Error Handling in C

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As such, C programming does not provide direct support for error handling, as there are no keywords in C that can prevent errors or exceptions from abruptly terminating the program. However, a programmer can use other functions for error handling.

You can use errno for efficient error handling in C. Additionally other functions that can be used for error handling include perror, strerror, ferror, and clearererr.

The errno Variable

C is a system programming language. It provides you access at a 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 set an error code errno. It is set as a 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. It is a good practice, to set errno to 0 at the time of initializing a program. A value of 0 indicates that there is no error in the program.

The following table shows the errno values and error messages associated with them −

Example

Take a look at the following example −

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

int main() {

   FILE* fp;

   // opening a file which does not exist
   fp = fopen("nosuchfile.txt", "r");
   printf("Value of errno: %d\n", errno);

   return 0;
}

Output

It will produce the following output −

Value of errno: 2

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.

void perror(const char *str);

Example

In the above example, the "errno = 2" is associated with the message No such file or directory, which can be printed with perror() function

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

int main(){

   FILE* fp;

   // opening a file which does not exist
   fp = fopen("nosuchfile.txt", "r");

   printf("Value of errno: %d\n", errno);
   perror("Error message:");

   return 0;
}

Output

When you run this code, it will produce the following output −

Value of errno: 2
Error message: No such file or directory

The strerror() Function

This returns a pointer to the textual representation of the current errno value.

char *strerror(int errnum);

Let us use this function to display the textual representation of errno=2

Example

Take a look at the following example −

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

int main() {

   FILE* fp;

   // opening a file which does not exist
   fp = fopen("nosuchfile.txt", "r");

   printf("Value of errno: %d\n", errno);
   printf("The error message is : %s\n", strerror(errno));

   return 0;
}

Output

Value of errno: 2
he error message is : No such file or directory

The ferror() Function

This function is used to check whether an error occurred during a file operation.

int ferror(FILE *stream);

Example

Here, we try to read from a file opened in ‘w’ mode. The ferror() function is used to print the error message

#include <stdio.h>

int main(){

   FILE *fp;
   fp = fopen("test.txt","w");
   char ch = fgetc(fp);  // Trying to read data, from writable file
   if(ferror(fp)){
      printf("File is opened in writing mode! You cannot read data from it!");
   }
   fclose(fp);
   
   return(0);
}

Output

Run the code and check its output −

File is opened in writing mode! You cannot read data from it!

The clearerr() Function

The clearerr() function is used to clear both end-of-file and error indicators for a file stream.

void clearerr(FILE *stream);

Example

Take a look at the following example −

#include <stdio.h>

int main(){

   FILE *fp;
   fp = fopen("test.txt","w");

   char ch = fgetc(fp);  // Trying to read data, from writable file

   if(ferror(fp)){
      printf("File is opened in writing mode! You cannot read data from it!\n");
   }

   // Clears error-indicators from the file stream 
   // Subsequent ferror() doesn't show error
   clearerr(fp);

   if(ferror(fp)){
      printf("Error again in reading from file!");
   }
   fclose(fp);
   
   return(0);
}

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.

Example 1

The following code fixes this error by checking if the divisor is zero before dividing −

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

int 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

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 program coming out after a successful operation. Here, EXIT_SUCCESS is a macro and it is defined as 0.

Example 2

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 the above program as follows −

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

int 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);
}

Output

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

Value of quotient: 4