Am I allowed to make circular references with constants structs?

Question

Am I allowed to do this in C99?

typedef struct dlNode {
    dlNode* next, prev;
    void* datum;
} dlNode;

const static dlNode head={
    .next = &tail,
    .prev = NULL,
    .datum = NULL
};

const static dlNode tail={
    .next = NULL,
    .prev = &head,
    .datum = NULL
};

I can make my program work without this. It'd just be convenient.

Answer 1

You can. You just have to forward declare tail to get it to work:

typedef struct dlNode {
    struct dlNode* next;
    struct dlNode* prev;
    void* datum;
} dlNode;

const static dlNode tail;

const static dlNode head={
    .next = &tail,
    .prev = NULL,
    .datum = NULL
};

const static dlNode tail={
    .next = NULL,
    .prev = &head,
    .datum = NULL
};

Answer 2

You are absolutely allowed to do it: add a forward declaration of tail, and C will merge it with a later definition:

typedef struct dlNode {
    const struct dlNode* next, *prev;
    void* datum;
} dlNode;

const static dlNode tail; // <<== C treats this as a forward declaration

const static dlNode head={
    .next=&tail,
    .prev=NULL,
    .datum=NULL
};

const static dlNode tail={ // This becomes the actual definition
    .next=NULL,
    .prev=&head,
    .datum=NULL
};

Note that you should fix your struct declaration to make next and prev constant, otherwise your definition would discard constant qualifiers.

Demo.