diff --git a/src/libutil/sync.hh b/src/libutil/sync.hh
index 611c900e0..3b2710f6f 100644
--- a/src/libutil/sync.hh
+++ b/src/libutil/sync.hh
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <cstdlib>
 #include <mutex>
 #include <condition_variable>
 #include <cassert>