diff --git a/os/xprintf.c b/os/xprintf.c
index 6fa530bd2..ee5d7daf6 100644
--- a/os/xprintf.c
+++ b/os/xprintf.c
@@ -30,6 +30,14 @@
 #include "os.h"
 #include <stdarg.h>
 #include <stdio.h>
+
+#ifndef va_copy
+# ifdef __va_copy
+#  define va_copy __va_copy
+# else
+#  error "no working va_copy was found"
+# endif
+#endif
     
 char *
 Xvprintf(const char *format, va_list va)