Roman numerals in C. Because, you know, yeah.

roman.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

const char *g_numeral_list[] = {"M", "CM", "D", "CD", "C", "XC",
                                "L", "XL", "X", "IX", "V", "IV", "I"};
const int g_value_list[]     = {1000, 900, 500, 400, 100, 90,
                                50, 40, 10, 9, 5, 4, 1};
const int g_list_length      = 13;
const size_t g_buffer_size   = 32;

void convert_to_numerals(int num, char **buffer) {
  int i;
  size_t total_buffer, max_chars;

  total_buffer = g_buffer_size;
  max_chars = g_buffer_size - 1;
  for (i = 0; i < g_list_length; ++i) {
    while (num >= g_value_list[i]) {
      if (max_chars < 2) {
        *buffer = (char *)realloc(*buffer, total_buffer + g_buffer_size);
        memset(*buffer + total_buffer * sizeof(char), 0, g_buffer_size);
        total_buffer += g_buffer_size;
        max_chars += g_buffer_size;
      }

      strncat(*buffer, g_numeral_list[i], max_chars);
      num -= g_value_list[i];
      max_chars -= strlen(g_numeral_list[i]);
    }
  }
}

int main(int argc, const char *argv[]) {
  int num;
  char *result;

  if (argc < 2) {
    printf("Usage: %s <num>\n", argv[0]);
    return 0;
  }

  num = atoi(argv[1]);
  if (num < 1) {
    fprintf(stderr, "Error: invalid input (zero, negative, or NaN).\n");
    return 1;
  }

  result = (char *)calloc(g_buffer_size, sizeof(char));
  convert_to_numerals(num, &result);
  printf("%s\n", result);
  free(result);

  return 0;
}

Haters gonna hate on my global variables and my running the algo twice to figure out how much memory I need. Because oversized buffers are for quitters.

And by that I mean I really need to learn to use realloc.