--- a/apps/s_apps.h	2020-04-05 13:52:44.967691150 +0100
+++ b/apps/s_apps.h	2020-04-05 13:52:50.147690999 +0100
@@ -122,6 +122,10 @@
 # define _kbhit kbhit
 #endif
 
+#if defined(OPENSSL_SYS_SERENITY)
+# include <sys/select.h>
+#endif
+
 #if defined(OPENSSL_SYS_VMS) && !defined(FD_SET)
 /*
  * VAX C does not defined fd_set and friends, but it's actually quite simple