diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index 84ee3f28a7..04c02aef9a 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -14,6 +14,11 @@
 
 #include "ruby.h"
 #include "rubyio.h"
+#if defined(HAVE_FCNTL_H) || defined(_WIN32)
+#include <fcntl.h>
+#elif defined(HAVE_SYS_FCNTL_H)
+#include <sys/fcntl.h>
+#endif
 
 #define STRIO_EOF FMODE_SYNC