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 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 code below fixes this by checking if the divisor is zero before dividing −
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 −
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.
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.
#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 program 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
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