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Lecture 4

InterprocessCommunication

31 August, 2010

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why IPC?

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message passing

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shared memory

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UNIX IPC

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signal

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#include

:

//pidistheprocessIDtosendthe

//signalto//sigistheIDofthesignal.

kill(pid,sig);

kill(pid,SIGTERM);

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at the receiver, default

actions are defined for

each signals

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but reaction to most

signals can be

customized

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Cfunction pointers

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//functiontakingina

//char*andreturninganint.

intfoo(char*name){...}

//declareafunctionpointer

//initializeafunctionpointer

//callthefunction

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return

type

var

name

arg

type(* ) ( )

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function pointers can be:

(i) passed into function,(ii) returned

(iii) defined as new type

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#include

void*wont_die(intsig){return;}:

void(*prev_handler)(int);

prev_handler=signal(SIGKILL,wont_die);

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#include

:

intfiles[2];

pipe(files);

//noweverythingwewriteintofile[1]

//canbereadfromfile[0].

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Flickr Photo Some rights reserved by Darwin Bell

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http://www.flickr.com/photos/darwinbell/http://www.flickr.com/photos/darwinbell/http://creativecommons.org/licenses/by/2.0/http://creativecommons.org/licenses/by/2.0/

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race conditions

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demo

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mutual exclusion

only one process can

access a sharedresource at a time

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critical region

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processes typically alternate

between critical and non-critical region.

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while (1) enter( )

critical_region leave( )

noncritical_region

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how to implemententer( ) and leave( )

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lock variable

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enter( )while (lock);

lock = 1;

leave( )lock = 0;

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while (lock);

lock = 1;

while (lock);

lock = 1;

process A process B

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interrupts

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enter( )disable interrupt

leave( )

enable interrupt

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PetersonsAlgorithm

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enter( )interested[A] = 1

turn =B

while (turn ==B&& interested[B]);

leave( )interested[A] = 0

process A

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enter( )interested[B] = 1

turn =A

while (turn ==A&& interested[A]);

leave( )interested[B] = 0

process B

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i [A] = 1

t = B

while (t is B && i [B]);

process A process B

i [B] = 1t = A

while (t is A && i [A]);

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i [A] = 1

t = B

while (t is B

&& i [B]);

process A process B

i [B] = 1

t = Awhile (t is A && i [A]);

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atomic instructions

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TSL R, lock

in one atomic step,

copy lock to R &set lock to 1

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enter( )TSLR,lock

CMPR,#0

JNEenter

RET

leave( )MOVlock,#0

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test&set(lock)

return lock and

set lock to 1

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enter( )while (test&set(lock));

leave( )

lock = 0;

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XCHG R, lock

in one atomic step, swap

values of two locations

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enter( )

MOVR,#1XCHGR,lock

CMPR,#0

JNEenter

RET

leave( )MOVlock,#0

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while (1) enter( )

critical_region leave( )

noncritical_region

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producer -

consumer

problem

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producer consumer

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while (1)

if (buffer is full) sleep

produce if (buffer was empty)

wake up consumer

producer

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while (1)

if (buffer is empty) sleep

consume if (buffer was full)

wake up producer

consumer

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while (1)

if (buffer is empty)

sleepconsume

if (buffer was full)

wakeup producer

while (1)

if (buffer is full)

sleep

produce if (buffer was empty)

wakeup consumer

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