diff --git a/async.c b/async.c
index b293ecd..f83e2f5 100644
--- a/async.c
+++ b/async.c
@@ -30,6 +30,7 @@
  */
 
 #include "fmacros.h"
+#include <stdlib.h>
 #include <string.h>
 #include <strings.h>
 #include <assert.h>