diff --git a/sdcc/sim/ucsim/cmd.src/cmdlex.cc b/sdcc/sim/ucsim/cmd.src/cmdlex.cc
index b567b98..7ba147c 100644
--- a/sdcc/sim/ucsim/cmd.src/cmdlex.cc
+++ b/sdcc/sim/ucsim/cmd.src/cmdlex.cc
@@ -32,6 +32,7 @@
 
 /* C99 systems have <inttypes.h>. Non-C99 systems may or may not. */
 
+#include <libio.h>
 #if defined __STDC_VERSION__ && __STDC_VERSION__ >= 199901L
 #include <inttypes.h>
 typedef int8_t flex_int8_t;