May 11, 2021 C
The C language does not provide direct support for error handling, but as a system programming language, it allows you to access the underlying data in the form of a return value. W hen an error occurs, most C or UNIX function calls return 1 or NULL, and an error code, errno, is set, which is a global variable that indicates that an error occurred during the function call. You can find a variety of error codes in the header file.
Therefore, the C programmer can decide which appropriate action to take by examining the return value and then deciding which one to take based on the return value. I t is a good programming practice for developers to set errno to 0 when the program is initialized. A value of 0 indicates that there are no errors in the program.
The C language provides perror() and strerror() functions to display text messages related to errno.
Let's simulate an error situation and try to open a file that doesn't exist. Y ou can output error messages in a number of ways, and here we use functions to demonstrate usage. It is also important to note that you should use the stderr file stream to output all 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 results:
Value of errno: 2 Error printed by perror: No such file or directory Error opening file: No such file or directory
When you do division, you do not check that the division is zero, which is a common problem for programmers when programming and can result in a run-time error.
To avoid this, the following code checks whether the division is zero before division:
#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 results:
Division by zero! Exiting...
Typically, a program successfully exits an operation with a value EXIT_SUCCESS. Here, EXIT_SUCCESS is a macro, which is defined as 0.
If there is an error condition in the program, when you exit the program, it comes with a status value EXIT_FAILURE, which is defined as -1. Therefore, the above program can be written as:
#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 results:
Value of quotient : 4