diff --git a/picojson.h b/picojson.h
index ef4b903..708a067 100644
--- a/picojson.h
+++ b/picojson.h
@@ -75,7 +75,9 @@ extern "C" {
 
 // experimental support for int64_t (see README.mkdn for detail)
 #ifdef PICOJSON_USE_INT64
+#ifndef __STDC_FORMAT_MACROS
 #define __STDC_FORMAT_MACROS
+#endif
 #include <cerrno>
 #if __cplusplus >= 201103L
 #include <cinttypes>