diff --git a/man2/pidfd_open.2 b/man2/pidfd_open.2
index 3fd813845..b67320708 100644
--- a/man2/pidfd_open.2
+++ b/man2/pidfd_open.2
@@ -133,7 +133,7 @@ returns, and indicates that the file descriptor is readable.
 #include <stdio.h>
 
 #ifndef __NR_pidfd_open
-#define __NR_pidfd_open 434
+#define __NR_pidfd_open 434   /* System call # on most architectures */
 #endif
 
 static int