C – Error Handling, Recursion and Variable Arguments

C – Error Handling

As such C programming does not provide direct support for error handling but being a system programming language, it provides you 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 errno is set which is global variable and indicates an error occurred during any function call. You can find various error codes defined in <error.h> header file.

So a C programmer can check the returned values and can take appropriate action depending on the return value. As a good practice, developer 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.

The errno, perror() and strerror()

The C programming language provides perror() and strerror() functions which can be used to display the text message associated with errno.

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

Let’s try to simulate an error condition and try to open a file which does not exist. Here I’m using both the functions to show the usage, but you can use one or more ways of printing your errors. Second important point to note is that you should use stderr file stream to output all the errors.

#include <stdio.h>
#include <errno.h>
#include <string.h>

extern int errno ;

int main ()
{
   FILE * pf;
   int errnum;
   pf = fopen ("unexist.txt", "rb");
   if (pf == NULL)
   {
      errnum = errno;
      fprintf(stderr, "Value of errno: %d\n", errno);
      perror("Error printed by perror");
      fprintf(stderr, "Error opening file: %s\n", strerror( errnum ));
   }
   else
   {
      fclose (pf);
   }
   return 0;
}

When the above code is compiled and executed, it produces the following result:

Value of errno: 2
Error printed by perror: No such file or directory
Error opening file: No such file or directory

Divide by zero errors

It is a common problem that at the time of dividing any number, programmers do not check if a divisor is zero and finally it creates a runtime error.

The code below fixes this by checking if the divisor is zero before dividing:

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

main()
{
   int dividend = 20;
   int divisor = 0;
   int quotient;
 
   if( divisor == 0){
      fprintf(stderr, "Division by zero! Exiting...\n");
      exit(-1);
   }
   quotient = dividend / divisor;
   fprintf(stderr, "Value of quotient : %d\n", quotient );

   exit(0);
}

When the above code is compiled and executed, it produces the following result:

Division by zero! Exiting...

Program Exit Status

It is a common practice to exit with a value of EXIT_SUCCESS in case of programming is coming out after a successful operation. Here, EXIT_SUCCESS is a macro and it is defined as 0.

If you have an error condition in your program and you are coming out then you should exit with a status EXIT_FAILURE which is defined as -1. So let’s write above program as follows:

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

main()
{
   int dividend = 20;
   int divisor = 5;
   int quotient;
 
   if( divisor == 0){
      fprintf(stderr, "Division by zero! Exiting...\n");
      exit(EXIT_FAILURE);
   }
   quotient = dividend / divisor;
   fprintf(stderr, "Value of quotient : %d\n", quotient );

   exit(EXIT_SUCCESS);
}

When the above code is compiled and executed, it produces the following result:

Value of quotient : 4

C – ┬áRecursion

Recursion is the process of repeating items in a self-similar way. Same applies in programming languages as well where if a programming allows you to call a function inside the same function that is called recursive call of the function as follows.

void recursion()
{
   recursion(); /* function calls itself */
}

int main()
{
   recursion();
}

The C programming language supports recursion, i.e., a function to call itself. But while using recursion, programmers need to be careful to define an exit condition from the function, otherwise it will go in infinite loop.

Recursive function are very useful to solve many mathematical problems like to calculate factorial of a number, generating Fibonacci series, etc.

Number Factorial

Following is an example, which calculates factorial for a given number using a recursive function:

#include <stdio.h>

int factorial(unsigned int i)
{
   if(i <= 1)
   {
      return 1;
   }
   return i * factorial(i - 1);
}
int  main()
{
    int i = 15;
    printf("Factorial of %d is %d\n", i, factorial(i));
    return 0;
}

When the above code is compiled and executed, it produces the following result:

Factorial of 15 is 2004310016

Fibonacci Series

Following is another example, which generates Fibonacci series for a given number using a recursive function:

#include <stdio.h>

int fibonaci(int i)
{
   if(i == 0)
   {
      return 0;
   }
   if(i == 1)
   {
      return 1;
   }
   return fibonaci(i-1) + fibonaci(i-2);
}

int  main()
{
    int i;
    for (i = 0; i < 10; i++)
    {
       printf("%d\t%n", fibonaci(i));
    }
    return 0;
}

When the above code is compiled and executed, it produces the following result:

0	1	1	2	3	5	8	13	21	34

C – Variable Arguments

Sometimes, you may come across a situation, when you want to have a function, which can take variable number of arguments, i.e., parameters, instead of predefined number of parameters. The C programming language provides a solution for this situation and you are allowed to define a function which can accept variable number of parameters based on your requirement. The following example shows the definition of such a function.

int func(int, ... ) 
{
   .
   .
   .
}

int main()
{
   func(1, 2, 3);
   func(1, 2, 3, 4);
}

It should be noted that function func() has last argument as ellipses i.e. three dotes () and the one just before the ellipses is always an int which will represent total number variable arguments passed. To use such functionality you need to make use of stdarg.h header file which provides functions and macros to implement the functionality of variable arguments and follow the following steps:

  • Define a function with last parameter as ellipses and the one just before the ellipses is always an int which will represent number of arguments.
  • Create a va_list type variable in the function definition. This type is defined in stdarg.h header file.
  • Use int parameter and va_start macro to initialize the va_list variable to an argument list. The macro va_start is defined in stdarg.h header file.
  • Use va_arg macro and va_list variable to access each item in argument list.
  • Use a macro va_end to clean up the memory assigned to va_list variable.

Now let us follow the above steps and write down a simple function which can take variable number of parameters and returns their average:

#include <stdio.h>
#include <stdarg.h>

double average(int num,...)
{

    va_list valist;
    double sum = 0.0;
    int i;

    /* initialize valist for num number of arguments */
    va_start(valist, num);

    /* access all the arguments assigned to valist */
    for (i = 0; i < num; i++)
    {
       sum += va_arg(valist, int);
    }
    /* clean memory reserved for valist */
    va_end(valist);

    return sum/num;
}

int main()
{
   printf("Average of 2, 3, 4, 5 = %f\n", average(4, 2,3,4,5));
   printf("Average of 5, 10, 15 = %f\n", average(3, 5,10,15));
}

When the above code is compiled and executed, it produces the following result. It should be noted that the function average() has been called twice and each time first argument represents the total number of variable arguments being passed. Only ellipses will be used to pass variable number of arguments.

Average of 2, 3, 4, 5 = 3.500000
Average of 5, 10, 15 = 10.000000