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

base_buffer.c

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

# define push_front(buff, c) (memmove(buff + 1, buff, (strlen(buff) + 2)), *buff = c)

char *convert(int o, char *buff, char const *base)
{
  *buff = 0;
  while (o > 0)
    {
      push_front(buff, base[o % strlen(base)]);
      o /= strlen(base);
    }
  return (buff);
}

int main(int ac, char **av)
{
  int o;
  char buff[sizeof(int) * 8 + 1];

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