Program written in C to swap specific elements in a list datatype

swap_list_element.c

#include <stdio.h>
#include <stddef.h>

typedef struct _List {
	struct _List *next;
	int val;
} List;

enum {
	SWAPSUCCESS,
	EMPTYLIST,
	INVALIDSWAP,
};

int swap_list_element(List **head, List *a, List *b) {
	// a, b identical
	if (a == b) { return SWAPSUCCESS; }
	// a, b not exist
	if (a == NULL || b == NULL) { return INVALIDSWAP; }
	// empty list
	if (*head == NULL) { return EMPTYLIST; }
	
	List dummy;
	dummy.next = *head;
	List *p1 = &dummy;
	while(p1->next) {
		if (p1->next == a || p1->next == b) {
			// find swap point
			List *target = (p1->next == a)? b : a;
			List *p2 = p1->next;
			while(p2->next) {
				if (p2->next == target) {
					// find swap point, do swap
					p2->next = p1->next;
					p1->next = target;
					target = target->next;
					p1->next->next = p2->next->next;
					p2->next->next = target;
					*head = dummy.next;
					return SWAPSUCCESS;
				}
				p2 = p2->next;
			}
			return INVALIDSWAP;
		}
		p1 = p1->next;
	}
	return INVALIDSWAP;
}

void
printList(List *p)
{
	while(p) {
		printf("%d->", p->val);
		p = p->next;
	}
	printf("NULL\n");
}

int main(int argc, char *argv[])
{
	List l[10];
	// build list
	for (int i = 0; i < 10; ++i) {
		l[i].val = i;
		l[i].next = &(l[i+1]);
	}
	l[9].next = NULL;
	//do swap
	List *p = l;
	printList(p);
	swap_list_element(&p, &l[0], &l[9]);
	printList(p);
	swap_list_element(&p, &l[3], &l[5]);
	printList(p);
	return 0;
}