What is the Bash command to create a hardlink to a directory in OS X?

#include <unistd.h>
#include <stdio.h>
int
main(int argc, char *argv[])
{
   if (argc != 3)
      return 1;
   int ret = link(argv[1], argv[2]);
   if (ret != 0)
      perror("link");
   return ret;
}

$ gcc -o hlink hlink.c -Wall

#include <stdio.h>
int
main(int argc, char *argv[])
{
   if (argc != 2)
      return 1;
   int ret = unlink(argv[1]);
   if (ret != 0)
      perror("unlink");
   return ret;
}

gcc -o hunlink hunlink.c

hlink source_folder target_folder

-d, -F, --directory
          allow the superuser to attempt to hard link  directories  (note:
          will  probably  fail  due  to  system restrictions, even for the
          superuser)

sudo  ln  -d  existing_dir  new_hard_link

unlink  new_hard_link

link: Operation not permitted

rsync -av --copy-dirlinks --delete ../htmlguide ~/src/