Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition

Lecture 2: OS Programming Interface

T. Yang, CS 170 2018

What to Learn?

n Operating System Services & Interfacen System Callsn System utilities

n OS Layersn Virtual Machines

A View of Operating System Services

Interface to OS: How to Interact with OS?

Shell Command InterpreterGUI: Graphical User Interface

OS Programming interface

Operating SystemServices

System Programs/Utilitiesn Categories of System programs/utilities

n Process status and managementn File /directory manipulation n File modification and text processingn Programming language support (compilers)n Program loading and executionn Communicationsn Application programs

n Most users’ view of the operation system is defined by system utilities programs, not the actual progrmaming API & system calls

Linux Utility Programs

Programming API – OS System Call

Role of system callsCompilers

Web Servers

Web Browsers

DatabasesEmail

Word Processing

Portable OS LibrarySystem Call

InterfacePortable OS Kernel

Platform support, Device Drivers

x86 ARMPowerPC

Ethernet 802.11 a/b/g/n SCSI IDE GraphicsPCI

Hardware

Software

System

UserOS

Application / Service

Linux Layers

Class Project: Nachos system Layers

Base Operating System (Linux for our class)

Nachos kernel threadsThread 1 Thread 2 Thread N

Nachos OS modules(Threads mgm, File System, Code execution/memory mapping,

System calls/Interrupt)

Simulated MIPS Machine(CPU, Memory, Disk, Console)

User processUser process

Projects 2&3

Project 1

Virtual machine

System Callsn System calls: Programming interface to the

services provided by the OSn Mostly accessed by programs via a high-

level Application Program Interface (API) rather than direct system call usen Three most common APIs are

n Win32 API for Windows,n POSIX API for POSIX-based systems (including

virtually all versions of UNIX, Linux, and Mac OS X), and

n Java API for the Java virtual machine (JVM)

n Why use APIs rather than system calls?

System Callsn System calls: Programming interface to the

services provided by the OSn Mostly accessed by programs via a high-

level Application Program Interface (API) rather than direct system call usen Three most common APIs are

n Win32 API for Windows,n POSIX API for POSIX-based systems (including

virtually all versions of UNIX, Linux, and Mac OS X), and

n Java API for the Java virtual machine (JVM)

n Why use APIs rather than system calls? Portability. Simplicity.

Transition from User to Kernel Mode

Standard C Library Examplen C program invoking printf() library call,

which calls write() system call

Kernel mode

User mode

Types of System Calls

n Process controln File managementn Device managementn Information maintenancen Communicationsn Protection

Examples of Windows and Unix System Calls

Unix I/O Calls with File DescriptorsnfileHandle = open(pathName, flags)

n A file handle (called file descriptor in Unix) is a small integer, pointing to a meta data structure about this file.

n Pathname: a name in the file system. n Flags: read only, read/write, append etc…

nerrorCode = close(fileHandle)

n Kernel will free the meta data structures associated

MemoryFile

540 1 2Metadata on a file

Yes

Does child process inherit file descriptors of parent process?

Parent ChildDuplicate

Unix I/O Calls nbyteCount = read(fileHandle, buf, count)

n Read at most count bytes from the device and put them in the byte buffer buf.

n Kernel can give the process fewer bytes, user process must check the byteCount to see how many were actually returned.

n A negative byteCount signals an error n byteCount = write(fileHandle, buf, count)

n Write at most count bytes from the buffer bufn Actual number written returned in byteCount n A negative byteCount signals an error

Seek offset

File content in bytes

Read count

Seek offset

File content in bytes

Write count

Where to store the seek offset?In-memory file meta data structure refered by file descriptor

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Example: Implement Command copy file1 file2

#include <stdio.h>#include <fcntl.h>#define BUF_SIZE 8192void main(int argc, char* argv[]) {

int input_fd, output_fd; int ret_in, ret_out;char buffer[BUF_SIZE]; /* Create input file descriptor */input_fd = open (argv [1], O_RDONLY);if (input_fd == -1) {

printf ("Error in openning the input file\n"); return;}

File1 File2

Memory buffer

5 6

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copy file1 file2/* Create output file descriptor */

output_fd = open(argv[2], O_WRONLY | O_CREAT, 0644);if(output_fd == -1){

printf ("Error in openning the output file\n"); return;}/* Copy process */while((ret_in = read (input_fd, &buffer, BUF_SIZE)) > 0){

ret_out = write (output_fd, &buffer, ret_in);if(ret_out != ret_in){ /* Write error */

printf("Error in writing\n");}

}close (input_fd); close (output_fd);

}

File1 File2

Memory buffer

5 6

Shelln A shell is a job control system

n Lets user execute system utilities/applicationsn Windows, MacOS, Linux all have shells

n Typical format:n cmd arg1 arg2 ... argnn i/o redirection <, >n filters & pipes

n ls | more

Proj 0

OS Design & Implementation

n Start by defining goals and specifications

n Affected by n Choice of hardwaren User goals –

n convenient to use, easy to learn, reliable, safe, and fast

n System goals –n easy to design, implement, and

maintain, as well as flexible, reliable, error-free, and efficient

OS Design Principles

n Separate policy (what to do) and mechanism (how to do)n Why?

n Layered structuren Modularn Monolithic kernel vs. Microkernel

Maximize flexibility

Layered Approachn The operating system is divided into a

number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.

MS-DOS: Simple Layer Structure n written to provide the most functionality in

the least space

Traditional UNIX System Structure

Modular approachn Object-orientedn Each core component is separaten Each talks to the others over known interfacesn Each is loadable as needed within the kernel

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Monolithic Kernel vs. Microkernel

Powerful kernel which can do everything

Thin kernel which offers minimial features

Microkernel System Structure n Moves as much from the kernel into “user” spacen Communication takes place between user modules

using message passingn Benefits:

n Easier to extend a microkerneln Easier to port the operating system to new

architecturesn More reliable (less code is running in kernel

mode)n More secure

n Weakness:n Performance overhead of user space to kernel

space communication

Mac OS X and iOS

UNIX

Virtual Machinesn A virtual machine takes the layered

approachn A virtual machine provides an interface

identical to the underlying bare hardware.n The host creates the illusion that each guest

has its own processor and virtual memory/storage.

Nachos can be considered as providing MIPS virtual machine running an Intel Linux machine

Example of virtual machine?

The Java Virtual Machine

Virtual Machines (Cont.)

(a) Non-virtual machine (b) virtual machine

VMware Architecture

Android (Linux-based)

What we have learned?

n Operating System Services & Interfacen System Callsn System utilities

n OS Layers and Virtual Machines: Discuss later

Role of system calls and utilitiesCompilers

Web Servers

Web Browsers

DatabasesEmail

Word Processing

Portable OS LibrarySystem Call

InterfacePortable OS Kernel

Platform support, Device Drivers

x86 ARMPowerPC

Ethernet 802.11 a/b/g/n SCSI IDE GraphicsPCI

Hardware

Software

System

UserOS

Application / ServiceSystem utilities