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Core Inter-Process Communication Mechanisms (Historically Important). Fred Kuhns (, Department of Computer Science and Engineering Washington University in St. Louis. Cooperating Processes.

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    1. Core Inter-Process Communication Mechanisms(Historically Important) Fred Kuhns (, Department of Computer Science and Engineering Washington University in St. Louis

    2. 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 Cs422 – Operating Systems Organization

    3. Purposes for IPC • Data Transfer • Sharing Data • Event notification • Resource Sharing and Synchronization • Process Control Cs422 – Operating Systems Organization

    4. 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 • Synchronization – primitives such as semaphores to higher level mechanisms such as monitors • Debugging • Event Notification - UNIX signals • We will defer a detailed discussion of synchronization mechanisms and concurrency until a later class • Here we want to focus on some common (and fundamental) IPC and event notification mechanisms Cs422 – Operating Systems Organization

    5. Message Passing • In a Message 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 communicationlink • exchange messages via send and receive • Implementation of communication link • physical (e.g., shared memory, hardware bus) • logical (e.g., syntax and semantics, abstractions) Cs422 – Operating Systems Organization

    6. 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 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? Cs422 – Operating Systems Organization

    7. 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 (blocking versus non-blocking) • Automatic or Explicit buffering • Send-by-copy or send-by-reference • fixed or variable sized messages Cs422 – Operating Systems Organization

    8. 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 Cs422 – Operating Systems Organization

    9. 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. • kernel 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. Cs422 – Operating Systems Organization

    10. Indirect Communication • Mailbox sharing: • P1, P2, andP3 share mailbox A. • P1, sends; P2andP3 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 Cs422 – Operating Systems Organization

    11. Synchronization • 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. Cs422 – Operating Systems Organization

    12. Buffering • All messaging system require framework to temporarily buffer messages. These queues are implemented in one of three ways: • Zero capacity – No messages may be queued within the link, requires sender to block until receives retrieves message. • Bounded capacity – Link has finite number of message buffers. If no buffers are available then sender must block until one is freed up. • Unbounded capacity – Link has unlimited buffer space, consequently send never needs to block. Cs422 – Operating Systems Organization

    13. Lets Get Practical Notes to help you with the first project Cs422 – Operating Systems Organization

    14. 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? Cs422 – Operating Systems Organization

    15. process 1 pipe 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 control target process Cs422 – Operating Systems Organization

    16. Signals - History • Unreliable Signals - Orignal System V (SVR2 and earlier) implementation. • Handlers are not persistent • recurring instances of signal are not masked, can result in race conditions. • Reliable Signals - BSD and SVR3. Fixed problems but approaches differ. • POSIX 1003.1 (POSIX.1) defined standard set of functions. Cs422 – Operating Systems Organization

    17. Signals Overview • Divided into asynchronous (CTL-C) and synchronous (illegal address) • Three phases to processing signals: • generation: event occurs requiring process notification • delivery: process recognizes and takes appropriate action • pending: between generation and delivery • SVR4 and 4.4BSD define 31 signals, original had 15. Some commercial system support > 32. • Signal to integer mappings differ between BSD and System V implementations Cs422 – Operating Systems Organization

    18. System call interface {read(), write(), sigaltstack() … } Signals - Virtual Machine Model signal handler stack Process X (Signal handles) instruction set register handles dispatch to handler kernel (restartable system calls) deliver signal I/O facilities filesystem scheduler Cs422 – Operating Systems Organization

    19. Actions • Handling, default actions • Abort: terminate process, generate core dump • Exit: terminate without generating core dump • Ignore: ignore signal • Stop: suspend process • Continue: resume process • User specified actions • Default action, • Ignore signal, • Catch signal - invoke user specified signal handler • User may not ignore, catch or block SIGKILL and SIGSTOP • User may change action at any time • User may block signal • signal remains pending until unblocked Cs422 – Operating Systems Organization

    20. Notes • Signal action may only be taken within context of receiving process • Process must be scheduled to run • kernel calls issig() on behalf of process to check for pending signals. issig is called when: • before returning to user mode from a system call or interrupt • before blocking in an interruptible system event • immediately after waking from an interruptible event • if pending signal and processes has handler, then kernel arranges to first run handler on returning to user mode, then resume the interrupted instruction. Cs422 – Operating Systems Organization

    21. Signal Generation • Exceptions - kernel notifies process with signal • Other Process - using kill or sigsend. • Terminal interrupts - stty allows binding of signals to specific keys, sent to foreground process • Job control - background processes attempt to read/write to terminal. Process terminate or suspends, kernels sends signal to parent • Quotas - exceeding limits • Notifications - event notification (device ready) • Alarms - process notified of alarm via signal reception Cs422 – Operating Systems Organization

    22. Reliable Signals - BSD • Persistent handlers • Masking signals • signals masked (blocked) temporarily • user can specify mask set for each signal • current signal is masked when handler invoked • Interruptible sleeps • Restartable system calls • Allocate separate stack for handling signals • why is this important? Cs422 – Operating Systems Organization

    23. Signals - A Few Details • Any process or interrupt can post a signal • set bit in pending signal bit mask • perform default action or setup for delivery • Signal typically delivered in context of receiving process. • exception is sending SIGSTOP, kernel may perform action directly • Pending signals are checked before returning to user mode and just before/after certain sleep calls. • Produce core dump or invoke signal handler Cs422 – Operating Systems Organization

    24. UNIX Pipes • Unidirectional, FIFO, unstructured data stream • Fixed maximum size • Simple flow control • pipe() system call creates two file descriptors. Why? • Implemented using filesystem, sockets or STREAMS (bidirectional pipe). Cs422 – Operating Systems Organization

    25. Named Pipes • Lives in the filesystem - that is, a file is created of type S_IFIFO (use mknod() or mkfifo()) • may be accessed by unrelated processes • persistent • less secure than regular Pipes. Why? Cs422 – Operating Systems Organization

    26. Process Tracing • ptrace() • used by debuggers such as dbx and gdb. • Parent process controls execution of child • child must notify kernel that it will be traced by parent • Modern systems use the /proc file system. • Allows process to trace unrelated processes Cs422 – Operating Systems Organization