Bugfix: out-of-range negative indices into lists.

We have a problem not handling negative list indices
which index beyond the beginning of the list.
Such accesses are just storing and retrieving the
first element!

* lib.c (listref): If a negative index is still negative
after the length of the list is added to it, then
return nil. Do not return car(list)!
(listref_l): Similary, do not return car_l(list) if
the index is less than zero, so that an error
occurs for an out-of-range negative index.

* txr.1: Update the documentation for ref
to describe these indexing constraints, and also
to include hashes.

1 files changed, 5 insertions, 2 deletions

diff --git a/lib.c b/lib.c
index 47b028cf..85ff5d33 100644
--- a/lib.c
+++ b/lib.c
@@ -520,8 +520,11 @@ val lazy_conses(val list)
 val listref(val list, val ind)
 {
   gc_hint(list);
-  if (lt(ind, zero))
+  if (lt(ind, zero)) {
     ind = plus(ind, length_list(list));
+    if (lt(ind, zero))
+      return nil;
+  }
   for (; gt(ind, zero); ind = minus(ind, one))
     list = cdr(list);
   return car(list);
@@ -537,7 +540,7 @@ loc listref_l(val list, val ind)
 
   for (; gt(ind, zero) && list; ind = minus(ind, one))
     list = cdr(list);
-  if (consp(list))
+  if (ge(ind, zero) && consp(list))
     return car_l(list);
 
   uw_throwf(error_s, lit("~s has no assignable location at ~s"),