Process Communication

1. Process Communication

  • Mechanism for processes to communicate and to synchronize their actions
  • Two models
    • communication through a shared memory
    • communication through message passing

2. Communication through message passing

  • Message system
    • processes communicate with each other without resorting to shared variables
    • A message passing facility must provide at least two operations
      • send(message, recipient)
      • receive(message, recipient)
    • with indirect communication, the message are sent to and received from mailboxes
  • Messaging passing can be either blocking (synchronous) or non-blocking (asynchronous)
    • blocking send: the sending process is blocked until the message is received by the receiving process or by the mail box
    • non-blocking send: the sending process resumes the operation as soon as the message is received by the kernel
    • blocking receive: the receiver blocks until the message is available
    • non-blocking receive: receive operation does not block, it either returns a valid message or a default value (null) to indicate a non-existing message
3. Communication through shared memory
  • The memory region to be shared must be explicitly defined
  • using system calls, in unix
    • shmget: creates a shared memory block
    • shmat: maps an existing shared memory block into a process’s address space
    • shmdt: removes (unmaps) a shared memory block from the process’s address space
    • shmctl: is a general-purpose function allowing various operations on the shared block (receive information about the block, set the permissions, lock in memory)
  • Problems with simultaneous access to the shared variables
  • Compilers for concurrent programming languages can provider direct support when declaring variables.

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