diff --git a/src/unix/PidFile.hxx b/src/unix/PidFile.hxx
index 3f97484a1..46b6fbb54 100644
--- a/src/unix/PidFile.hxx
+++ b/src/unix/PidFile.hxx
@@ -26,6 +26,7 @@
 
 #include <assert.h>
 #include <unistd.h>
+#include <stdlib.h>
 #include <fcntl.h>
 
 class PidFile {