--- a/src/output-plugins/spo_alert_unixsock.c
+++ b/src/output-plugins/spo_alert_unixsock.c
@@ -56,6 +56,8 @@
 #include "spo_alert_unixsock.h"
 #include "barnyard2.h"
 
+#include <stdlib.h>
+
 #define UNSOCK_FILE "barnyard2_alert"