File Handling in C

File – place on disc where group of related data is stored.
E.g. your C programs, executable, word file, excel file etc.

High-level programming languages support file operations.

• Naming
• Opening
• Reading
• Writing
• Closing 

When to use File Handling?

• Large data volumes
• E.g. physical experiments, human genome, population records etc.
• Need for store/retrieve data

Defining and opening file:-

To store data file in secondary memory (disc) must specify to OS.

1. File name (e.g. fun.c, result.data).
2. Data structure (e.g. FILE).
3. Purpose (e.g. reading, writing, appending).

Different modes:-

• Writing mode(w) :- if file already exists then contents are deleted, else new file with specified name created.

• Appending mode(a):- if file already exists then file opened with contents safe else new file created.

• Reading mode(r):- if file already exists then opened with contents safe else error occurs.  

Input/Output operations on files:

C provides several different functions for reading/writing.

• getc() – read a character
• putc() – write a character
• fprintf() – write set of data values 
• fscanf() – read set of data values
• getw() – read integer 
• putw() – write integer

Errors that occur during I/O

Typical errors that occur

• trying to read beyond end-of-file.
• trying to use a file that has not been opened.
• perform operation on file not permitted by ‘fopen’ mode.
• open file with invalid filename.
• write to write-protected file.

C Program to demonstrate File handling program

#include<conio.h>
#include<stdio.h>
void main()
{
char c ;
FILE *fptr1 ;
clrscr() ;
printf("Enter the text to be stored in the file.\n") ;
printf("Use ^Z or F6 at the end of the text and pressENTER: \n\n") ;
fptr1 = fopen("myfile.DAT","w") ;
while((c = getc(stdin)) != EOF)
fputc(c, fptr1) ;
fclose(fptr1) ;
printf("\nThe content of the file is : \n\n") ;
fptr1 = fopen("myfile.DAT", "r") ;
do
{
c = fgetc(fptr1) ;
putchar(c) ;
} while(c != EOF) ;
fclose(fptr1) ;
getch() ;
}

/* opening the contents of the file */
#include <stdio.h>
#include <conio.h>
void main()
{
FILE *fp;
int c ;
fp = fopen("file2.c", "r");
c = getc( fp ) ;
while (  c != EOF )
{
putchar(c);
c=getc (fp);
}
fclose(fp) ;
}

/* open the file based on user input */
#include <stdio.h>
#include <conio.h>
void main()
{
FILE *fopen(), *fp;
int c ;
char filename[40] ;
clrscr();
printf("Enter file to be displayed:");
gets(filename) ;
fp=fopen(filename,"r");
c=getc(fp) ;
while(c!=EOF )
{
putchar(c);
c=getc(fp );
}
fclose(fp);
getch();
}

/* Count no. of characters in a file*/
#include <stdio.h>
#include <conio.h>
void main() 
/* Prompt user for file and count number of characters 
       and lines in it*/
{
FILE *fp;
int c , nc, nlines;
char filename[40] ;
clrscr();
nlines=0 ;
nc=0;

printf("Enter file name: ");
gets( filename );

fp=fopen( filename, "r" );

if(fp == NULL)
{
printf("Cannot open %s for reading \n", filename );
exit(0);      /* terminate program */
}
c = getc( fp ) ;
while (  c != EOF )
{
if ( c  ==  '\n'  )
nlines++ ;
nc++ ;
c = getc ( fp );
}

fclose( fp );

if ( nc != 0 )
{
printf("There are %d characters in %s \n", nc, filename );
printf("There are %d lines \n", nlines );
}
else
printf("File: %s is empty \n", filename );
getch();
}

/* compare two files */
#include <conio.h>
#include <stdio.h>
void main()
{
FILE *fp1, *fp2;
int ca, cb;
char fname1[40], fname2[40] ;
printf("Enter first filename:") ;
gets(fname1);
printf("Enter second filename:");
gets(fname2);
fp1 = fopen( fname1,  "r" );       /* open for reading */
fp2 = fopen( fname2,  "r" ) ;      /* open for reading */
if ( fp1 == NULL )      /* check does file exist etc */
{
printf("Cannot open %s for reading \n", fname1 );
exit(0);    /* terminate program */
}
else if ( fp2 == NULL )
{
printf("Cannot open %s for reading \n", fname2 );
exit(0);    /* terminate program */
}
else        /* both files opened successfully  */
{
ca  =  getc( fp1 ) ;
cb  =  getc( fp2 ) ;
while ( ca != EOF   &&   cb != EOF   &&   ca == cb  )
{
ca  =  getc( fp1 ) ;
cb  =  getc( fp2 ) ;
}
if (  ca == cb )
printf("Files are identical \n");
else if ( ca !=  cb )
printf("Files differ \n" );
fclose ( fp1 );
fclose ( fp2 );
}
getch();
}

/* Copy file3.c to file3.txt */ 
#include <conio.h>
#include <stdio.h>
void main()  
{
FILE *fp1, *fp2; 
int c ; 

fp1 = fopen( "file3.c",  "r" );
fp2 = fopen( "file3.txt", "w" );
if ( fp1 == NULL )
{
printf("Cannot open file3.c for reading \n" );
exit(1);
}
else if ( fp2 == NULL )
{
printf("Cannot open file3.txt for writing \n");
exit(1);
}
else
{
c = getc(fp1) ;
while ( c != EOF)
{
putc( c,  fp2);
c =  getc( fp1 ) ;
}
fclose ( fp1 );
fclose ( fp2 );
printf("Files successfully copied \n");
}
getch();
}

/*Program using fprintf & fscanf functions in file Handling */ 
#include<stdio.h>  
#include<conio.h>  
main()  
{  
FILE *fp;  
int num,qty,I;  
float price,value;  
char item[10],filename[10];  
printf(“Enter filename”);  
scanf(“%s”,filename);  
fp=fopen(filename,”w”);  
printf(“Input Some data\n\n”0;  
printf(“Enter Values for Item name number price quantity\n”);  
for (I=1;I< =3;I++)  
{  
fscanf(stdin,”%s%d%f%d”,item,&number,&price,&quality);  
fprintf(fp,”%s%d%f%d”,itemnumber,price,quality);  
}  
fclose (fp);  
fprintf(stdout,”\n\n”);  
fp=fopen(filename,”r”);  
/*Reading data from file*/  
printf(“Item name number price quantity value”);  
for(I=1;I< =3;I++)  
{  
fscanf(fp,”%s%d%f%d”,item,&number,&prince,&quality);  
value=price*quantity”);  
fprintf(“stdout,”%s%d%f%d%d\n”,item,number,price,quantity,value);  
}  
fclose(fp);  
} 

/*C Program to handle mixed data types*/ 
#include<stdio.h> 
#include<conio.h> 
main() 
{ 
FILE *fp; 

int num,qty,I; 

float price,value; 

char item[10],filename[10]; 

printf("Input filename"); 
scanf("%s", filename); 

fp=fopen(filename,"w"); 
printf("Input inventory datann"); 
printf("Item name, number, price, quantity\n"); 
for I=1;I< =3;I++) 
{ 
fscanf(stdin,"%s%d%f%d",item,&number,&price,&quality); 
fprintf(fp,"%s%d%f%d",itemnumber,price,quality); 
} 
fclose (fp); 
fprintf(stdout,"nn"); 
fp=fopen(filename,"r"); 
printf("Item name number price quantity value"); 
for(I=1;I< =3;I++) 
{ 
fscanf(fp,"%s%d%f%d",item,&number,&prince,&quality); 
value=price*quantity”); 
fprintf(stdout,"%s%d%f%d%dn",item,number,price,quantity,value); 
} 
fclose(fp); 
}

Pointers ==>>