Operating Systems Yasir Kiani. 13-Sep-20062 Agenda for Today Review of previous lecture Interprocess communication (IPC) and process synchronization UNIX/Linux

Operating Operating SystemsSystems

Yasir KianiYasir Kiani

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Agenda for Today Review of previous lecture Interprocess communication (IPC)

and process synchronization UNIX/Linux IPC tools (pipe, named

pipe—FIFO, socket, message queue, shared memory)

Use of pipe Recap of the lecture

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Review of Lecture 7

The wait and exec system calls and sample code

Cooperating processes Producer-consumer problem

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Interprocess Interprocess Communication (IPC)Communication (IPC)

Mechanism for processes to Mechanism for processes to communicate and to synchronize communicate and to synchronize their actions.their actions.

Message system – processes Message system – processes communicate with each other communicate with each other without resorting to shared without resorting to shared variables.variables.

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IPC facility provides two operations:IPC facility provides two operations:Send Send ((messagemessage) – message size ) – message size

fixed or variable fixed or variable Receive Receive ((messagemessage))


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If If PP and and QQ wish to communicate, wish to communicate, they need to:they need to:establish a establish a communicationcommunication linklink

between thembetween themexchange messages via exchange messages via

send/receivesend/receive


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Implementation of communication Implementation of communication linklinkphysicalphysical (e.g., shared memory, (e.g., shared memory,

hardware bus)hardware bus) logicallogical (e.g., logical properties) (e.g., logical properties)


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Implementation Implementation QuestionsQuestions

How are links established?How are links established? Can a link be associated with more Can a link be associated with more

than two processes?than two processes? How many links can there be How many links can there be

between every pair of between every pair of communicating processes?communicating processes?

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Implementation Implementation QuestionsQuestions

What is the capacity of a link?What is the capacity of a link? Is the size of a message that the Is the size of a message that the

link can accommodate fixed or link can accommodate fixed or variable?variable?

Is a link unidirectional or bi-Is a link unidirectional or bi-directional?directional?

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Direct CommunicationDirect Communication

Processes must name each other Processes must name each other explicitly:explicitly:sendsend ( (P, messageP, message) – send a ) – send a

message to process Pmessage to process PReceive Receive ((Q, messageQ, message) – receive a ) – receive a

message from process Qmessage from process Q

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Direct CommunicationDirect Communication

Properties of communication linkProperties of communication link Links are established automatically.Links are established automatically. A link is associated with exactly one A link is associated with exactly one

pair of communicating processes.pair of communicating processes. Between each pair there exists Between each pair there exists

exactly one link.exactly one link. The link may be unidirectional, but is The link may be unidirectional, but is

usually bi-directional.usually bi-directional.

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Indirect CommunicationIndirect Communication

Messages are directed and Messages are directed and received from mailboxes (also received from mailboxes (also referred to as ports).referred to as ports). Each mailbox has a unique id.Each mailbox has a unique id. Processes can communicate only if Processes can communicate only if

they share a mailbox.they share a mailbox.

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Indirect Communication …Indirect Communication … Properties of communication linkProperties of communication link

Link established only if processes Link established only if processes share a common mailboxshare a common mailbox

A link may be associated with many A link may be associated with many processes.processes.

Each pair of processes may share Each pair of processes may share several communication links.several communication links.

Link may be unidirectional Link may be unidirectional or bi-directional.or bi-directional.

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OperationsOperations create a new mailboxcreate a new mailbox send and receive messages through send and receive messages through

mailboxmailbox destroy a mailboxdestroy a mailbox

Primitives are defined as:Primitives are defined as:send send ((A, messageA, message) )

receive receive ((A, messageA, message))

Indirect Communication …Indirect Communication …

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Mailbox sharingMailbox sharingPP11, P, P22,, and and P P33 share mailbox A. share mailbox A.

PP11, sends; , sends; PP22 andand P P33 receive. receive.

Who gets the message?Who gets the message?


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SolutionsSolutionsAllow a link to be associated with Allow a link to be associated with

at most two processes.at most two processes.Allow only one process at a time Allow only one process at a time

to execute a receive operation.to execute a receive operation.Allow the system to select Allow the system to select

arbitrarily the receiver. Sender is arbitrarily the receiver. Sender is notified who the receiver was.notified who the receiver was.


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SynchronizationSynchronization Message passing may be either Message passing may be either

blocking or non-blocking.blocking or non-blocking. BlockingBlocking is considered is considered

synchronoussynchronous Non-blockingNon-blocking is considered is considered

asynchronousasynchronous sendsend and and receivereceive primitives may primitives may

be either blocking or non-blocking.be either blocking or non-blocking.

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BufferingBuffering Queue of messages attached to the Queue of messages attached to the

link; implemented in one of three ways.link; implemented in one of three ways.

Zero capacityZero capacity – No messages – No messages Sender Sender must wait for receivermust wait for receiver

Bounded capacityBounded capacity – – nn messages messagesSender must wait if link full.Sender must wait if link full.

Unbounded capacityUnbounded capacity – infinite length – infinite length Sender never waits.Sender never waits.

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UNIX/Linux IPC ToolsUNIX/Linux IPC Tools PipePipe Named pipe (FIFO)Named pipe (FIFO) BSD SocketBSD Socket TLITLI Message queueMessage queue Shared memoryShared memory Etc.Etc.

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UNIX/Linux Pipe Important system calls

pipe, read, write, close pipe: Create a pipe for IPC read: Read from a pipe write: Write data to a pipe close: Close/destroy a pipe

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File Descriptors in UNIX/Linux

An integer returned by open() system call

Used as an index in the per process file descriptor table (PPFDT)

Used in read, write, and close calls

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File Descriptors in UNIX/Linux

Size of PPFDT is equal to the number of files that a process can open simultaneously (OPEN_MAX in Linux—see <linux/limits.h>

Used as an index in the per process file descriptor table (PPFDT)

Used in read, write, and close calls

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UNIX/Linux Pipe Important characteristics of a pipe

Stream of bytes Used as half-duplex channel Bounded buffer Maximum data written is PIPE_BUF

(defined in <sys/param.h> under UNIX and in <linux/param.h> in Linux)—5120 and 4096, respectively

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#include <unistd.h> int pipe (int filedes[2]);

Synopsis of pipe System Call

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Example

P Pfork

parent child

Write end

Read end

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/* Parent creates pipe, forks a child, child writes into

pipe, and parent reads from pipe */

#include <stdio.h>

#include <sys/types.h>

#include <sys/wait.h>

main()

{

int pipefd[2], pid, n, rc, nr, status;

char *testString = "Hello, world!\n“, buf[1024];

Sample Code

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rc = pipe (pipefd);

if (rc < 0) {

perror("pipe");

exit(1);

}

pid = fork ();

if (pid < 0) {

perror("fork");

exit(1);

}

Sample Code

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if (pid == 0) { /* Child’s Code */

close(pipefd[0]);

write(pipefd[1], testString, strlen(testString));

close(pipefd[1]);

exit(0);

}

Sample Code

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/* Parent’s Code */

close(pipefd[1]);

n = strlen(testString);

nr = read(pipefd[0], buf, n);

rc = write(1, buf, nr);

wait(&status);

printf("Good work child!\n");

return(0);

Sample Code

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Recap of Lecture Review of previous lecture Interprocess communication

(IPC) and process synchronization

UNIX/Linux IPC tools (pipe, FIFO, socket, message queue, shared memory, etc.)

Use of UNIX pipe Recap of the lecture

Documents

Operating Systems Yasir Kiani. 13-Sep-20062 Agenda for Today Review of previous lecture Interprocess communication (IPC) and process synchronization UNIX/Linux