I/O Systems and Operations

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https://sepang2.tistory.com/39

I/O Systems and Operations

Contents

• An Overview of I/O Subsystem and Operations

• Stream Model – I/O functions – Standard streams

• Buffering – Block buffering vs. Line buffering vs. Unbuffered

• Pipes • File I/O – File pointer – File Attributes

• Device I/O – Device Drivers – Block devices vs. Character devices vs. Network devices

 

Reminder: A View of OS Services

I/O (Input/Output)

•  The main job of a computer is I/O

– Its another (major) job, Computing or Processing, is merely secondary

 

• The role of OS in computer I/O is to control devices and to manage I/O operations

=> I/O 디바이스 컨트롤, I/O operation들을 manage

 

The Role of OS: I/O Device Control

• I/O devices vary so widely in their function and speed

-> Varied methods are needed to control them

– These methods form the “I/O subsystem of the OS kernel”, which separates the rest of the kernel from the complexities of managing I/O devices

 

• The basic I/O hardware elements, such as ports, buses, and device controllers, accommodate a wide variety of I/O devices

– Port: A connection point between a device and a computer (e.g., a serial port)

– Bus: A common set of wires, shared by devices, and a rigidly defined protocol that specifies a set of messages that can be sent on the wires (I/O 디바이스와 CPU와 연결) 

=> I/O 디바이스는 매우 다양하므로 method 또한 매우 다양

 

• To encapsulate the details and oddities of different devices, the OS kernel is structured to use device-driver modules

– Device Driver: A uniform device-access interface to the I/O subsystem, much as system calls provide a standard interface between the application and the OS

=> I/O 컨트롤을 하는 매개체

 

A Typical PC Bus Structure

main BUS : PCI Bus

A Kernel I/O Structure

kernel layer와 HW layer 중간에 있는 device driver

The Role of OS: I/O Operation Management

• I/O operations or transactions occur when a program receives bytes from a source (e.g., a keyboard, mouse, file, and sensor) or sends bytes to a destination (e.g., a display, file, printer, and actuator)

-> Most modern OS, including Unix/Linux, use the “stream model” to control I/O

 

Stream Model

• Any input or output transaction can be viewed as a flow of bytes (=stream) from a source to a destination

• The operating system creates and manages streams based upon the function calls made by the program – An I/O stream connection can be established using the fopen() function, and broken using the fclose() function

=> source에서 dest로 byte sequence(stream)을 전송, fopen으로 stream 열기, fclose으로 stream 닫기

 

The Steps in Creating, Using, and Closing a Stream

• When a stream is created, one can think of it as having an address from which bytes are sent and an address at which bytes are received.

– Each address is at a memory location controlled by the OS

=> OS은 이미 source의 address와 destination의 address를 알기에 data transfer를 해준다.

 

Transporting Bytes on Streams

• There are several functions that can be used to transport bytes on streams:

– These include the familiar fprintf() and fscanf() functions, which are specialized to work only with ASCII formatted text data (byte tranfer + byte manipulation)

– The generic functions for this purpose are fread() and fwrite() (byte tranfer에만 신경씀)

    ->At the heart of both fprintf() and fscanf() functions are calls to fread() and fwrite() to actually move the bytes after the desired byte manipulations have been completed

– There are a number of other functions as well, such as fgetc(), fputc(), gets(), and puts()

 

fprintf()

• Bytes are being sent down a stream using the fprintf() function

fscanf()

• Bytes are being transported on a stream using the fscanf() function

- 하나의 string을 공백 문자나 개행문자로 구분!

- 데이터 가공: string 타입으로 써야하므로 끝에 \0을 붙여줌!

fscanf(fpt,"%s",text); =fpt라는 stream connection으로 string 타입의 데이터를 받아와 text라는 문자 배열에 저장해라!

fscanf()

• fscanf() and fprintf() functions perform byte manipulations(fprintf도 수행) besides the byte transfers.

int 타입으로 fscanf하기에 byte manipulations 수행

fread() & fwrite()

• Synopsis

– size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);

– size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);

 

• Description

– The function fread() reads nmemb elements of data, each size bytes long, from the stream pointed to by stream, storing them at the location given by ptr.

– The function fwrite() writes nmemb elements of data, each size bytes long, to the stream pointed to by stream, obtaining them from the location given by ptr.

 

• Return value

– On success, fread() and fwrite() return the number of items read or written (=fread(),fwrite() : element의 개수가 리턴됨)

      -> An indicator of the number of bytes actually moved along the stream

– If an error occurs, the return value is a short item count (or zero). (=전송된 element의 개수가 리턴됨)

 

fread()

• Bytes are read from a source into the program regardless of what the bytes represent.

fread(text,1,15,fpt) (=fpt로부터 text에 1byte size의 것을 15개 읽어서 저장) , 문자 배열 text이기에 널 캐릭터가 마지막에 있어야 하는데, fread()는 그런 기능이 없으므로, 프로그래머가 직접 추가해줘야함 (text[15]=0;)

fwrite()

• Bytes are moved from the program into a destination

  – The parameters of the fwrite() are identical to those in the fread() function

문자열을 text에 저장후, text의 내용을 data2.txt에 저장

– Executing this code will produce a file data2.txt including “Fortytwo 42 …”

– Note that this example uses the strlen() function to determine the number of cells in the array to send on the stream, which in this case is 40.

 

Short Question…

• What would you expect here?

40byte짜리 문자열을 70 byte 읽어들이므로 printf()에서는 40이 찍힐 것, 만약 fread(text,2,70,fpt)라면 printf()에는 20이 찍힐 것 (얼마나 읽혀졌는지를 리턴하기때문)

System I/O Functions

• Another set of functions that can be used for I/O

– These functions include open(), close(), read(), and write() (<= C libraryX Lib by OS O)

– They look very similar to the f-versions of the I/O functions, i.e., fopen(), fclose(), fread(), and fwrite() functions, which are standardized in the C library and therefore expected to behave similarly regardless of the underlying operating system

*** open(){ sys-open()} =wrapper function

• The non-f I/O functions are “system calls” (open(),close()..등은 시스템콜으로 취급,OS type에 따라 동작이 달라질 수 O)

– Depending on which OS they are called from, they may behave differently

– A general rule of thumbs is that the non-f versions are NOT buffered, while the f-versions are buffered

 

Standard Streams

• Most programs get user input from a keyboard and display text output to a screen. 

=> 모든 application이 keyboard로 user input을 받을 수 있어야 하고, 스크린으로 output을 나타낼 수 있어야 함.

Therefore, every time a program is started, the OS automatically creates three streams:

  – Standard in (stdin): It connects keyboard to program

  – Standard out (stdout): It connects program to display

  – Standard error (stderr)

      It connects program to a secondary display that is intended only for displaying errors 일반적으로는 stdout과 동일한 display

      stderr stream is intended as a backup output stream for programs

• The standard streams are most commonly used by calling the scanf() and printf() functions

    – While fscanf() can receive bytes from any stream, scanf() is “hardwired” to the stdin stream. The same is true with regards to printf() and fprintf()

 

Example

• In the following example, the pair of scanf() & fscanf() and printf() & fprintf() function calls perform the exact same operation; => scanf와 fscanf 동일하게 동작, printf()와 fprintf() 동일하게 동작

– The definitions for the standard streams can be found in the include file stdio.h

Buffering

• A buffer is a temporary storage or the “middlemen” between sender and receiver of bytes on a stream (i.e., between two devices or between a device and an application)

=> sender -> BUFFER -> receiver

  – An additional piece of memory that is used to moderate the flow of bytes from the source to the destination

 

• Buffering is done for 3 reasons: (Buffering 이유)

  – To cope with device speed mismatch

       What if the sender puts bytes into the stream faster than the receiver can handle?

       What if a program is in the middle of a calculation and is not prepared to receive any bytes?

  – To cope with device data-transfer size mismatch

       Common in computer networking, where buffers are used widely for fragmentation and reassembly of messages

  – To support “copy seman tics” for application I/O (=> write()호출했을 때의 data가 그대로 copy되어야하기 때문)

 

Three Types of Buffering

• Based on how the temporary storage is “flushed”

   – Block buffering

       The buffer is flushed when it receives a certain amount of data (e.g. 1KB, 16KB, 64KB, etc…)

       Commonly used for large data transfers, such as file I/O

   – Line buffering

       The buffer is flushed when it receives a newline character(‘\n’)

       Typically used for text-based I/O, such as when interacting with a user

   – Unbuffered

       The buffer doesn’t act as a buffer and is flushed every bytes

       Buffering may be completely turned off when responsiveness is critical

 

• The type of buffering on an existing stream can be changed by the setvbuf() or setbuf() function

 

Example code