diff --git a/libcxx/include/__support/ibm/xlocale.h b/libcxx/include/__support/ibm/xlocale.h
--- a/libcxx/include/__support/ibm/xlocale.h
+++ b/libcxx/include/__support/ibm/xlocale.h
@@ -310,7 +310,12 @@
   }
 
   va_list ap_copy;
+  // va_copy may not be provided by the C library in C++ 03 mode.
+#if defined(_LIBCPP_CXX03_LANG) && __has_builtin(__builtin_va_copy)
+  __builtin_va_copy(ap_copy, ap);
+#else
   va_copy(ap_copy, ap);
+#endif
   int str_size = vsnprintf(*strp, buff_size, fmt,  ap_copy);
   va_end(ap_copy);