Oops, I often call C++ by the wrong name. But while the code was C++, the argument does work for C as well. Here's the C version of the C++ snippets:

HASH* demonstration(void* p)
   HASH* hash_ptr = hash_new()
   hash_add(hash_ptr, "demo_data", p);

   HASH* outer_hash_ptr = hash_new();
   hash_add(outer_hash_ptr, "demonstration_data", hash_ptr);

   return outer_hash_ptr;
}
[download]

and

HASH* demonstration(void* p)
   HASH hash;
   hash_init(&hash);
   hash_add(&hash, "demo_data", p);

   HASH* outer_hash_ptr = hash_new();
   hash_add(outer_hash_ptr, "demonstration_data", &hash);  XXX BUG

   return outer_hash_ptr;
}
[download]