diff --git a/io.h b/io.h
index bbe060180..83a64a65c 100644
--- a/io.h
+++ b/io.h
@@ -23,6 +23,9 @@
 #include <stdint.h>
 #include <stdbool.h>
 #include <stdio.h>
+#ifdef __cplusplus
+#define __STDC_FORMAT_MACROS 1
+#endif
 #include <inttypes.h>
 #include "endian.h"
 #include "ethash.h"