File Structures 1

Organizing file for better performance of disk

File Structures

Introduction to File Structures Applications of File Structures File structures in software's File organizer in windows File organizing in ubuntu Chapter 1 : File processing operations Road map of Syllabus Text book

Topics under discussionTopics under discussion

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Is a combination of representations for data in files

Operations for accessing data Allows applications to read, write, modify data Very important in needs of applications and tremendous variety of applications

File structure

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For representing a city region telephone network. store a set of fixed key words which are referenced very frequently.

represent an image in the form of a bitmap. implement back functionality in the internet browser. implement printer spooler so that jobs can be printed in the order of their arrival.

record the sequence of all the pages browsed in one session.

store information about the directories and files in a system.

Applications

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File structures in software's

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Programming files – c ,cpp,java,html,xml…. Database files - dbase ,foxpro,oracle-log,dmp… Log data and software files … Text files – doc,txt,docx…

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File organizer in windows

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In ubuntu

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Why Study File Structure Design?I. Data Storage

Computer data can be stored in three kinds of locations:

Primary Storage ==> Memory [Computer Memory] Secondary Storage

[Online Disk/ Tape/ CDRom that can be accessed by the computer]

Tertiary Storage ==> Archival Data

[Offline Disk/Tape/ CDRom not directly available to the computer.]

OurFocus

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Why Study File Structure Design?II. Memory versus Secondary Storage

Secondary storage such as disks can pack thousands of megabytes in a small physical location.

Computer Memory (RAM) is limited. However, relative to Memory, access to secondary storage is extremely slow [E.g., getting information from slow RAM takes 120. 10-9 seconds (= 120 nanoseconds) while getting information from Disk takes 30. 10-3 seconds (= 30 milliseconds)]

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Physical file and logical file Opening files Closing files Reading and writing Seeking Special characters in unix Unix directory structure Physical devices and logical files File related header files Unix file system commands

Chapter 1 : file processing operations

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Physical file and logical file

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C FILE *fp fp=fopen(“student.txt”,”r”); student.txt

CPP fstream f1; f1.open(“student.txt”,ios::in);

In c and cpp

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Physical file : Actually exists on the storage File known by computer OS and appears in the directory Logical file : file seen by the program Allows to describe the operations performed on physical

file

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Opening Files

Once we have a logical file identifier hooked up to a physical file or device,

need to declare what we intend to do with the file:

Open an existing file Create a new file

That makes the file ready to use by the programWe are positioned at the beginning of the file and are ready to read or write.

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Opening Files in C and C++ : unix syntax fd = open(filename, flags [, pmode]);

fd = file descriptor filename = physical file name flags = O_APPEND, O_CREAT, O_EXCL, O_RDONLY,

O_RDWR, O_TRUNC, O_WRONLY. pmode = rwe rwe rwe

owner group world

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FILE *fopen( const char * filename, const char * mode );

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Closing Files

Makes the logical file name available for another physical file (it’s like hanging up the telephone after a call).

Ensures that everything has been written to the file [since data is written to a buffer prior to the file].

Files are usually closed automatically by the operating system (unless the program is abnormally interrupted).

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Reading

Read(Source_file, Destination_addr, Size)

Source_file = location the program reads from, i.e., its logical file name

Destination_addr = first address of the memory block where we want to store the data.

Size = how much information is being brought in from the file (byte count).

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Writing

Write(Destination_file, Source_addr, Size)

Destination_file = the logical file name where the data will be written.

Source_addr = first address of the memory block where the data to be written is stored.

Size = the number of bytes to be written.

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Seeking A program does not necessarily have to read through a

file sequentially: It can jump to specific locations in the file or to the end of file so as to append to it.

The action of moving directly to a certain position in a file is often called seeking.

Seek(Source_file, Offset) Source_file = the logical file name in which the seek will occur Offset = the number of positions in the file the pointer is to be

moved from the start of the file.

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Special Characters in Files I

Sometimes, the operating system attempts to make “regular” user’s life easier by automatically adding or deleting characters for them.

F6 Ctrl – z

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The Unix Directory Structure

The Unix File System is a tree-structured organization of directories. With the root of the tree represented by the character “/”.

Each directory can contain regular files or other directories.

The file name stored in a Unix directory corresponds to its physical name.

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Physical Devices and Logical Files Magnetic disks or tapes can be thought of as files and so can the keyboard and the console.

No matter what the physical form of a Unix file (real file or device), it is represented in the same way in Unix: by an integer.

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Stdout, Stdin, Stderr

Stdout --> Console fwrite(&ch, 1, 1, stdout); Stdin --> Keyboard fread(&ch, 1, 1, stdin); Stderr --> Standard Error (again, Console) [When the compiler detects an error, the error message is written in this file]

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I/O Redirection and Pipes < filename [redirect stdin to “filename”] > filename [redirect stdout to “filename”] program1 | program2 [take any stdout output from program1 and use it in place of any stdin input to program2.

E.g., list | sort

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Unix System Commands

cat filenames --> Print the content of the named textfiles. tail filename --> Print the last 10 lines of the text file. cp file1 file2 --> Copy file1 to file2. mv file1 file2 --> Move (rename) file1 to file2. rm filenames --> Remove (delete) the named files. chmod mode filename --> Change the protection mode on the named file.

ls --> List the contents of the directory. mkdir name --> Create a directory with the given name. rmdir name --> Remove the named directory.

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Text book

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Look up the operations equivalent to open,close,create,read,write, seek in COBOL,Ada,Fortran. compare them with c and c++

How do you use fseek, illustrate What is the difference between pmode,O_RDWR A couple of years ago a company bought a new cobol compiler . One difference between the new compiler and old one is , it did not automatically close files when program execution terminates , where as old compiler did . What sorts of problems would this cause ?

Assignment

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Look up the unix command wc . Execute the following in unix and explain why it gives no.of files in ls|wc-c

In some typical environments such as unix , dos, all the following represent to move the data from one place to another : scanf, fgetc , read , cat , fscanf , gets , < ,main(argc,argv) , getc, fgets , |

Describe which are quite useful , which of them belong to c++ and OS.

Implement tail –n command in your own way

Date of submission : 07-07-14

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