[C] Roman Numerals Calculation Example

Code:
#include <stdio.h>
// #include <stdlib.h>
#include <stdbool.h>
#include <string.h>

static char _roman_chars[] = "IVXLCDMvxlcdm";
static int _numerals[] = {1, 5, 10, 50, 100, 500, 1000, 5000, 10000, 50000, 100000, 500000, 1000000};

int get_roman_numeric_value(char numeral)
{
   char *p = strchr(_roman_chars, numeral);
   if (p != NULL) return _numerals[p - _roman_chars];
   return -1;
}

int get_int_from_roman(char *roman)
{
   size_t len = strlen(roman);
   if (len < 1) return -1;
   int result = get_roman_numeric_value(roman[0]);
   for (size_t i = 1; i < len; ++i)
   {
      int num = get_roman_numeric_value(roman[i]);
      int prev = get_roman_numeric_value(roman[i-1]);
      if (num < 0 || prev < 0) return -1;
      // printf("%d\n", num);
      if (prev < num)
         result = result - (2 * prev) + num;
      else
         result += num;
   }
   return result;
}

bool valid_roman_number(char *roman)
{
   return strlen(roman) == strspn(roman, _roman_chars);
}

int main()
{
   char roman[] = "MCXIV";
   if (!valid_roman_number(roman))
   {
      printf("Invalid roman number specified.\n");
      return 1;
   }

   int val = get_int_from_roman(roman);
   if (val < 0)
   {
      printf("An error ocurred when trying to get the numeric value from the roman format.\n");
      return 1;
   }
   printf("Roman Value: %d\n", val);
}