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The response and recovery procedures from error conditions present in a software application are referred to as error handling.

C programming does not provide direct error handling support, but as a system programming language, it does provide access at a lower level in the form of return values.

In the event of an error, most C or Unix function calls return -1 or NULL and set the error code errno. It is declared as a global variable and indicates that an error occurred during any function call. The error.h> header file defines various error codes.

strerror, errno, and perror() ()

The C programming language includes perror() and strerror() functions for displaying the text message associated with errno.

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

Example:

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

extern int errno ;

int main () {

   FILE * pf;
   int errnum;
   pf = fopen ("file.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;
}

Output:

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 when dividing any number, programmers do not check to see if the divisor is zero, resulting in a runtime error.

The code below corrects this by first checking to see 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);
}

Output:

Division by zero! Exiting...

Exit Status:

The C standard defines two constants, EXIT SUCCESS and EXIT FAILURE, which can be passed to exit() to indicate a successful or unsuccessful termination. These are macros defined in the header file stdlib.h.

#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <stdlib.h>
  
int main ()
{
    FILE * fp;
    fp = fopen ("filedoesnotexist.txt", "rb");
  
    if (fp == NULL)
    {
        printf("Value of errno: %d\n", errno);
        printf("Error opening the file: %s\n",
                             strerror(errno));
        perror("Error printed by perror");
  
        exit(EXIT_FAILURE);
        printf("I will not be printed\n");
    }
  
    else
    {
        fclose (fp);
        exit(EXIT_SUCCESS);
        printf("I will not be printed\n");
    }
    return 0;
}

Output:

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

Note: also read about the File handling in C

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