#### Hexadecimal to Octal 2

```/**
* @file
* @brief Convert hexadecimal number to octal number (with decimal intermediary)
* @details
* The input is valid from 0 to 0xFFFF_FFFF_FFFF_FFFF.
*
* At first, this program converts a hex string to an unsigned long long
* decimal, and then to an octal string.
*
* When there is an invalid character in input string, this program stops
* parsing and converts the string until that character.
*
*/

#include <stdio.h>  /// for printf() and fgets()
#include <string.h> /// for memset()

/**
* @brief Convert a hexadecimal number to octal number.
* @param hex Hexadecimal number to convert.
* @returns A pointer to the converted octal string.
*/
const char *hex_to_oct(const char *hex)
{
#define MAX_OCT_STR_LEN 23 /* 17_7777_7777_7777_7777_7777 */
static char octal[MAX_OCT_STR_LEN];
memset(octal, '\0', MAX_OCT_STR_LEN); // Initialize as NULL string

unsigned long long decimal = 0;
int i = 0;
int len;

if (hex == NULL)
{
// Return an empty string
return octal;
}

/* Hexadecimal to decimal conversion */
while (*hex != '\n' && *hex != '\0')
{
char ch = *hex;

if (ch >= '0' && ch <= '9')
{
ch -= '0';
}
else if (ch >= 'a' && ch <= 'f')
{
ch = ch - 'a' + 10;
}
else if (ch >= 'A' && ch <= 'F')
{
ch = ch - 'A' + 10;
}
else
{
printf("Invalid hexadecimal input: %c\n", ch);
break;
}

decimal *= 16;
decimal += ch;
hex++;
}

/* Decimal to octal conversion */
if (decimal == 0)
{
octal[0] = '0';
len = 1;
}
else
{
i = 0;
while (decimal > 0)
{
octal[i] = '0' + decimal % 8;
i++;
decimal /= 8;
}

len = i;
}

octal[len] = '\0';

/* Reverse the octal string */
for (i = 0; i < len / 2; i++)
{
char tmp = octal[i];
octal[i] = octal[len - i - 1];
octal[len - i - 1] = tmp;
}

return octal;
}

/**
* @brief Main function
* @returns 0 on exit
*/
int main()
{
#define MAX_HEX_STR_LEN 17 /* FFFF_FFFF_FFFF_FFFF */
char hex[MAX_HEX_STR_LEN];

/* Input hexadecimal number from user */
printf("Enter any hexadecimal number: ");
fgets(hex, MAX_HEX_STR_LEN, stdin);

const char *octal = hex_to_oct(hex);

printf("Hexadecimal number = %s\n", hex);
printf("Octal number = %s\n", octal);

return 0;
}
```