diff --git a/libio/ioprims.c b/libio/ioprims.c
index 5d31e070d9a..e3e0d7e75cd 100644
--- a/libio/ioprims.c
+++ b/libio/ioprims.c
@@ -31,6 +31,7 @@ the executable file might be covered by the GNU General Public License. */
 #include "libioP.h"
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <unistd.h>
 
 #ifdef TODO
 /* Add open, isatty */