Question

typedef struct hash_table_data
{
  int key;
  int data;
  struct hash_table_data* next;
  struct hash_table_data* prev;
}hash_table_data;

typedef struct hash_table
{
  int num_entries;
  struct hash_table **entries;
}hash_table;

VERSUS

struct hash_table_data_
{
  int key,data;
  struct hash_table_data_ *next,*prev;
};
typedef struct hash_table_data_ hash_table_data;

struct hash_table_
{
  int num_entries;
  struct hash_table_data_ **entries;
};
typedef struct hash_table_ hash_table;

Answer 1

In the second example, you have an extra names in action: struct hash_table_data_ and struct hash_table_

In the first example, you effectively hide the name struct hash_table_data with a typedef which renames struct hash_table_data to the shorter hash_table_data.

Think of it like so:

typedef (something that happens to be struct X) (to) X

vs

define a struct X
typedef struct X (to) X

Now in the latter example, you actually don’t do exactly what the former example does. In the latter example, you do

define a struct X_
typedef struct X_ (to) X

The key here is that you have an “extra” struct X_ which could be used directly.