diff --git a/src/basic/missing_wait.h b/src/basic/missing_wait.h
index 3965b5bdbfb..05648779e3f 100644
--- a/src/basic/missing_wait.h
+++ b/src/basic/missing_wait.h
@@ -5,6 +5,7 @@
 
 #include "macro.h"
 
+/* since glibc-2.36 */
 #ifndef P_PIDFD
 #  define P_PIDFD 3
 #else