Understanding BSD queue.h single linked list implementation

slqueue.c

/*
 * See queue(3) or https://www.freebsd.org/cgi/man.cgi?query=queue&sektion=3 for reference.
 */
#include <stdio.h>
#include <sys/queue.h>

struct Pattern {
	char name[100];

	/* Following expands to:
	 *   struct { struct Pattern *sle_next; } entries;
	 */
	SLIST_ENTRY(Pattern) entries;
};

int main(int argc, char **argv) {

	/* Following expands to
	 *   struct yylisthead { struct Pattern *slh_first; } head = { ((void *)0) };
	 */
	SLIST_HEAD(yylisthead, Pattern) head = SLIST_HEAD_INITIALIZER(head);

	/* Following expands to
	 *   do { (((&head))->slh_first) = ((void *)0); } while (0);
	 * which can be simplified to:
	 *   (&head)->slh_first = NULL;
	 */
	SLIST_INIT(&head);

	struct Pattern p1 = { .name = "A", .entries = { NULL } };

	/* Following expands to
	 *   do { (((&p1))->entries.sle_next) = (((&head))->slh_first); (((&head))->slh_first) = (&p1); } while (0);
	 * which can be simplified to:
	 *   (&p1)->entries.sle_next = (&head)->slh_first; (&head)->slh_first = (&p1)
	 */
	SLIST_INSERT_HEAD(&head, &p1, entries);

	struct Pattern *e;

	/* Following expands to
	 *   for ((e) = (((&head))->slh_first); (e); (e) = (((e))->entries.sle_next))
	 * which can be simplified to:
	 *   for (e = (&head)->slh_first; e != NULL; e = e->entries.sle_next)
	 */
	SLIST_FOREACH(e, &head, entries) {
        	printf("1\tname: %s\n", e->name);
	}
	puts("");

	struct Pattern p3 = { .name = "C", .entries = { NULL } };
	SLIST_INSERT_HEAD(&head, &p3, entries);

	SLIST_FOREACH(e, &head, entries) {
        	printf("2\tname: %s\n", e->name);
	}
	puts("");

	struct Pattern p2 = { .name = "B", .entries = { NULL } };
	SLIST_INSERT_AFTER(&p3, &p2, entries);

	SLIST_FOREACH(e, &head, entries) {
        	printf("3\tname: %s\n", e->name);
	}
	puts("");

        struct Pattern *etmp;
	/* Following expands to
	 *   for ((e) = (((&head))->slh_first); (e) && ((etmp) = (((e))->entries.sle_next), 1); (e) = (etmp))
	 * which can be simplified to:
	 *   for (e = (&head)->slh_first; (e != NULL) && (etmp = e->entries.sle_next, 1); e = etmp)
	 */
	SLIST_FOREACH_SAFE(e, &head, entries, etmp) {
		if (e == &p2) {
			/* Following expands to
			 *   do {
			 *     if ((((&head))->slh_first) == (e)) {
			 *       do {
			 *         ((((&head)))->slh_first) = ((((((&head)))->slh_first))->entries.sle_next);
			 *       } while (0);
			 *     } else {
			 *       struct Pattern *curelm = (((&head))->slh_first);
			 *       while (((curelm)->entries.sle_next) != (e))
			 *         curelm = ((curelm)->entries.sle_next);
			 *       do {
			 *         ((curelm)->entries.sle_next) = ((((curelm)->entries.sle_next))->entries.sle_next);
			 *       } while (0);
			 *     };
			 *   } while (0) ;
			 * which can be simplified to:
			 *   if (&head)->slh_first == e {
			 *     (&head)->slh_first = (&head)->slh_first->entries.sle_next;
			 *   } else {
			 *     struct Pattern *curelm = (&head)->slh_first;
			 *     while (curelm->entries.sle_next != e)
			 *       curelm = curelm->entries.sle_next;
			 *     curelm->entries.sle_next = curelm->entries.sle_next->entries.sle_next;
			 *   };
			 */
			SLIST_REMOVE(&head, &p2, Pattern, entries);
			continue;
		}
        	printf("4\tname: %s\n", e->name);
	}

	/* Following expands to
	 *   while (!((&head)->slh_first == ((void *)0)))
	 * which can be simplified to:
	 *   while ((&head)->slh_first != NULL)
	 */
	while (!SLIST_EMPTY(&head)) {
		/* Following expands to
		 *   e = ((&head)->slh_first);
		 * which can be simplified to:
		 *   e = (&head)->slh_first;
		 */
		e = SLIST_FIRST(&head);

		/* Following expands to
		 *   do { (((&head))->slh_first) = (((((&head))->slh_first))->entries.sle_next); } while (0);
		 * which can be simplified to:
		 *   (&head)->slh_first = (&head)->slh_first->entries.sle_next;
		 */
		SLIST_REMOVE_HEAD(&head, entries);
	}
}