Many list funcs should work with constant lists.

ccan/list/list.c

@@ -2,9 +2,9 @@
 #include <stdlib.h>
 #include "list.h"
 
-struct list_head *list_check(struct list_head *h, const char *abortstr)
+struct list_head *list_check(const struct list_head *h, const char *abortstr)
 {
-	struct list_node *n, *p;
+	const struct list_node *n, *p;
 	int count = 0;
 
 	if (h->n.next == &h->n) {
@@ -15,7 +15,7 @@ struct list_head *list_check(struct list_head *h, const char *abortstr)
 				abortstr, h);
 			abort();
 		}
-		return h;
+		return (struct list_head *)h;
 	}
 
 	for (p = &h->n, n = h->n.next; n != &h->n; p = n, n = n->next) {
@@ -29,5 +29,5 @@ struct list_head *list_check(struct list_head *h, const char *abortstr)
 			abort();
 		}
 	}
-	return h;
+	return (struct list_head *)h;
 }

ccan/list/list.h

@@ -62,7 +62,7 @@ struct list_head
  *			printf(" -> %s\n", c->name);
  *	}
  */
-struct list_head *list_check(struct list_head *h, const char *abortstr);
+struct list_head *list_check(const struct list_head *h, const char *abortstr);
 
 #ifdef CCAN_LIST_DEBUG
 #define debug_list(h) list_check((h), __func__)
@@ -166,7 +166,7 @@ static inline void list_del(struct list_node *n)
  * Example:
  *	assert(list_empty(&parent->children) == (parent->num_children == 0));
  */
-static inline bool list_empty(struct list_head *h)
+static inline bool list_empty(const struct list_head *h)
 {
 	(void)debug_list(h);
 	return h->n.next == &h->n;

ccan/list/test/compile_ok-constant.c

@@ -0,0 +1,49 @@
+#include "list/list.h"
+#include "tap/tap.h"
+#include "list/list.c"
+#include <stdbool.h>
+#include <stdio.h>
+
+struct child {
+	const char *name;
+	struct list_node list;
+};
+
+static bool children(const struct list_head *list)
+{
+	return !list_empty(list);
+}
+
+static const struct child *first_child(const struct list_head *list)
+{
+	return list_top(list, struct child, list);
+}
+
+static const struct child *last_child(const struct list_head *list)
+{
+	return list_tail(list, struct child, list);
+}
+
+static void check_children(const struct list_head *list)
+{
+	list_check(list, "bad child list");
+}
+
+static void print_children(const struct list_head *list)
+{
+	const struct child *c;
+	list_for_each(list, c, list)
+		printf("%s\n", c->name);
+}
+
+int main(void)
+{
+	LIST_HEAD(h);
+
+	children(&h);
+	first_child(&h);
+	last_child(&h);
+	check_children(&h);
+	print_children(&h);
+	return 0;
+}