What are error numbers in C?

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:

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

#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;
}

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

What are error numbers in C?

Example