How to properly use typedef for structs in C

Question

I have a struct for a linked list in my C code. When the list is defined like this:

typedef struct LinkedList {
    Ht_item* item;
    LinkedList* next;
}LinkedList;

I get the Compilation Error: "error: unknown type name ‘LinkedList’" alongside other related error messages

However, when i define my LinkedList this way:

typedef struct LinkedList LinkedList;

struct LinkedList {
    Ht_item* item;
    LinkedList* next;
};

I get no errors.

I do not understand why the Error happens in the first instance?

Answer 1

In this definition:

typedef struct LinkedList {
    Ht_item* item;
    LinkedList* next;
}LinkedList;

You're using the type LinkedList before it's been defined or declared.

This works:

typedef struct LinkedList LinkedList;

struct LinkedList {
    Ht_item* item;
    LinkedList* next;
};

Because LinkedList has been declared (but not fully defined) by the time it's used, and because you're creating a pointer to that type it doesn't need to be fully defined yet.

This would also work:

typedef struct LinkedList {
    Ht_item* item;
    struct LinkedList* next;
}LinkedList;

For the same reason the prior piece of code works.