diff --git a/src/semaphore.hh b/src/semaphore.hh
index 68f5ca3d..d3ce9f94 100644
--- a/src/semaphore.hh
+++ b/src/semaphore.hh
@@ -25,6 +25,7 @@
 #define SEMAPHORE_HH
 
 #include <cerrno>
+#include <cstdlib>
 #include <cstring>
 #include <stdexcept>