diff --git a/document.c b/document.c
index 8bfb1b1..897da11 100644
--- a/document.c
+++ b/document.c
@@ -6,6 +6,7 @@
 
 #include <stdlib.h>
 #include <stdio.h>
+#include <unistd.h>
 #include <string.h>
 #include <limits.h>
 #include <sys/types.h>