent

Stack

Heap


Code(text segment)


(Reserved for OS)Child #1

Stack

Heap


Code(text segment)

Uninitialized vars

(BSS segment)


Stack

Heap


Code(text segment)


(Reserved for OS)

Child #3

Stack

Heap


Code(text segment)



Stack

Heap


Code(text segment)

Uninitialized vars

(BSS segment)

(Reserved for OS)

Child #5

Stack

Heap


Code(text segment)



Stack

Heap


Code(text segment)


(Reserved for OS)

Child #7

Stack

Heap


Code(text segment)



Stack

Heap


Code(text segment)

Uninitialized vars

(BSS segment)


Stack

Heap


Code(text segment)



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Problematic?

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Problematic?

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Memory concerns

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Memory concerns OS aggressively tries to share memory between processes.

Especially processes that are fork()'d copies of each other

Copies of a parent process do not actually get a private copyof the address space...

... Though that is the illusion that each process gets.

Instead, they share the same physical memory, until one of them makesa change.

The virtual memory system is behind these shenanigans. We will discuss this in much detail later in the course

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exit: Ending a process

void exit(int status)

exits a process

Normally return with status 0

atexit()registers functions to be executed upon exit

void cleanup(void) {printf("cleaning up\n");

}

void fork6() {atexit(cleanup);

fork();exit(0);

}

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Zombies

Idea

When process terminates, still consumes system resources Various tables maintained by OS

Called a zombie

Living corpse, half alive and half dead

Reaping Performed by parent on terminated child

Parent is given exit status information

Kernel discards process

What if parent doesnt reap? If any parent terminates without reaping a child, then child will be

reaped by initprocess

So, only need explicit reaping in long-running processes

e.g., shells and servers

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linux> ./forks 7 &[1] 6639Running Parent, PID = 6639

Terminating Child, PID = 6640linux>psPID TTY TIME CMD6585 ttyp9 00:00:00 tcsh6639 ttyp9 00:00:03 forks6640 ttyp9 00:00:00 forks

6641 ttyp9 00:00:00 pslinux>kill 6639[1] Terminatedlinux>psPID TTY TIME CMD6585 ttyp9 00:00:00 tcsh6642 ttyp9 00:00:00 ps

Zombie

Example

psshows child process asdefunct

Killing parent allows child to be

reaped by init

void fork7(){

if (fork() == 0) {/* Child */printf("Terminating Child, PID = %d\n",

getpid());exit(0);

} else {printf("Running Parent, PID = %d\n",

getpid());while (1)

; /* Infinite loop */

}}

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linux> ./forks 8Terminating Parent, PID = 6675Running Child, PID = 6676linux>ps

PID TTY TIME CMD6585 ttyp9 00:00:00 tcsh

6676 ttyp9 00:00:06 forks6677 ttyp9 00:00:00 pslinux>kill 6676linux>ps

PID TTY TIME CMD6585 ttyp9 00:00:00 tcsh

6678 ttyp9 00:00:00 ps

Nonterminating

Child Example

Child process still active even

though parent has terminated

Must kill explicitly, or else will keeprunning indefinitely

void fork8(){

if (fork() == 0) {/* Child */printf("Running Child, PID = %d\n",

getpid());while (1)

; /* Infinite loop */} else {

printf("Terminating Parent, PID = %d\n",getpid());

exit(0);}

}

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wait: Synchronizing with Children

int wait(int *child_status)

suspends current process until one of its children terminates

return value is thepidof the child process that terminated

if child_status!= NULL, then the object it points to will be setto a status indicating why the child process terminated

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wait: Synchronizing with Children

void fork9() {int child_status;

if (fork() == 0) {printf("HC: hello from child\n");

}else {

printf("HP: hello from parent\n");wait(&child_status);printf("CT: child has terminated\n");

}printf("Bye\n");exit();

}

HP

HC Bye

CT Bye

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wait()Example If multiple children completed, will take in arbitrary order

Can use macros WIFEXITED and WEXITSTATUS to get information aboutexit status

void fork10(){

pid_t pid[N];int i;int child_status;for (i = 0; i < N; i++)

if ((pid[i] = fork()) == 0)exit(100+i); /* Child */

for (i = 0; i < N; i++) {pid_t wpid = wait(&child_status);

if (WIFEXITED(child_status))printf("Child %d terminated with exit status %d\n",

wpid, WEXITSTATUS(child_status));else

printf("Child %d terminate abnormally\n", wpid);}

}

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waitpid(): Waiting for a Specific Process

waitpid(pid, &status, options)

suspends current process until specific process terminates

various options (that we wont talk about)

void fork11(){

pid_t pid[N];

int i;int child_status;for (i = 0; i < N; i++)

if ((pid[i] = fork()) == 0)exit(100+i); /* Child */

for (i = 0; i < N; i++) {

pid_t wpid = waitpid(pid[i], &child_status, 0);if (WIFEXITED(child_status))printf("Child %d terminated with exit status %d\n",

wpid, WEXITSTATUS(child_status));else

printf("Child %d terminated abnormally\n", wpid);}

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fork() and execve()

How do we start a new program, instead of just a copy ofthe old program?

Use the UNIX execve() system call

int execve(const char *filename,

char *const argv [], char *const envp[]);

filename: name of executable file to run

argv: Command line arguments

envp: environment variable settings (e.g., $PATH, $HOME, etc.)

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fork() and execve()

execve() does not fork a new process! Rather, it replaces the address space and CPU state of the current

process

Loads the new address space from the executable file and starts it

from main()

So, to start a new program, use fork() followed by execve()

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execl andexecFamily

int execl(char *path, char *arg0, char *arg1, , 0)

Loads and runs executable atpathwith args arg0, arg1, pathis the complete path of an executable object file

By convention,arg0is the name of the executable object file Real arguments to the program start with arg1, etc.

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

Environment taken from char **environ, which points to an arrayof name=value strings:

USER=ganger

LOGNAME=ganger

HOME=/afs/cs.cmu.edu/user/ganger

Returns -1if error, otherwise doesnt return!

Family of functions includesexecv, execve(basefunction),execvp, execl, execle, and execlp

Carnegie Mellon

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exec:Loading and Running Programs

main() {if (fork() == 0) {execl("/usr/bin/cp", "cp", "foo", "bar", 0);

}wait(NULL);printf("copy completed\n");exit();

}

Carnegie Mellon

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Summary

Exceptions

Events that require nonstandard control flow

Generated externally (interrupts) or internally (traps and faults)

Processes

At any given time, system has multiple active processes

Only one can execute at a time, though

Each process appears to have total control of

processor + private memory space

Carnegie Mellon

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Summary (cont.)

Spawning processes

Call to fork

One call, two returns

Process completion

Call exit

One call, no return Reaping and waiting for Processes

Callwaitorwaitpid

Loading and running Programs

Call execl(or variant) One call, (normally) no return

Date post:	03-Jun-2018
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CENG334-2013-W02a-ECF

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