This union was right in the middle of the buf struct:

        union {
            caddr_t b_addr;             /* low order core address */
            int *b_words;               /* words for clearing */
            struct filsys *b_filsys;    /* superblocks */
            struct dinode *b_dino;      /* ilist */
            daddr_t *b_daddr;           /* indirect block */
        } b_un;
There were a number of other places that did the same thing.   It's
OFFICIALLY now in undefined behavior by the standard (though of course that
didn't exist in the BSD days) ,
to store in one element of the union and retrieve it via another.