diff --git a/siena_diff.c b/siena_diff.c
index 032640dc1e46dc9bed7f142f74778f0a03513ec9..bfb8ad63c62d51e7d2da2a9229fa24e59c9e20c3 100644
--- a/siena_diff.c
+++ b/siena_diff.c
@@ -14,6 +14,7 @@
 #include "libss/libss.h"
 #include "libss/libavw.h"
 
+#include <unistd.h>
 
 /* }}} */
 /* {{{ usage */