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error handling

What are error numbers in C?

Abdul Monum

Error numbers in C language refer to the list of constant integer values or error codes defined by the ISO C, POSIX, and the C++ standard libraries. The error codes represent different types of errors. When a function fails in C, it returns -1 or NULL, and the global variable errno (defined in the errno.h header file) is set to the respective error code.

Below is a table showing a sample list of error codes and their respective meaning in the Linux operating system:

Errno value

Error code

Meaning

2

ENOENT

No such file or directory

5

EIO

I/O error

7

E2BIG

Argument list too long

11

EAGAIN

Try again

33

EDOM

Math argument out of domain of func

34

ERANGE

Math result not representable

84

EILSEQ

Illegal byte sequence

Error handling in C can be done by tracking the value of errno after each function call. The errno.h header file also provides functions such as perror() and strerror() to display the error message corresponding to the errno value. The function declarations are as follows:

void perror(const char *s);
  • s: custom error message

  • Output: custom error message followed by colon, space, and implementation-defined error message

char* strerror(int errnum);
  • errnum: error code

  • Output: pointer to the implementation-defined error message string

Example

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

int main()
{
    FILE * fp;
    fp = fopen("file.txt", "r");
  
    printf("1. Opening a file which does not exist \n");
    printf("errno value: %d\n", errno);
    printf("%s \n", strerror(errno));
    perror("Error message is");
  
    double a = log(-10);
    printf("\n2. Taking log of negative value \n");
    printf("errno value: %d\n", errno);
    printf("%s \n", strerror(errno));
    perror("Error message is");
    
    double b = log(0.0);
    printf("\n3. Taking log of zero \n");
    printf("errno value: %d\n", errno);
    printf("%s \n", strerror(errno));
    perror("Error message is");    
    
    return 0;
}             

In the above program, we see three types of errors with their corresponding errno values and implementation-defined error messages using perror() and strerror().

  • In line 9, fopen() raises an error because the file does not exist, and therefore errno is set to 2

  • In line 16, log(-10) results in NaNNaN, and therefore errno is set to 33

  • In line 22, log(0.0) results in \infty, and therefore errno is set to 34

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error handling

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