Specific C fifo implementation confusion

Question

I need to use a FIFO for a project I'm working on. I found this implementation on the internet and I can't figure out what's going on in this function; I'm hoping someone can enlighten me.

/* Free an entire fifo */
void fifo_free(fifo_t *f, void (*freefn)(void *))
{
    fifonode_t *fn = f->f_head;
    fifonode_t *tmp;

    if (freefn == NULL)
        freefn = fifo_nullfree;

    while (fn) {
        (*freefn)(fn->fn_data);

        tmp = fn;
        fn = fn->fn_next;
        free(tmp);
    }

    free(f);
}

...

/*ARGSUSED*/
static void fifo_nullfree(void *arg)
{
    /* this function intentionally left blank */
}

1. See how it uses void (*freefn)(void *) as a parameter? Why? Why is that pointer passed into the function? What's the purpose of (*freefn)(fn->fn_data)

2. What's the point of these lines?

   if (freefn == NULL)
       freefn = fifo_nullfree;
   

   Wouldn't a comment in this situation be just as effective?

You can see a full copy of this implementation here

Answer 1

1. The fifo_free function only knows how to free up memory used for fifonode_t items, it doesn't know what to do with the data (pointed to by fn_data) that is in those items. This is where freefn come in.

   The parameter void (*freefn)(void *) is a pointer to a function. The function takes a void pointer. This is used to free the data that is stored against each fifonode_t in your FIFO.

2. In some cases you might not need to do anything about freeing the extra data (say you were storing pointers to some other static data). In that case, you could pass NULL for freefn and the function will use fifo_nullfree as the function, which basically does nothing.

   Note: the fifo_free function could just as easily have done the if test inside the loop and only called the freefn function if it wasn't NULL. Possibly they have some reason not to.