TUTORIAL: ERROR HANDLING IN C

Beginners Tutorial: Handling errors in C ( Use of errno.h )

Hello CEans! In the following tutorial, you will learn how to handle the wrong data entered by users.

INTRODUCTION:

There are many exceptions regarding the range of a particular datatype in C. Each datatype has a certain range, and if the range is exceeded, the compiler fails to recognize the datatype and produces a garbage value to the variable (to know about garbage values in C, #-Link-Snipped-#) and hence, departs from the expected output and sometimes the program even crashes. Definitely, we don't want our software to crash (taking in consideration of big projects). So, to handle these errors or wrong data or to display alerts when the wrong data is entered, we use error exception concept.

EXAMPLE:

Let us consider the following code snippet of fibonacci series. Normally, people write the fibonacci
sequence program, as shown below.

#include <stdio.h>
int main()
{
  int  range, i, a = 0, b = 1, c;
  printf("Enter the range: ");
  scanf("%d",&range);
  printf("Fibonacci sequence for %d range is \n\n",range);
  printf("%d %d ",a,b);
  for(i = 3; i <= range; i++)
  {
      c = a+ b;
      a = b;
      b = c;
      printf("%d ",c);
  }
  fflush(stdin);
  getchar();
  return 0;
}

The program produces perfect output for many ranges. But there are some exceptions.
1) What if the range is -ve?
2) What if the range is less than or equal to 1?
3) What if the range is big?? Suppose above 50?
These are the outputs produced for the above exceptions, which are surely not the outputs
which we expected.





HOW TO OVERCOME:

These are the steps to be followed to overcome these exceptions!
1) checking the condition for range.
2) printing the error.
3) Re- starting the program to get a desired range.

1> To check the error.
we all know we can use conditional statements if-else. To know about it <a href="https://www.tutorialspoint.com/ansi_c/c_control_statements.htm" target="_blank" rel="nofollow noopener noreferrer">C - Flow Control Statements</a>

2> Printing the error.
So, this is what the tutorial is all about. How to print the error??
Using printf() ?? No, Why to type our own error statements, when the C environment provides the inbuilt error statements.
Just include the <errno.h> header file and print the error using perror(). 😀
To use perror(), you must know what error statements will be printed for whant kind of error. Have a look at the <errno.h> in my system. This may vary in your system.

#define EPERM        1    /* Operation not permitted */
#define    ENOFILE        2    /* No such file or directory */
#define    ENOENT        2
#define    ESRCH        3    /* No such process */
#define    EINTR        4    /* Interrupted function call */
#define    EIO        5    /* Input/output error */
#define    ENXIO        6    /* No such device or address */
#define    E2BIG        7    /* Arg list too long */
#define    ENOEXEC        8    /* Exec format error */
#define    EBADF        9    /* Bad file descriptor */
#define    ECHILD        10    /* No child processes */
#define    EAGAIN        11    /* Resource temporarily unavailable */
#define    ENOMEM        12    /* Not enough space */
#define    EACCES        13    /* Permission denied */
#define    EFAULT        14    /* Bad address */
/* 15 - Unknown Error */
#define    EBUSY        16    /* strerror reports "Resource device" */
#define    EEXIST        17    /* File exists */
#define    EXDEV        18    /* Improper link (cross-device link?) */
#define    ENODEV        19    /* No such device */
#define    ENOTDIR        20    /* Not a directory */
#define    EISDIR        21    /* Is a directory */
#define    EINVAL        22    /* Invalid argument */
#define    ENFILE        23    /* Too many open files in system */
#define    EMFILE        24    /* Too many open files */
#define    ENOTTY        25    /* Inappropriate I/O control operation */
/* 26 - Unknown Error */
#define    EFBIG        27    /* File too large */
#define    ENOSPC        28    /* No space left on device */
#define    ESPIPE        29    /* Invalid seek (seek on a pipe?) */
#define    EROFS        30    /* Read-only file system */
#define    EMLINK        31    /* Too many links */
#define    EPIPE        32    /* Broken pipe */
#define    EDOM        33    /* Domain error (math functions) */
#define    ERANGE        34    /* Result too large (possibly too small) */
/* 35 - Unknown Error */
#define    EDEADLOCK    36    /* Resource deadlock avoided (non-Cyg) */
#define    EDEADLK        36
/* 37 - Unknown Error */
#define    ENAMETOOLONG    38    /* Filename too long (91 in Cyg?) */
#define    ENOLCK        39    /* No locks available (46 in Cyg?) */
#define    ENOSYS        40    /* Function not implemented (88 in Cyg?) */
#define    ENOTEMPTY    41    /* Directory not empty (90 in Cyg?) */
#define    EILSEQ        42    /* Illegal byte sequence */

These are the error codes provided for various problems in the programming.
For, the program of fibonacci sequence the error code I will be using is EINVAL and ERANGE
For using this, you just need to declare a few things as,

errno = EINVAL;              // errno is inbuilt name. No need to be declared in main()
perror(NULL);                  // prints the error for EINVAL

Similarly,

errno = ERANGE;
perror(NULL);

You are even allowed to write a small statement in perror() as

perror("BIG RANGE");

The output for this error will print "BIG RANGE: result too large"

3> Re-starting the program to get a desired range.
This can be done in two ways! Using goto or by just using loops!
As goto causes ambiguity in big programs, we avoid using it.


IMPLEMENTATION IN PROGRAM:

So after knowing about handling the exceptions, we just implement it in our program as below.. 😀

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
 
int main()
{
  int n, a = 0, b = 1, c, i, done = 0;
  while(!done)        /* error detection loop. If detected, program restarts*/
  {
  printf("Enter the number of terms\n");
  scanf("%d",&n);
  if(n <= 0)
  {
    errno = EINVAL; 
    perror("Range must be positive");
    fflush(stdin);
    getchar();
    done = 0;
    system("cls");
  }
  else if(n > 20)
    {
      errno = ERANGE; 
      perror("BIG RANGE! ");
      fflush(stdin);
      getchar();
      done = 0;
      system("cls");
    }
  else
    done = 1;
  }
 
  printf("First %d terms of Fibonacci series are : \n",n);
  if(n>1)
  {       
    printf("%d %d ",a,b);
    for ( i = 2 ; i < n ; i++ )
    {
      c = a + b;
      a = b;
      b = c;
      printf("%d ",c);
    }
  }
  else
  printf("0");
 
  fflush(stdin);
  getchar();
  return 0;
}

OUTPUT FOR THE THREE EXCEPTIONS: