**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:

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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:
```C
void perror(const char *s);
```
- `s`: custom error message

- **Output**: custom error message followed by colon, space, and implementation-defined error message
```C
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;
}             

gcc compiler

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 $NaN$, and therefore `errno` is set to `33`

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

gcc.tar.gz

What are error numbers in C?

Error numbers in C are constant integer values representing different error types, set in errno. Functions perror() and strerror() display corresponding messages.

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