diff --git a/critbit.h b/critbit.h
index 11d096b..f9ceb3c 100644
--- a/critbit.h
+++ b/critbit.h
@@ -5,11 +5,18 @@
 extern "C" {
 #endif
 
+/* for __pure__ if we are compiling under dietlibc */
+#include <stddef.h>
+
+#ifndef __pure__
+#define __pure__
+#endif
+
 typedef struct {
   void *root;
 } critbit0_tree;
 
-int critbit0_contains(critbit0_tree *t, const char *u);
+int critbit0_contains(critbit0_tree *t, const char *u) __pure__;
 int critbit0_insert(critbit0_tree *t, const char *u);
 int critbit0_delete(critbit0_tree *t, const char *u);
 void critbit0_clear(critbit0_tree *t);