Inter-Process Communication Mechanisms

CSE331Operating Systems Design

Cooperating Processes• Independent process cannot affect or be affected by the execution of another

process.

• Cooperating process can affect or be affected by the execution of another process

• Advantages of process cooperation– Information sharing – Computation speed-up– Modularity– Convenience

• Dangers of process cooperation– Data corruption, deadlocks, increased complexity– Requires processes to synchronize their processing

Purposes for IPC

• Data Transfer• Sharing Data• Event notification• Resource Sharing and

Synchronization• Process Control

IPC Mechanisms

• Mechanisms used for communication and synchronization– Message Passing

• message passing interfaces, mailboxes and message queues• sockets, STREAMS, pipes

– Shared Memory: Non-message passing systems

• Common examples of IPC– Synchronization using primitives such as semaphores to higher level mechanisms such

as monitors. Implemented using either shared memory or message passing.– Debugging– Event Notification - UNIX signals

Message Passing• In a Message passing system there are no shared

variables. IPC facility provides two operations for fixed or variable sized message:– send(message)– receive(message)

• If processes P and Q wish to communicate, they need to:– establish a communication link– exchange messages via send and receive

• Implementation of communication link– physical (e.g., shared memory, hardware bus)– logical (e.g., syntax and semantics, abstractions)

Implementation Questions• How are links established?

• Can a link be associated with more than two processes?

• How are links made known to processes?

• How many links can there be between every pair/group of communicating processes?

• What is the capacity of a link?

• Is the size of a message that the link can accommodate fixed or variable?

• Is a link unidirectional or bi-directional?

Message Passing Systems•Exchange messages over a communication link

•Methods for implementing the communication link and primitives (send/receive):

1.Direct or Indirect communications (Naming)2.Symmetric or Asymmetric communications3.Automatic or Explicit buffering4.Send-by-copy or send-by-reference5.fixed or variable sized messages

Direct Communication – Internet and Sockets• Processes must name each other explicitly:– Symmetric Addressing

• send (P, message) – send to process P• receive(Q, message) – receive from Q

– Asymmetric Addressing• send (P, message) – send to process P• receive(id, message) – rx from any; system sets id = sender

• Primitives:– send(A, message) – send a message to mailbox A– receive(A, message) – receive a message from mailbox A

• Properties of communication link– Links established automatically between pairs– processes must know each others ID– Exactly one link per pair of communicating processes

• Disadvantage: a process must know the name or ID of the process(es) it wishes to communicate with

Indirect Communication - Pipes

• Messages are sent to or received from mailboxes (also referred to as ports).– Each mailbox has a unique id.– Processes can communicate only if they share a mailbox.

• Properties of communication link– Link established only if processes share a common

mailbox– A link may be associated with more than 2 processes.– Each pair of processes may share several communication

links.• Ownership:– process owns (i.e. mailbox is implemented in user space):

only the owner may receive messages through this mailbox. Other processes may only send. When process terminates any “owned” mailboxes are destroyed.

– system owns – then mechanisms provided to create, delete, send and receive through mailboxes. Process that creates mailbox owns it (and so may receive through it) but may transfer ownership to another process.

Indirect Communication

• Mailbox sharing:– P1, P2, and P3 share mailbox A.– P1, sends; P2 and P3 receive.– Who gets the message?

• Solutions– Allow a link to be associated with at most two processes.– Allow only one process at a time to execute a receive operation.– Allow the system to select arbitrarily the receiver. Sender is

notified who the receiver was

Synchronizing Message Flow

• Message passing may be either blocking or non-blocking.– blocking send: sender blocked until message received by

mailbox or process– nonblocking send: sender resumes operation immediately

after sending– blocking receive: receiver blocks until a message is

available– nonblocking receive: receiver returns immediately with

either a valid or null message.

Conventional View

pro

cess

1

pro

cess

2

pro

cess

n

kernel

user

Protection domains - (virtual address space)

How can processes communicate with each other and the kernel?

Universal IPC Facilities

pro

cess

1

pro

cess

2

kerneluser

pipe

handler

handle event

stop

db

x

• Universal Facilities in UNIX– Signals - asynchronous or synchronous event notification.– Pipes - unidirectional, FIFO, unstructured data stream.– Process tracing - used by debuggers to control target process

UNIX Pipes

• Pipe sets up communication channel between two (related) processes.

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Two processes connected by a pipe

• One process writes to the pipe, the other reads from the pipe. • Looks exactly the same as reading from/to a file. • System call:

int fd[2] ;pipe(&fd[0]) ;

fd[0] now holds descriptor to read from pipefd[1] now holds descriptor to write into pipe

Simple Example

#include <unistd.h> #include <fcntl.h>#include <stdio.h>char *message = "This is a message!!!" ;main(){ char buf[1024] ; int fd[2]; pipe(fd); /*create pipe*/ if (fork() != 0) { /* I am the parent */ write(fd[1], message, strlen (message) + 1) ; } else { /*Child code */ read(fd[0], buf, 1024) ; printf("Got this from MaMa!!: %s\n", buf) ; }}

Create a Pipeline

• Sometimes useful to connect a set of processes in a pipeline.

Process

c

Process

DPipePipe

Process A writes to pipe AB, Process B reads from AB and writes to BC

Process C reads from BC and writes to CD …..

Shared MemoryCommon chunk of read/write memory

among processes

Proc. 1 Proc. 2

ptrAttach

Proc. 3 Proc. 4 Proc. 5

ptr ptr ptr

ptrAttach

Create

Shared Memory(unique key)

0

MAX

Creating Shared Memory

int shmget(key_t key, size_t size, int shmflg);

Example:key_t key;

int shmid;

key = ftok(“<somefile>", ‘A');

shmid = shmget(key, 1024, 0644 | IPC_CREAT);

Here’s an example: shm_create.c.

Attach and DetachShared Memory

void *shmat(int shmid, void *shmaddr, int shmflg);int shmdt(void *shmaddr);

Example:key_t key; int shmid; char *data;

key = ftok("<somefile>", ‘A'); shmid = shmget(key, 1024, 0644); data = shmat(shmid, (void *)0, 0);

shmdt(data);

Here’s an shm_attach.c

Deleting Shared Memory

int shmctl(int shmid, int cmd, struct shmid_ds *buf);

shmctl(shmid, IPC_RMID, NULL);

Example: Shm_delete.c

Command-line IPC control

• ipcs – Lists all IPC objects owned by the user

• ipcrm– Removes specific IPC object

Shared Semaphores

• Managing concurrent access to shared memory segment.

• Using Shared Semaphores

– Creation: semget( … )

– Incr/Decr/Test-and-set : semop(…)

– Deletion: semctl(semid, 0, IPC_RMID, 0);