diff --git a/include/os.h b/include/os.h
index e32421610..c7108a5d3 100644
--- a/include/os.h
+++ b/include/os.h
@@ -88,7 +88,9 @@ extern void ddxBeforeReset(void);
 #endif
 
 #ifdef DDXOSVERRORF
-extern _X_EXPORT void (*OsVendorVErrorFProc) (const char *, va_list args);
+extern _X_EXPORT void (*OsVendorVErrorFProc) (const char *,
+                                              va_list args)
+_X_ATTRIBUTE_PRINTF(1, 0);
 #endif
 
 extern _X_EXPORT int WaitForSomething(int *     /*pClientsReady */