Explain C Error handling functions

File is collection of records or is a place on hard disk, where data is stored permanently.

Operations on files

The operations on files in C programming language are as follows −

• Naming the file
• Opening the file
• Reading from the file
• Writing into the file
• Closing the file

Syntax

The syntax for opening a file is as follows −

FILE *File pointer;

For example, FILE * fptr;

The syntax for naming a file is as follows −

File pointer = fopen ("File name", "mode");

For example,

fptr = fopen ("sample.txt", "r");
FILE *fp;
fp = fopen ("sample.txt", "w");

Error Handling in Files

Some of the errors in files are as follows −

• Trying to read beyond an end of file.
• Device over flow.
• Trying to open an invalid file.
• Performing an invalid operation by opening a file in a different mode.

ferror( )

It is used for detecting an error while performing read / write operations.

The syntax for ferror() function is as follows −

Syntax

int ferror (file pointer);

For example,

Example

FILE *fp;
if (ferror (fp))
printf ("error has occurred");

It returns zero, if success and a non- zero, otherwise.

Program

Following is the C program for using ferror() function −

#include<stdio.h>
int main(){
   FILE *fptr;
   fptr = fopen("sample.txt","r");
   if(ferror(fptr)!=0)
      printf("error occurred");
   putc('T',fptr);
   if(ferror(fptr)!=0)
      printf("error occurred");
   fclose(fptr);
   return 0;
}

Output

When the above program is executed, it produces the following result −

error occurred
Note: try to write a file in the read mode results an error.

perror ( )

It is used for printing an error.

The syntax for perror() function is as follows −

Syntax

perror (string variable);

For example,

Example

FILE *fp;
char str[30] = "Error is";
perror (str);

The output is as follows −

Error is: error 0

Program

Following is the C program for using perror() function −

#include<stdio.h>
int main ( ){
   FILE *fp;
   char str[30] = "error is";
   int i = 20;
   fp = fopen ("sample.txt", "r");
   if (fp == NULL){
      printf ("file doesnot exist");
   }
   else{
      fprintf (fp, "%d", i);
      if (ferror (fp)){
         perror (str);
         printf ("error since file is opened for reading only");
      }
   }
   fclose (fp);
   return 0;
}

Output

When the above program is executed, it produces the following result −

error is: Bad file descriptor
error since file is opened for reading only

feof( )

It is used for checking whether an end of the file has been reached (or) not.

The syntax for feof() function is as follows −

Syntax

int feof (file pointer);

For example,

Example

FILE *fp;
if (feof (fp))
printf ("reached end of the file");

It returns a non-zero, if success and zero, otherwise.

Program

Following is the C program for using feof() function −

 Live Demo

#include<stdio.h>
main ( ){
   FILE *fp;
   int i,n;
   fp = fopen ("number. txt", "w");
   for (i=0; i<=100;i= i+10){
      putw (i, fp);
   }
   fclose (fp);
   fp = fopen ("number. txt", "r");
   printf ("file content is");
   for (i=0; i<=100; i++){
      n = getw (fp);
      if (feof (fp)){
         printf ("reached end of file");
         break;
      }else{
         printf ("%d", n);
      }
   }
   fclose (fp);
   getch ( );
}

Output

When the above program is executed, it produces the following result −

File content is
10 20 30 40 50
60 70 80 90 100
Reached end of the file.