C Error Handling

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C programming doesn't provide any direct support for the error handling but being a system programming language, it provides you to access at lower level in the form of return values. Most of the C or even Unix function calls return -1 or NULL in case of any error and sets an error code.

C developers should set errno to 0 at the time of initialization of the program. A value of 0 indicates that there is no error in the program.

C provides the two functions named perror() and strerror(), can be used to display the text message associated with the errno.

The function perror(), displays the string you pass to it, followed by a colon, a space, and then the textual representation of the current errno value
The function strerror(), which returns a pointer to the textual representation of the current errno value

Handle Error in Opening a File in C

The following C program illustrates how to handle the error that occurs in opening a file.

/* C Error Handling - Error Handling Example Program */
      
#include<stdio.h>
#include<errno.h>
#include<string.h>
#include<conio.h>

extern int errno;

void main()
{
   FILE *fp;
   int errnum;
   char fname[20];
   clrscr();

   printf("Enter filename (not exist in the current directory): ");
   gets(fname);
   fp=fopen(fname, "r");
   if(!fp)
   {
      errnum = errno;
      fprintf(stderr, "Value of errno: %d\n", errno);
      perror("Error printed by perror");
      fprintf(stderr, "Error in opening file: %s\n", strerror( errnum ));
   }
   else
   {
      fclose(fp);
   }

   getch();
}

Here is the sample output of the above C program.

Handle Divide by Zero Error in C

It is a good practice to check divide by zero error. Because sometimes, user might enter a value 0 to the denominator. Here this program illustrates the concept of checking the divide by zero error.

/* C Error Handling - This program illustrates
 * the concept of error handling in C programming
 */

#include<stdio.h>
#include<stdlib.h>
#include<conio.h>

void main()
{
   int numerator;
   int denominator;
   int quotient;
   clrscr();

   printf("Enter the value of numerator and denominator: ");
   scanf("%d%d", &numerator, &denominator);

   if(denominator == 0)
   {
      fprintf(stderr, "Divide by zero error..!!");
      printf("\nPress any key to exit...\n");
      getch();
      exit(-1);
   }
   quotient = numerator/denominator;
   fprintf(stderr, "Value of quotient: %d\n", quotient );

   getch();
}

Here is the two sample run of this C program. The first one for no error and the second one for divide by zero error.

C Program Exit Status

There are the following two type of exit status:

EXIT_SUCCESS - defined as 0
EXIT_FAILURE - defined as -1

Here, this C program illustrates the concept of program exit status i.e., EXIT_FAILURE and EXIT_SUCCESS.

/* C Error Handling - This program illustrates
 * the EXIT_SUCCESS and EXIT_FAILURE
 */

#include<stdio.h>
#include<stdlib.h>
#include<conio.h>

void main()
{
   int num;
   int den;
   int quot;
   clrscr();

   printf("Enter the value of numerator and denominator: ");
   scanf("%d%d", &num, &den);

   if(den == 0)
   {
      fprintf(stderr, "Divide by zero error..!!");
      printf("\nPress any key to exit...\n");
      getch();
      exit(EXIT_FAILURE);
   }
   quot = num/den;
   fprintf(stderr, "Value of quotient: %d\n", quot);

   printf("Press any key to exit...");
   getch();
   exit(EXIT_FAILURE);
}

On compile and executing the above code, it will produce the following result:

Now let's take a look at the mnemonics.

_doserrno

Variable indicating actual DOS error code. Here is the syntax to declare _doserrno:

int _doserrno

When an MS-DOS system call results in an error, _doserrno is set to the actual DOS error code. Here the following table lists the mnemonics for the actual DOS error codes which _doserrno can be set:

Mnemonic	DOS error code
E2BIG	Bad environ
EACCES	Access denied
EACCES	Bad access
EACCES	Is current dir
EBADF	Bad handle
EFAULT	Reserved
EINVAL	Bad data
EINVAL	Bad function
EMFILE	To many open
ENOENT	No such file or directory
ENOEXEC	Bad format
ENOMEM	Mcb destroyed
ENOMEM	Out of memory
ENOMEM	Bad block
EXDEV	Bad drive
EXDEV	Not same device

errno

Variable indicating type of error. Here is the syntax to declare errno:

extern int errno;

Whenever an error in a system call occurs, errno is set to a value indicating the type of error. Here value can be:

Error numbers in errno

Here the following table lists the mnemonics and meanings for the error numbers found in errno. Each value listed can be used to index into the sys_errlist array for displaying messaged. Also, perror will display message.

Mnemonic	Meaning
EZERO	Error 0
EINVFNC	Invalid function number
ENOFILE	File not found
ENOPATH	Path not found
ECONTR	Memory blocks destroyed
EINVMEM	Invalid memory block address
EINVENV	Invalid environment
EINVFMT	Invalid format
EINVACC	Invalid access code
EINVDAT	Invalid data
EINVDRV	Invalid drive specified
ECURDIR	Attempt to remove CurDir
ENOTSAM	Not same device
ENMFILE	No more files
ENOENT	No such file or directory
EMFILE	Too many open files
EACCES	Permission denied
EBADF	Bad file number
ENOMEM	Not enough memory
ENODEV	No such device
EINVAL	Invalid argument
E2BIG	Arg list too long
ENOEXEC	Exec format error
EXDEV	Cross-device link
EDOM	Math argument
ERANGE	Result too large
EFAULT	Unknown error
EEXIST	File already exists

C Online Test

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