simple base conversion with realloc [compilation: gcc -W -Wall -Wextra -std=c89 base_realloc.c]

base_realloc.c

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

void push_front(char **str, char c)
{
  char *ret;

  ret = malloc(sizeof(char) * (strlen(*str) + 2));
  *ret = c;
  memmove(ret + 1, *str, strlen(*str));
  free(*str);
  *str = ret;
}

char *convert(int o, char const *base)
{
  char *ret;

  ret = malloc(sizeof(char));
  *ret = 0;
  while (o > 0)
    {
      push_front(&ret, base[o % strlen(base)]);
      o /= strlen(base);
    }
  return (ret);
}

int main(int ac, char **av)
{
  int o;

  if (ac != 2)
    return (1);
  o = atoi(av[1]);
  printf("%s\n", convert(o, "0123456789ABCDEF"));
  printf("%s\n", convert(o, "01"));
  return (0);
}