diff --git a/src/print-system-constants.c b/src/print-system-constants.c
index 4db74e5..1119147 100644
--- a/src/print-system-constants.c
+++ b/src/print-system-constants.c
@@ -1,3 +1,6 @@
+/**
+ * see: https://en.cppreference.com/w/c/types/limits
+ */
 #include <limits.h>
 #include <stdint.h>
 #include <stdio.h>