+os_vm_address_t undefined_alien_address = 0;
+
+static void
+ensure_undefined_alien(void) {
+ os_vm_address_t start = os_validate(NULL, os_vm_page_size);
+ if (start) {
+ os_protect(start, os_vm_page_size, OS_VM_PROT_NONE);
+ undefined_alien_address = start;
+ } else {
+ lose("could not allocate guard page for undefined alien");
+ }
+}
+