Error Handling During File Operations

ERROR HANDLING DURING FILE OPERATIONS

It is quite common that an error may occur while reading data from or writing data to a file. For example, an error may arise

• when trying to read a file beyond EOF indicator

• when trying to read a file that does not exist

• when trying to use a file that has not been opened

• when trying to use a file in an inappropriate mode, i.e., writing data to a file that has been opened for reading

• when writing to a file that is write-protected (i.e., trying to write to a read-only file)

If we fail to check for errors, then the program may behave abnormally. Therefore, an unchecked error may result in premature termination of the program or incorrect output.

Programming Tip: An error will be generated if you try to read a file that is opened in w mode and vice versa.

In C, the library function ferror() is used to check for errors in the stream. Its prototype can be given as tream). to

int ferror (FILE *stream);

The ferror() function checks for any errors in the stream. It returns value zero if no errors have occurred and a non-zero value if there is an error. The error indication will last until the file is closed unless it is cleared by the clearerr() function. Look at the code given below which uses the ferror ().

#include <stdio.h>

main()

{

FILE *fp;

char feedback [100];

int i;

fp = fopen("Comments.TXT", "w");

if (fp==NULL)           

{

printf("\n The file could not be opened");

exit(1);

printf("\n Provide feedback on this book: ");

gets (feedback);

for (i 0; i < feedback [i]; i++)

fputc(feedback [i], fp);

if (ferror (fp))

{

printf("\n Error writing in file");

exit(1);

}

fclose(fp);

}

When you execute this code and an error occurs while writing the feedback, the program will terminate and a message indicating 'Error writing in file' will be displayed on the screen.

clearerr()

The function clearerr () is used to clear the end-of- file and error indicators for the stream. Its protoype can be given as

void clearerr (FILE *stream);

The clearerr() clears the error for the stream pointed to by stream. The function is used because error indicators are not automatically cleared; once the error indicator for a specified stream is set, operations on that stream continues to return an error value until clearerr, fseek, fsetpos, or rewind is called. Look at the code given below which use the clearerr()

#include <stdio.h>

#include <stdlib.h>

#include <errno.h>

main()

{

FILE *fp;

fp=fopen("Comments.TXT", "w");

if (fp==NULL)

{

perror("OOPS ERROR");

printf("\n error no = %d", errno);

exit(1);

}

printf("\n Kindly give the feedback on this book: ");

gets (feedback);

for (i = 0; i < feedback [i]; i++)

{

fputc(feedback [i], fp);

if (ferror (fp))

{

clearerr (fp);

break;

/* clears the error indicators and jump out of for loop */

}

}

// close the file

fclose(fp);

}

perror()

The function perror () stands for print error. In case of an error, the programmer can determine the type of error that has occurred using the perror () function. The perror () function defined in stdio.h header file is used to handle errors in C programs. When called, perror () displays a message on stderr describing the most recent error that occurred during a library function call or system call. The prototype of perror () can be given as

void perror (char *msg);

The perror() takes one argument msg which points to an optional user-defined message. This message is printed first, followed by a colon, and the implementation-defined message that describes the most recent error.

If a call to perror() is made when no error has actually occurred, then a 'No error' will be displayed. The most important thing to remember is that a call to perror () does nothing to deal with the error condition. It is entirely up to the program to take action. For example, the program may prompt the user to do something such as terminate the program.

Usually the program's action will be determined by checking the value of errno and the nature of the error. In order to use the external constant errno, you must include the header file ERRNO. H. The program given below illustrates the use of perror (). Here we assume that the file Comments.TXT does not exist.

#include <stdio.h>

#include <stdlib.h>

#include <errno.h>

main()

{

FILE *fp;

fp = fopen("Comments.TXT", "w");

if (fp==NULL)

{

perror("OOPS ERROR");

printf("\n error no = %d", errno);

exit(1);

}

printf("\n Provide feedback on this book: ");

gets (feedback);

for (i=0; i<feedback [i]; i++)

fputc(feedback [i], fp);

fclose(fp);

}

Output

OOPS ERROR: No such file or directory 

errno =2