Circular includes

a.h

#ifndef A_DOT_H_INCLUDED
#define A_DOT_H_INCLUDED

#include "b.h"

struct a
{
    struct b *b;
};

#endif /* A_DOT_H_INCLUDED */

b.h

#ifndef B_DOT_H_INCLUDED
#define B_DOT_H_INCLUDED

#include "a.h"

struct b
{
    struct a *a;
};

#endif /* B_DOT_H_INCLUDED */

main.c

#include "a.h"

int main(void)
{
    struct a a;
    (void) a;

    return 0;
}

$ gcc -Wall -Wextra -Werror main.c  && echo "OK"
OK