1. Washington WASHINGTON UNIVERSITY IN ST LOUIS Core Inter-Process Communication Mechanisms (Historically Important) Fred Kuhns (fredk@arl.wustl.edu, http://www.arl.wustl.edu/~fredk) Department of Computer Science and Engineering Washington University in St. Louis

2. 2 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts 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

3. 3 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts Purposes for IPC Data Transfer Sharing Data Event notification Resource Sharing and Synchronization Process Control

4. 4 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts 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 memoru or message passing. –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

5. 5 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts 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)

6. 6 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts 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?

7. 7 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts 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 communications 3.Automatic or Explicit buffering 4.Send-by-copy or send-by-reference 5.fixed or variable sized messages

8. 8 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts 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

9. 9 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts 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.

10. 10 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts Indirect Communication Mailbox sharing: –P 1, P 2, and P 3 share mailbox A. –P 1, sends; P 2 and P 3 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

11. 11 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts 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.

12. 12 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts Buffering All messaging system require framework to temporarily buffer messages. These queues are implemented in one of three ways: 1.Zero capacity – No messages may be queued within the link, requires sender to block until receives retrieves message. 2.Bounded capacity – Link has finite number of message buffers. If no buffers are available then sender must block until one is freed up. 3.Unbounded capacity – Link has unlimited buffer space, consequently send never needs to block.

13. 13 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts Lets Get Practical Notes to help you with the first project

14. 14 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts Conventional View process 1 process 2 process n kernel user Protection domains - (virtual address space) How can processes communicate with each other and the kernel?

15. 15 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts Universal IPC Facilities process 1 process 2 kernel user pipe handler handle event stop dbx 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

16. 16 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts 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.

17. 17 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts 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

18. 18 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts Signals - Virtual Machine Model System call interface {read(), write(), sigaltstack() … } kernel scheduler I/O facilities filesystem instruction set (restartable system calls) Process X (Signal handles) register handlers deliver signal dispatch to handler signal handler stack

19. 19 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts 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 –change action at any time –block signal: signal remains pending until unblocked

20. 20 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts Notes Signal action may only be taken within context of receiving process Process must be scheduled to run Checks for pending signals 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 process has handler, then kernel arranges to first run handler on returning to user mode, then resume the interrupted instruction.

21. 21 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts 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

22. 22 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts 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?

23. 23 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts 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

24. 24 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts 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). 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?

25. 25 Fred Kuhns (12/4/2015)CS422 – Operating Systems Concepts 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