How do I make these 2 macros into a function?

/* Given block ptr bp, compute address of next and previous free blocks */

#define NEXT_FREEP(bp)(*(void **)(bp + 16))

#define PREV_FREEP(bp)(*(void **)(bp))

here bp is a block pointer (void* bp)

I want to have this macro as a function so I can call it like this

static char *free_listp = 0;/* Pointer to the first free block */

static void insertAtFront(void *bp){

NEXT_FREEP(bp) = free_listp; //Sets next ptr to start of free list

PREV_FREEP(free_listp) = bp; //Sets current's prev to new block

PREV_FREEP(bp) = NULL; // Sets prev pointer to NULL

free_listp = bp; // Sets start of free list as new block

}