FILE HANDLING IN C

INTRODUCTION

Computer stores programs and data in secondary memory in the form of files. Storing programs and data in primary memory is not preferred due to its volatility nature which loses its contents when the power goes off.

Interaction between program console and file

File

It is collection of related data stored on secondary storage devices like disk.

Files represent programs and data. Data may be numeric, alphabetic or alphanumeric.

A file is named and is referred to by its name.

It is collection of bits, bytes, lines or records.

Types of Files

Text Files

Binary Files

Secondary memory Data files

Primary memory

Programs+Data

Keyboard o/p screen

get data from Write data

Write data Read data Program-file

Program-console

Text Files

They are the files that contains letters, digits and symbols. These Files contains the characters coded from ASCII Character set.

Binary Files

It is sequential access file in which data are stored and read back one after the another in binary format instead of ASCII format.

Operations on Files

The common operations that can be performed on files are

Naming File

Opening and Closing of files

Reading and Writing of Files

Naming of File

The file name must be specified for particular file. The file name is string of characters. The valid file names are

File1.txt, File2.c, File3.h, File4.dat and so on.

If the file is used for storing text .txt extension is used. If the file stores C program .c extension is used. If the file is header file .h extension is used.

Opening and Closing of Files

fopen()

The function fopen() is used for opening file. Every File that is opened returns a pointer to particular file. The syntax is given below

fptr=fopen(“filename or path of file”,mode);

filename or path of file is the name of file being opened. If path of file is not mentioned file exists in current directory.

Eg: If you are working in C:\Tc and if the path of file is not mentioned then file exists in C:\Tc directory.

Mode of opening can be any one of following:

Text Files

r Opens File for reading only.

w Opens File for writing only.

a Opens File for appending only.

r+ Opens File for reading and writing.

w+ Opens File for reading and writing.

a+ Appends file for read/write

Binary Files

rb Opens File for reading only.

wb Opens File for writing only.

ab Opens File for appending only.

rb+ Opens File for reading and writing.

wb+ Opens File for reading and writing.

ab+ Appends file for read/write

‘fptr’ is pointer variable that contains the address of structure FILE that has been defined in stdio.h header file.

Eg

FILE *fptr;

fptr=fopen(“text.txt”,”r”);

the text.txt file is opened for reading only.

fclose()

fclose() is used for closing file which is being opened. The syntax is

fclose(fptr);

Eg:

FILE *fptr;

Fptr=fopen(“text.txt”,”r”);

/* perform operations on file */

fclose(fp);

FILE I/O FUNCTIONS(READING AND WRITING OF FILES)

Two Categories of File I/O functions

1. Unformatted ( No format for I/O)

2. Formatted

Unformatted File I/O functions

Character oriented File I/O functions( fgetc() and fputc())

String oriented File I/O functions(fgets() and fputs())

Formatted File I/O functions

fprintf() and fscanf()

Character Oriented File I/O functions

fgetc()

It is used to read character from file. Syntax is given below

ch=fgetc(fptr);

ch is character variable and fptr is pointer to FILE. This function reads the character from file and returns character value which is collected by variable ‘ch’

Example explanation of fgetc()

Step 1: opens File in read mode which is already been created.

Step2: Read individual character from the file till end of file is reached.

Step 3: Close the file.

Creating File say ‘file1.txt’.

Left click ->new->text document

Write data to file if data does not exist manually or using program

Program

#include<stdio.h>#include<conio.h>main(){ FILE *fp; char ch; clrscr(); fp=fopen("text.txt","r"); /* opens file for reading */ ch=fgetc(fp);/* read character from file and assign it to ch */ printf("%c",ch); /* Loop until the end of file is reached */ while(ch!=EOF) { ch=fgetc(fp); printf("%c",ch);

} fclose(fp);}

O/P

fputc()

It is used for writing character to file. The syntax is as follows

fputc(ch,fptr);

This function writes the contents of character ch to file represented by fptr.

Example Explanation for fputc()

Step 1: opens file in write mode which is already existed in current directory or create file which is not existing in current directory.

Step2: Write individual character one by one from program to the file.

Step3: Close the file.

Previous data in text.txt file without writing content

Program

#include<stdio.h>#include<conio.h>main(){ FILE *fp; char ch; clrscr(); fp=fopen("text.txt","w"); /* opens file for writing */ /* when the file is opened in write mode in already existing file the data in previous file will be overwritten with new data */ ch=getchar();/* read character from keyboard inorder to write data to file*/ /* Loop until the end of file is reached */ while(ch!=EOF) { fputc(ch,fp); ch=getchar(); } fclose(fp);}

Input from keyboard to write data to file

After writing content to file contents of text.txt file are overwritten with new data

Writing content to file which is not existing in current directory

Program

#include<stdio.h>#include<conio.h>main(){ FILE *fp; char ch; clrscr(); fp=fopen("text1.txt","w"); /* opens file for writing */ /* when the file is opened in write mode in already existing file the data in previous file will be overwritten with new data */ ch=getchar();/* read character from keyboard inorder to write data to file*/ /* Loop until the end of file is reached */ while(ch!=EOF) { fputc(ch,fp); ch=getchar(); } fclose(fp);}After compiling and running the above program new file text1.txt will be created.

The contents of text1.txt file are empty

Take input from keyboard to write data to file

Now the contents of text1.txt file are

Appending the data to file say text1.txt which is already existing in current directory and data in that file is not empty.

Program

#include<stdio.h>#include<conio.h>main(){ FILE *fp; char ch; clrscr(); fp=fopen("text1.txt","a"); /* opens file for appending data at end */ /* when the file is opened in append mode in already existing file the data will be added to the previous file at the end */ ch=getchar();/* read character from keyboard inorder to append data to file*/ /* Loop until the end of file is reached */ while(ch!=EOF) { fputc(ch,fp); ch=getchar(); } fclose(fp);}

Input from Keyboard

Contents of text1.txt file after appending are

String oriented I/O functions

fputs()

This function is used to write string to file. The syntax is given below

fputs(buffer,fptr);

buffer is the name of character array and fptr is pointer to FILE type. This function writes the string represented by buffer to file pointed to by fptr.

Example Explanation for fputs()

Step 1: Creates the file which is not existing in current directory or opens the file for writing string to file which is existing in current directory.

Step 2: Write the string to file.

Step 3: Close the file.

Program

#include<stdio.h>#include<conio.h>main(){ FILE *fp; char str[10]; int n,i; clrscr(); fp=fopen("text2.txt","w"); /* opens file for writing */ /* when the file is opened in write mode in already existing file the data in previous file will be overwritten with new data */ printf("enter number of students"); scanf("%d",&n); fflush(stdin);/* remove the previous data from input buffer */ /* loop through the number of students to enter names of students to write data to file */ for(i=1;i<=n;i++) { gets(str); fputs(str,fp); } fclose(fp);}

Input from keyboard to write String to text2.txt file

The contents of text2.txt file after writing string are

fgets()

This function is used to read string from file upto its specified size. The syntax of fgets() is given below

fgets(buffer,size,fptr);

buffer is name of character array,size is integer value, fptr is pointer to FILE type. This function reads string of maximum size-1 characters from file pointed to by fptr and copies data to the memory area denoted by buffer.

Example Explanation of fgets()

Step 1: Opens the file in read mode.

Step 2: Read the contents from file upto its maximum size.

Step 3: Close the file.

Program#include<stdio.h>#include<conio.h>main(){ FILE *fp; char str[10]; int size; clrscr(); fp=fopen("text2.txt","r"); /* opens file for reading string from file */ printf("enter size"); scanf("%d",&size); fgets(str,size,fp);/* reads string of size -1 characters from file pointed to by fp and store data in str */ puts(str);/* prints the string contents */ fclose(fp);}

O/P

Formatted I/O functions

fprintf()

This function is used to write data items which may be of different types to file. The syntax is given below

fprintf(fp,”control string”,argument-list);

fp is file pointer pointing to FILE type,control string specifies format specifiers and argument list contains the comma separated variables, values of which are to be written to the file.

Example Explanation of fprintf() function

Program#include<stdio.h>#include<conio.h>struct student{int rollno;char name[100];};main(){ struct student s; FILE *fp; int i,n; clrscr(); fp=fopen("text3.txt","w"); /* opens file for writing data types to file */ printf("enter number of student records"); scanf("%d",&n); for(i=1;i<=n;i++) { scanf("%d%s",&s.rollno,&s.name);/* scan student details */ fprintf(fp,"%d%s",s.rollno,s.name);/* write student data to file */ }

fclose(fp);}

Input from keyboard to write items of different types to file

The contents of text3.txt after writing are

fscanf()

This function is used to read multiple data items which may be of different types from file. The syntax is given below

fscanf(fptr,”control string”,arguments list);

fp is file pointer pointing to FILE type,control string specifies format specifiers and argument list contains the comma separated variables preceded by & symbol into which data read from file are to be copied.

Example Explantion of fscanf()

Program#include<stdio.h>#include<conio.h>struct student{int rollno;char name[100];};main(){ struct student s; FILE *fp; int i,n; clrscr(); fp=fopen("text3.txt","r"); /* opens file for reading data types from file */ while(!feof(fp)) /* loop through end of file is reached */ { fscanf(fp,"%d%s",&s.rollno,&s.name);/* scan student details from the file */ printf("%d%s",s.rollno,s.name);/* print student data */ } fclose(fp);}

O/P

Reading and Writing data to and from binary files

The main disadvantage of fprintf() is all data types are treated as characters for eg 234 occupies two bytes in memory but when data is stored in disk it occupies 3 bytes of memory. Thus large amount of data requires large space on disk. Hence storing data in text mode on disk turns out to be inefficient method.

To overcome this drawback the files should be read and written in binary mode. The functions used for reading and writing data in binary mode are

fread() fwrite()

fwrite() This function is used for writing structure block to file. The syntax is given below.

fwrite(&structure-variable,sizeof(structure),integer,fptr);

The first argument is address of structure to be written to disk, the second argument specifies size of structure in bytes, integer is number of records and fptr is pointer to file type.Example Explanation of fwrite()Program#include<stdio.h>#include<conio.h>struct student{int rollno;char name[100];};main()

{ struct student s; FILE *fp; int i,n; clrscr(); fp=fopen("text4.txt","wb"); /* opens binary file for writing structure of data*/ printf("enter the number of records"); scanf("%d",&n); for(i=1;i<=n;i++) { scanf("%d%s",&s.rollno,&s.name);/* scan student details */ fwrite(&s,sizeof(s),1,fp);/* write structure to file */ } fclose(fp);}Input from keyboard to write structure block to file

The contents of text4.txt are

fread() This function is used for reading structure block from file. The syntax is given below.

fread(&structure-variable,sizeof(structure),integer,fptr);

The first argument is address of structure to be read from disk, the second argument specifies size of structure in bytes, integer is number of records and fptr is pointer to file type.Example Explanation of fread()Program#include<stdio.h>#include<conio.h>struct student{int rollno;char name[100];};main(){ struct student s; FILE *fp; clrscr(); fp=fopen("text4.txt","rb"); /* opens binary file for reading structure block from file*/ while(fread(&s,sizeof(s),1,fp)) /* read strucure data from file till there is no content in file */ { printf("%d%s",s.rollno,s.name); }

fclose(fp);}O/P

Random Accessing of Files

The functions used for randomly accessing data from file are

fseek() ftell() rewind()

fseek() This function is used for setting pointer position in file at specified position. The syntax is given below

fseek(fp,displacement,pointer_position);

fp is pointer to FILE type. Displacement is positive or negative. This is number of bytes which are skipped backward(if negative) or forward(if positive) from current pointer position. Pointer_position can take one of 3 values 0,1,2. The meanings of values are given below.

Pointer_position Macro name Meaning0 SEEK_SET Beginning of file1 SEEK_CUR Current position of file2 SEEK_END End of file

Eg:fseek(fp,0,1)-Stay at the current positionfseek(fp,n,1)-move pointer forward by n bytesfseek(fp,-n,1)-go backward by n bytes from current position.

When operation is successful fseek returns zero.If an error occurs fseek returns -1.

ftell()

This function gives the value of current position pointer in file.

Syntaxftell(fp)

rewind()

This function is used for moving pointer to beginning of file

Example Explanation of functions(fseek(),ftell())

The contents of text1.txt file are

Program#include<stdio.h>#include<conio.h>main(){ FILE *fp; char ch; clrscr(); fp=fopen("text1.txt","r"); /* opens file for reading*/ fseek(fp,5,1); printf("File pointer position is %d",ftell(fp)); printf("\n");

while(!feof(fp)) { ch=fgetc(fp); printf("%c",ch);

} fclose(fp);}

O/P

Error Handling FunctionsThe situations under which I/O operations fail are

Trying to use file that has not been opened. Trying to open file which does not exist for reading purpose. Trying to open file for writing purpose when there is no disk space. Trying to write to read only file

The error handling functions are ferror() feof()

ferror()

This function takes file pointer as argument and returns non zero value if error is found such as writing data to file which is opened in read mode etc and zero otherwise.

Syntaxferror(fp)

feof()This function avoids reading file after the end of file mark.Syntaxfeof(fp)

Example Explanation

Trying to open file which doesnot exist in current directoryProgram#include<stdio.h>#include<conio.h>#include<stdlib.h>main(){ FILE *fp; char ch; clrscr(); fp=fopen("text5.txt","r"); if(fp==NULL) { printf("The file doesnot exist"); exit(1); } fclose(fp);}

Program

#include<stdio.h>#include<conio.h>#include<stdlib.h>main(){ FILE *fp,*fp1; char ch; clrscr(); fp=fopen("text1.txt","r"); if(fp==NULL) { printf("The file doesnot exist"); exit(1); } fclose(fp); fp1=fopen("text2.txt","r"); fputc('c',fp1); if(ferror(fp1)) { printf("The file is opened in read mode.it is not used for writing"); exit(1); } fclose(fp1);}O/P