1. Operating Systems
Lecture 8

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Agenda for Today
Review of previous lecture
Interprocess communication (IPC) and process synchronization
UNIX/Linux IPC tools (pipe, named pipe—FIFO, socket, message queue, shared memory)
Use of pipe
Recap of the lecture
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Review of Lecture 7
The wait and exec system calls and sample code
Cooperating processes
Producer-consumer problem
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4. Interprocess Communication (IPC)
Mechanism for processes to communicate and to synchronize their actions.
Message system – processes communicate with each other without resorting to shared variables.
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5. Interprocess Communication (IPC)
IPC facility provides two operations:
Send (message) – message size fixed or variable
Receive (message)
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6. Interprocess Communication (IPC)
If P and Q wish to communicate, they need to:
establish a communication link between them
exchange messages via send/receive
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7. Interprocess Communication (IPC)
Implementation of communication link
physical (e.g., shared memory, hardware bus)
logical (e.g., logical properties)
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8. Implementation Questions
How are links established?
Can a link be associated with more than two processes?
How many links can there be between every pair of communicating processes?
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9. Implementation Questions
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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Direct Communication
Processes must name each other explicitly:
send (P, message) – send a message to process P
Receive (Q, message) – receive a message from process Q
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Direct Communication
Properties of communication link
Links are established automatically.
A link is associated with exactly one pair of communicating processes.
Between each pair there exists exactly one link.
The link may be unidirectional, but is usually bi-directional.
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12. Indirect Communication
Messages are directed and received from mailboxes (also referred to as ports).
Each mailbox has a unique id.
Processes can communicate only if they share a mailbox.
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13. Indirect Communication …
Properties of communication link
Link established only if processes share a common mailbox
A link may be associated with many processes.
Each pair of processes may share several communication links.
Link may be unidirectional or bi-directional.
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14. Indirect Communication …
Operations
create a new mailbox
send and receive messages through mailbox
destroy a mailbox
Primitives are defined as:
send (A, message)
receive (A, message)
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15. Indirect Communication …
Mailbox sharing
P1, P2, and P3 share mailbox A.
P1, sends; P2 and P3 receive.
Who gets the message?
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16. Indirect Communication …
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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Synchronization
Message passing may be either blocking or non-blocking.
Blocking is considered synchronous
Non-blocking is considered asynchronous
send and receive primitives may be either blocking or non-blocking.
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Buffering
Queue of messages attached to the link; implemented in one of three ways.
Zero capacity – No messages Sender must wait for receiver
Bounded capacity – n messages Sender must wait if link full.
Unbounded capacity – infinite length Sender never waits.
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UNIX/Linux IPC Tools
Pipe
Named pipe (FIFO)
BSD Socket
TLI
Message queue
Shared memory
Etc.
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UNIX/Linux Pipe
Important system calls
pipe, read, write, close
pipe: Create a pipe for IPC
read: Read from a pipe
write: Write data to a pipe
close: Close/destroy a pipe
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21. File Descriptors in UNIX/Linux
An integer returned by open() system call
Used as an index in the per process file descriptor table (PPFDT)
Used in read, write, and close calls
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22. File Descriptors in UNIX/Linux
Size of PPFDT is equal to the number of files that a process can open simultaneously (OPEN_MAX in Linux—see <linux/limits.h>
Used as an index in the per process file descriptor table (PPFDT)
Used in read, write, and close calls
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UNIX/Linux Pipe
Important characteristics of a pipe
Stream of bytes
Used as half-duplex channel
Bounded buffer
Maximum data written is PIPE_BUF (defined in <sys/param.h> under UNIX and in <linux/param.h> in Linux)—5120 and 4096, respectively
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24. Synopsis of pipe System Call
#include <unistd.h>
int pipe (int filedes[2]);
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Example
child
parent
fork P P
Read end
Write end
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Sample Code
/* Parent creates pipe, forks a child, child writes into
pipe, and parent reads from pipe */
#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>
main() {
int pipefd[2], pid, n, rc, nr, status;
char *testString = "Hello, world!\n“, buf[1024];
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Sample Code
rc = pipe (pipefd);
if (rc < 0) {
perror("pipe");
exit(1); }
pid = fork ();
if (pid < 0) {
perror("fork");
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Sample Code
if (pid == 0) { /* Child’s Code */
close(pipefd[0]);
write(pipefd[1], testString, strlen(testString));
close(pipefd[1]);
exit(0); }
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Sample Code
/* Parent’s Code */
close(pipefd[1]);
n = strlen(testString);
nr = read(pipefd[0], buf, n);
rc = write(1, buf, nr);
wait(&status);
printf("Good work child!\n");
return(0);
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Recap of Lecture
Review of previous lecture
Interprocess communication (IPC) and process synchronization
UNIX/Linux IPC tools (pipe, FIFO, socket, message queue, shared memory, etc.)
Use of UNIX pipe
Recap of the lecture
27 December 2018
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