Pointer on a struct in C

Question

typedef struct st {
    double d ;
    struct st *n ;
} st, *List ;

On this part:

} st, *List ;

What is this second "st"? Is it an object created (why does it have the same name as the type)? I don't understand the "*List". Does it use typedef to say that a "List" is a pointer to an "st"? Thank you.

It take me less than one minute to figure out your answer because this question is a double with this one http://stackoverflow.com/questions/1543713/c-typedef-of-pointer-to-structure. Next time, consider searching one google before posting. — Luc DUZAN, May 02 '14 at 13:29

score 4 · Accepted Answer · answered May 02 '14 at 13:28

The part

typedef struct st {...} st;

creates a new symbol st that can be used to declare instances of struct st without the keyword struct. You can read this article for more information - it has some examples.

The second declaration *List creates a pointer type to struct st so you can declare a pointer to the structure without having to write struct st*.

typedef essentially creates an alias that can be used in place of the original type.

score 2 · Answer 2 · answered May 02 '14 at 13:28

2

The first st is part of struct st and creates a named struct. The second st is part of typedef … st and creates a type as an alias for struct st. The * List creates a type List as a pointer to struct st.

answered May 02 '14 at 13:28

ckruse

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score 2 · Answer 3 · answered May 02 '14 at 13:30

typedef lets you define multiple type aliases. For example, you can do this:

typedef int Number, *NumberPtr;

This defines Number to be an alias for int, and NumberPtr to be an alias for int*.

Your declaration does the same, except it uses struct st instead of an int. In other words, it defines two type names - st for the struct st, and List for struct st*.

This lets you write

st s;
List p;

instead of

struct st s;
struct st *p;

score 1 · Answer 4 · answered May 02 '14 at 13:32

This does three things: The first is it defines a struct called struct st that contains a double and a pointer to another struct st. This first task could be accomplished on its own like so:

struct st {
    double d ;
    struct st *n ;
};

The typedef is doing two more things. It is defining st to be struct st and also defining List to be struct st *. A more simple example of this 2x typedef is something like:

typedef int Integer, *PointerToInteger;

Pointer on a struct in C

4 Answers4