File Organization and Processing CS 215 2 nd Term 2010-2011 Basic file operations Cairo University...

File Organization and Processing

CS 215 2nd Term 2010-2011

Basic file operationsBasic file operations

Cairo University

Faculty of Computers and Information

CS215 – File Structures - Sherif Khattab 22nd Term 2010-2011

course objectives• learn how to use files in your programs

efficiently• understand how database management

systems (e.g., oracle, mysql, sql server) work• learn fundamental problems in computing and

analyze and evaluate their alternative solutions


Why file structures?

• get data from files in as few disk accesses as possible

– disk access is about 1 million times slower than RAM access

• increase the possibility of caching (i.e., reusing the data you fetch from disk into RAM)


Book approach

• study the history of file structure design (since 1970)

– analyze the problems faced and solutions proposed

• history repeats itself :) most of the file design problems that you will face are similar to old problems

• develop two toolkits– conceptual: the common fundamental solutions– object-oriented: a set of c++ classes to

implement these solutions


object-oriented design review

what’s wrong in the above example?– yes, class variables should go into the private

section!– the book is somehow “old” :)


object-oriented design review

class has a pointer, hence the big three are there (destructor, copy constructor, and assignment operator).

– can you write the implementation of these functions?


conversion operator

• we have seen operator overloading of assignment (=), arithmetic (+, -, *, etc.), and boolean (<, >, ==, etc.) operators

• type conversion operators are needed to convert from the class type to other data types

without a conversion operator from String to char *, the third line in the above code gives a compilation error

– strcpy function expects two char * parameters


conversion operator

operator char * is a type conversion operator from String to char *

• the implementation returns a copy (using strdup function) of the string private variable

• in modern c++ compilers, the operator has no return

– operator char *() instead of char * operator char *()


physical and logical files

• phone sets = file handles in the program (e.g., fstream objects)

• the number of files that a program opens concurrently (in the same time) is limited


Feedback


Feedback

• slides• code samples• course is easy, we need more


3 ways to open, read/write, and close files

• in c++, there are 3 ways that a program can use files:

– open, read or write, and close• each method provides a certain level of

abstraction (hiding of details)– 1st method: lowest-level (more details)– 2nd method: a little less details– 3rd method: more details are hidden


1st method (fcntl.h)

• using integer file descriptors– the integer represent the phone set number :)– reading, writing, and closing use that integer

not the file name• flags tell the operating system how you want

your file opened


1st method

• flags can be combined by bit-wise ORING (the | operator)

• the protection mode tells the operating system who and how users can access your file (permissions)


2nd method (stdio.h)

• 2nd method is called C streams• the file descriptor integer is encapsulated in a

sturcture named struct_iobuf or FILE• the type string encapsulates flags of the 1st

method


3rd method (fstream.h)

• 3rd method is called c++ streams• file descriptor is encapsulated in fstream

object• mode defined in the ios class encapsulates

the flags– ios::in, ios::out, ios::noreplace, ios::binary,

ios::nocreate


program to display file contents


1st method#include <fcntl.h>

#include <sys/uio.h>

#include <sys/types.h>

#include <unistd.h>

#include <string.h>

#include <stdio.h>

int main( ) {

char ch;

int file; // file descriptor

char filename[20];

printf("Enter the name of the file: "); // Step 1

fgets(filename, 20, stdin); // Step 2

filename[strlen(filename)-1] = 0;

file = open(filename, O_RDONLY); // Step 3

while (read(file, &ch, 1) != 0) // Step 4a

printf("%c", ch); // Step 4b

close(file); // Step 5

return 0;

}


2nd method


3rd method


comments

• gets is unsafe. why?– use fgets– safer but?

• the return type of the main function• check that the file was opened correctly• check code samples on the course web site!

File Organization and Processing CS 215 2 nd Term 2010-2011 Basic file operations Cairo University...

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