On OpenBSD, fd_set is declared in <sys/select.h>.
diff --git a/emu/util.c b/emu/util.c
index fcaed91..4b1af68 100644
--- a/emu/util.c
+++ b/emu/util.c
@@ -3,6 +3,7 @@
#include <ctype.h>
#include <sys/socket.h>
+#include <sys/select.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
#include <unistd.h>
On OpenBSD, fd_set is declared in <sys/select.h>.
pdp6/emu/util.c
Line 22 in 5b3e2f0