Inter Process Communication:

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• Inter Process Communication
• ? It is an essential aspect of process
  management.
• By allowing processes to communicate with
• each other
• We can synchronize the execution of the
• processes relative to each other.
• 2. Share important information.

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General IPC Commands

• IPCS report inter-process communication
  facilities statusIPCRM remove the specified
  message queues, semaphore sets and shared memory
  segmentsDifferent IPC Mechanisms In
  UnixSEMAPHORESSHARED MEMORYPIPESMESSAGE
  QUEUES

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Semaphore

• ?Semaphore is a resource that contains a integer
  value.
• ?This integer value is stored in the kernel.
• ? Semaphores are a mean of synchronizing
  processes by testing and setting.

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• Semaphores enable multiple processes to perform
  mutual
• exclusion on critical section.
• Critical Section
• ?Each process has a segment of code in which it
  changes common variable, writing to a file,
  updating table and so onThis section is called a
  critical section.
• Mutual Exclusion
• ? Only one process at a time may be in the
  critical section.

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• Semaphores are used to avoid starvation and
  deadlock.
• StarvationIt occurs where a process is
  constantly denied access to resource it needs.
• Deadlock
• It occurs when two or ore processes each hold a
  resource that the other need, and will not
  release the until each process obtains the
  desired resource.? Its value is set to the
  number of resources it controls.For example, if
  we are sharing 3 printers then the semaphore
  value will be 3.

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• Two types of operation can be carried out on
  semaphores
• Wait OperationIf the semaphore value greater
  than zero, the resource is granted to the
  requesting process which decrements the semaphore
  value by the number of resources it acquired.If
  the semaphore value is zero or less, the
  requesting process decrements the semaphore value
  and blocks.
• Signal OperationWhen a processes releases a
  shared resource, it increments the semaphore
  value by the number of resources it released. ?
  The kernel notifies the queued process that the
  shared resource is now available and allows it to
  acquire the resource.

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Shared Memory

• ? SM can be used in a uniprocessor as well as in
  multiprocessor.? It can be used to communicate
  between different processes of the same user,
  different processes of the different user, and
  even between processes that runs at different
  times.? It is also possible to broadcast data
  from one process to many others and collect data
  from many to one hence we can say that SM is a
  multidimensional communication media.? SM allows
  multiple processes to share virtual memory space
  to exchange and change its content. This requires
  process synchronization to coordinate the access
  to the shared memory segment.

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• The following are the steps involved in the
  client server file copying program. ?Server
  gets access to a shared memory segment using a
  semaphore. ?The server reads from the input file
  into the shared memory segment. ?When the read
  is complete, the server again notifies the client
  using a semaphore. ?The client writes the data
  from the shared memory segment to the output file
  

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Unix Shared Memory System Calls

• UNIX provides the following system calls to
  perform the above action.Shmget- obtain a shared
  memory identifierShmat attach to shared
  memorySmdat detach from shared memoryShmcnl
  shared memory control

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PIPES

• ?Pipes provides a one-way flow of data.
• A pipe is created by the pipe system call.Int
  pipe (int fd 2)Here fd is an array of
  integers to store the file descriptor values of a
  pipe. A file descriptor is an integer that
  identifies the reading and writing end of the
  pipe. If the system call fails, it returns 1. if
  it is successful it returns to integer values
  fd0 and fd1.

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• ?When using pipes kernel provides the
  synchronization between the reading and writing
  processes.
• ?when a process tries to write to a pipe that is
  full the calling process blocks until the pipe is
  able to receive more data.
• ?if a process attempts to read from an empty
  pipe, the calling process blocks until the pipe
  receives some data or the pipe closes.
• ?if a pipe is open for reading but not for
  writing then the calling process (the reading
  end) blocks.

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• ?Data stored in the pipe is read on a First in
  First Out basis.
• ?The disadvantage with pipes is that they can
  only be used between processes that have a parent
  process in common. This is because a pipe is
  passed from one process to another by the fork
  system call.  There is no way for two totally
  unrelated processes to create a pipe between them
  and use it for IPC.

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• Two processes communicate with each other
  using pipes in the following way.Before fork

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After Fork
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• ?First, process creates a pipe and then forks to
  create a copy of itself.
• Next, the process closes the read end of the pipe
  and the child process closes the write end of the
  pipe.
• ?When the writing process finishes sending its
  data it should close its writing end to avoid
  blocking the reading process. By closing the
  writing end, an end-of-file (EOF) character will
  be put in the pipe. The (EOF) informs the reading
  process that there is no more data coming from
  the writing process, hence avoiding it to block.

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• Message Queues
• ? It is like a pipe and is used to transfer
  messages between processes in Unix system.
• ? A message queue is a queue onto which messages
  can be placed. A message is composed of message
  type and message data.

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• ?First, process creates a pipe and then forks to
  create a copy of itself.
• ?Next, the process closes the read end of the
  pipe and the child process closes the write end
  of the pipe.
• ?When the writing process finishes sending its
  data it should close its writing end to avoid
  blocking the reading process. By closing the
  writing end, an end-of-file (EOF) character will
  be put in the pipe. The (EOF) informs the reading
  process that there is no more data coming from
  the writing process, hence avoiding it to block.

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• Message Queues
• ?It is like a pipe and is used to transfer
  messages between processes in Unix system.
• ?A message queue is a queue onto which messages
  can be placed. A message is composed of message
  type and message data.

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• ?It can be either private or public. If it
  is private, it can be accessed only by its
  creating process or child processes of that
  creator. If it is public, it can be
  accessed by any process that knows the queues
  key.
• ?Message may be read by type, and hence there is
  no need to read them in a FIFO order, which is a
  case with the pipe.

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• The msgget() system call is used to create a
  message queue.
• This system call accepts two parameters 1)     
  A queue key2)      FlagsThe key may be one of
• IPC_PRIVATE - used to create a private message
  queue
• Positive integer - used to create a publicly
  accessible message queue

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Mach IPC

• ?UNIX, provide communication between processes,
  and between hosts with fixed, global names
  (Internet addresses).
• ?There is no location independence of facilities,
  because any remote system needing to use a
  facility must know the name of the system
  providing that facility.
• ?Mach simplifies this picture by sending messages
  between location-independentPorts.
• ?The messages contain typed data for ease of
  interpretation

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• IPC in MACH is done through
• Ports
• Messages.?Almost everything in mach is an object
  and all objects are addressed via their
  communication ports.?Messages are sent to this
  port to initiate the operation on the
  objects.When object is created, its creator also
  allocates a port to represent an object and
  obtains an access right to that port.Rights can
  be given out by the creator of the object and
  passed in the message.

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• Ports
• ? Ports are used to provide a one-way channel of
  communication for message passing.
• ? A port has a single receiver and multiple
  senders.
• ? Only one entity the kernel or user task can
  receive messages from a port, but anyone with
  permission may send to it.
• ? A port is implemented as a protected, bounded
  queue within the kernel on which the object
  resides. They are not manipulated directly by
  the user.
• ? To the kernel, a port is an object, which
  includes information about the port properties
  as well as internal data structures like locks,
  a message queue, and pointers to related
  objects.

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• A port is referred by using a port name in the
  port name
• space. Every task has a port name space and it
  contains a list
• of port names.
• ?The port name space are also kernel protected
  means that a user can not simply add and remove
  entries in the port name space, he has to rely on
  the kernel to do it on request.
• ?If a queue is full, a sender may abort the send,
  wait for a slot to become available in the queue,
  or have the kernel deliver the message for it.

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• Messages?A message is a collection of typed
  data.
• Available types are         Pure-data
  (integers, floats, strings)         Port
  rights Memory regions

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Port Rights

• ? It indicates to access a specific port.Three
  kinds of port rights1) Receive RightIt allows
  task to receive the messages from this port. Only
  one receive right exists for a port.2) Send
  RightIt allows task to send the messages to
  this port. The number of send rights per port is
  not restricted.3) Send-one RightIt allows task
  to send one message to this port. The send-once
  right is destroyed after the message is sent.

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• A message consists of a fixed-length header and a
  variable number of typed data objects.
• ?The header contains the destinations port name,
  the name of the reply port to which return
  messages should be sent, and the length of the
  message.
• ?There are two types of messages in
  mach.1.       Small messages2.       Large
  messages
• ?The length of small messages is less than 128
  bytes.
• ?Large messages carries one or two pages of
  out-of-line data.
• ?Small messages are used for most requests and
  replies.
• ? Large message are used for transferring data
  including file and device access and paging
  traffic.

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