--- a/src/_csrc/sha2.c
+++ b/src/_csrc/sha2.c
@@ -34,6 +34,7 @@
  * $From: sha2.c,v 1.1 2001/11/08 00:01:51 adg Exp adg $
  */
 
+#include <stdlib.h>
 #include "pycabcrypt.h"
 #include "sha2.h"