defining typedef for a struct for 2 times

Question

I saw in a code such a thing like:

struct listnode{
char data;
struct listnode *nextptr;}

typedef struct listnode LISTNODE;

typedef LISTNODE *listnodeptr;

so why did he use typedef for 2 times???

Answer 1

The first typedef declares an alias for the structure itself

typedef struct listnode LISTNODE;
        ^^^^^^^^^^^^^^^

The second typedef declares an alias for pointer to object of the structure type.

typedef LISTNODE * listnodeptr;
        ^^^^^^^^^^

It is matter of style whether to use several typedefs of just one.

All these three declarations could be combined in one declaration

typedef struct listnode{
char data;
struct listnode *nextptr;
} LISTNODE, *listnodeptr;

Answer 2

Two typedef s here are not repetitive, they alias two different types.

• typedef struct listnode LISTNODE; creates the type LISTNODE which is an alias of struct listnode
• typedef LISTNODE *listnodeptr; creates the type listnodeptr which is an alias of LISTNODE * , ie, struct listnode *