diff --git a/openaes/src/oaes_lib.c b/openaes/src/oaes_lib.c
index 6e20b47..0cc0394 100644
--- a/openaes/src/oaes_lib.c
+++ b/openaes/src/oaes_lib.c
@@ -37,6 +37,7 @@
 #include "ftime.h"
 #include <malloc.h>
 #include <string.h>
+#include <unistd.h>
 
 #ifdef WIN32
 #include <process.h>