Inter-Process Communication Mechanisms

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Inter-Process Communication Mechanisms

• CSE331
• Operating Systems Design

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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

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Purposes for IPC

• Data Transfer
• Sharing Data
• Event notification
• Resource Sharing and Synchronization
• Process Control

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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

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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)

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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?

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Message Passing Systems

• Exchange messages over a communication link
• Methods for implementing the communication link
  and primitives (send/receive)
• Direct or Indirect communications (Naming)
• Symmetric or Asymmetric communications
• Automatic or Explicit buffering
• Send-by-copy or send-by-reference
• fixed or variable sized messages

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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

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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.

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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

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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.

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Conventional View
Protection domains - (virtual address space)
user
process 2
process n
process 1
kernel
How can processes communicate with each other
and the kernel?
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Universal IPC Facilities
handler
user
process 2
dbx
kernel
stop
handle event

• 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

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UNIX Pipes

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

Two processes connected by a pipe
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• 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 fd2
• pipe(fd0)
• fd0 now holds descriptor to read from pipe
• fd1 now holds descriptor to write into pipe

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Simple Example

• include ltunistd.hgt
• include ltfcntl.hgt
• include ltstdio.hgt
• char message "This is a message!!!"
• main()
• char buf1024
• int fd2
• pipe(fd) /create pipe/
• if (fork() ! 0) / I am the parent /
• write(fd1, message, strlen (message)
  1)
• else /Child code /
• read(fd0, buf, 1024)
• printf("Got this from MaMa!! s\n", buf)
  

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Create a Pipeline

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

Process
Process
Pipe
Pipe
c
D
Process A writes to pipe AB, Process B reads from
AB and writes to BC Process C reads from BC and
writes to CD ..
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Shared Memory

• Common chunk of read/write memory
• among processes

Proc. 2
Proc. 1
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Creating Shared Memory

• int shmget(key_t key, size_t size, int shmflg)
• Example
• key_t key
• int shmid
• key ftok(ltsomefilegt", A')
• shmid shmget(key, 1024, 0644 IPC_CREAT)
• Heres an example shm_create.c.

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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("ltsomefilegt", A')
• shmid shmget(key, 1024, 0644)
• data shmat(shmid, (void )0, 0)
• shmdt(data)
• Heres an shm_attach.c

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Deleting Shared Memory

• int shmctl(int shmid, int cmd, struct shmid_ds
  buf)
• shmctl(shmid, IPC_RMID, NULL)
• Example Shm_delete.c

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Command-line IPC control

• ipcs
• Lists all IPC objects owned by the user
• ipcrm
• Removes specific IPC object

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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)