--- mruby-3.0.0/mrbgems/mruby-io/src/io.c	2021-08-20 00:27:04.837317416 -0400
+++ mruby-3.0.0.serenity/mrbgems/mruby-io/src/io.c	2021-08-20 00:28:51.960390750 -0400
@@ -49,6 +49,7 @@
   #include <sys/wait.h>
   #include <sys/time.h>
   #include <unistd.h>
+  #include <sys/select.h>
   typedef size_t fsize_t;
   typedef time_t ftime_t;
   typedef suseconds_t fsuseconds_t;