CS 149: Operating Systems January 29 Class Meeting Department of Computer Science San Jose State University Spring 2015 Instructor: Ron Mak mak.

CS 149: Operating SystemsJanuary 29 Class Meeting

Department of Computer ScienceSan Jose State University

Spring 2015Instructor: Ron Mak

www.cs.sjsu.edu/~mak

http://www.cs.sjsu.edu/~mak

2Computer Science Dept.Spring 2015: January 29

CS 149: Operating Systems© R. Mak

Example System Call: fork()#include <stdio.h>

main(){ printf("Parent: Process started.\n"); printf("Parent: Forking a child.\n");

if (fork() != 0) { int status; printf("Parent: Waiting for child to complete.\n"); waitpid(-1, &status, 0); printf("Parent: Child has completed.\n"); printf("Parent: Terminating.\n"); } else { printf("Child: Process started.\n"); printf("Child: Start 10 second idle:");

int i; for (i = 10; i >= 0; i--) { printf("%3d", i); fflush(stdout); sleep(1); } printf(" done!\n"); printf("Child: Terminating.\n"); }}

Executed by the parent.

Executed by the child.

The two processes fork at this point.

forktest.c



The waitpid() System Call

What are the parameters?

Use the command-line UNIX man (for “manual”) command to get documentation of any system command or API call._

waitpid(-1, &status, 0);

Demo

Pass the address of variable status(i.e., pass by reference).



Shell: I/O Redirection

For a program run on the command line: Standard input (stdin) is the keyboard. Standard output (stdout) is the terminal.

#include <stdio.h>

#define MAX_LENGTH 256

main(int argc, char *args[]){ char *progname = args[0]; char line[MAX_LENGTH];

while (gets(line) != NULL) { printf("%s: %s\n", progname+2, line); }}

This program reads lines of textfrom stdin and echos them tostdout after prepending eachline with the program name.

echo.c

Why the +2?



Shell: I/O Redirection

You can redirect a program’s stdin on the command line with the < operator:

Now the echo program will read its own source file.

You can redirect a program’s stdout on the command line with the > operator:

Now the echo program will write to file echo.txt. Use the >> operator to append to an output file.

You can redirect both stdout and stdin.

./echo < echo.c

./echo < echo.c > echo.txt

Demo



OS Concept: Pipes

Pipes are a mechanism for interprocess communication (IPC).

One process writes to one end of a pipe. Another process reads from the other end. Each process believes it is doing file I/O.

Operating Systems: Design and Implementation, 3rd ed. Andrew Tanenbaum & Albert Woodhull (c) 2006 Prentice-Hall, Inc. 0-13-142938-8



Shell: Piping

You can pipe a program’s output to another program with the | operator:

The first program’s stdout becomes the stdin for the second program.

What’s the result of this command?

./echo1 | ./echo2

./echo1 < echo.txt | ./echo2 | ./echo3

Demo



The POSIX Standard

POSIX (Portable Operating System Interface)

IEEE standard for system calls.

Maintain compatibility among operating systems. UNIX and non-UNIX



Process Management System Calls

POSIX system calls.

Most modern operating systems have system calls that perform these functions. However, details may differ.

Modern Operating Systems, 3rd ed.Andrew Tanenbaum(c) 2008 Prentice-Hall, Inc.. 0-13-600663-9All rights reserved



File Management System Calls




File System Management System Calls




Miscellaneous System Calls




Kernel Mode vs. User Mode

An OS such as UNIX executes its critical system software in kernel mode. This is one way the OS protects itself.

As a user, you run utility programs (such as editors, compilers, and browsers) and application programs in user mode. System calls made by your programs can invoke

some OS code that is executed in kernel mode.

On the command line, you can become “super user” and bypass some of the OS protection mechanisms.



Workflow of a System Call

1. Push nbytes2. Push &buffer3. Push fd4. Invoke the read

library routine.5. Put the system call code

for read in a register.6. TRAP instruction switches

to kernel mode to access the dispatcher in theOS kernel.

7. Dispatcher accesses the read handler.

8. Execute the read handler.9. Return to the user program

at the point after the TRAP instruction.

Invoke the system call count = read(fd, buffer, nbytes)




Operating System Structure

There are various ways to organize the code of an operating system.

Monolithic Layered Virtual machines Client-server Distributed

_



OS Structure: Monolithic

The original classic way an OS was designed.

No structure. A collection of routines. Each routine can call any other routine. No information hiding.

A challenge to build and link.

An even bigger challenge to maintain and debug.

Every software engineer should read the bookThe Mythical Man-Month about the development of IBM OS/360.



OS Structure: Layered

Organize the OS as a hierarchy of layers. Each layer built on top of the one below it.




OS Structure: Layered

Layers of the THE operating system.

For a Dutch computer in the late 1960s. The bottommost layer 0 provided multiprogramming.

_Operating Systems: Design and Implementation, 3rd ed. Andrew Tanenbaum & Albert Woodhull (c) 2006 Prentice-Hall, Inc. 0-13-142938-8



OS Structure: Virtual Machines

Virtual machines (VMs) were first developed for the IBM 370 in the late 1960s.

Each virtual machine behaves like a separate physical machine.

Controlled by a virtual machine monitor.Operating Systems: Design and Implementation, 3rd ed. Andrew Tanenbaum & Albert Woodhull (c) 2006 Prentice-Hall, Inc. 0-13-142938-8



OS Structure: Virtual Machines

Modern VM monitors include:

VirtualBox VMware KVM OpenVZ Xen



OS Structure: Client-Server

Put as much OS code as possible outside of the kernel and into user space.

OS services run as separate processes. Processes send messages to each other to request services. A “microkernel” serves as the communications bus.




OS Structure: Distributed

Like client-server, except that server processes run on different machines on the network.

Service request messages go over the network. User programs do not need to know

where the services are provided._




Processes

A process is basically an abstraction of a running program. The most central concept in any operating system.

Each process runs in its own address space._



Process Models

Multiprogramming withmultiple programs.

One program counterper CPU.

Each program has itsown virtual CPU.

The real CPU rapidlyswitches from oneprogram to another. Process switching

AKA context switchingOperating Systems: Design and Implementation, 3rd ed. Andrew Tanenbaum & Albert Woodhull (c) 2006 Prentice-Hall, Inc. 0-13-142938-8



Process Models, cont’d

Only one program is active on a CPU at any instant.




Context Switching

A process’s stateinformation is keptin its process controlblock (PCB).




Process Creation

Principal events that cause processes to be created:

System initialization.

Execution of a process creation system call by a running process. fork()

A user request to create a new process.

Initiation of a batch job.



Process Termination

Conditions that cause a process to terminate:

Normal exit (voluntary). Error exit (voluntary). Fatal error (involuntary). Killed by another process (involuntary).

Cascading termination On some operating systems, when a process

terminates, so do its child processes.



Process Creation and Termination

What does the exec()system call do?




Process States

Running Actually using the CPU at that instant.

Ready Runnable, but temporarily stopped to let another process run.

Blocked Unable to run until some external event happens.


CS 149: Operating Systems January 29 Class Meeting Department of Computer Science San Jose State University Spring 2015 Instructor: Ron Mak mak.

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