diff --git a/reentr.h b/reentr.h
index cf41900683..78b8518710 100644
--- a/reentr.h
+++ b/reentr.h
@@ -41,6 +41,12 @@
 
 #ifdef USE_REENTRANT_API
 
+/* For thread-safe builds, alternative methods are used to make calls to this
+ * safe. */
+#ifdef USE_THREAD_SAFE_LOCALE
+#   undef HAS_SETLOCALE_R
+#endif
+ 
 /* Deprecations: some platforms have the said reentrant interfaces
  * but they are declared obsolete and are not to be used.  Often this
  * means that the platform has threadsafed the interfaces (hopefully).
diff --git a/regen/reentr.pl b/regen/reentr.pl
index f2c1d7cf3c..f5788c7ad9 100644
--- a/regen/reentr.pl
+++ b/regen/reentr.pl
@@ -91,6 +91,12 @@ print $h <<EOF;
 
 #ifdef USE_REENTRANT_API
 
+/* For thread-safe builds, alternative methods are used to make calls to this
+ * safe. */
+#ifdef USE_THREAD_SAFE_LOCALE
+#   undef HAS_SETLOCALE_R
+#endif
+ 
 /* Deprecations: some platforms have the said reentrant interfaces
  * but they are declared obsolete and are not to be used.  Often this
  * means that the platform has threadsafed the interfaces (hopefully).