--- ruby-3.0.2/include/ruby/internal/intern/select/posix.h
+++ ruby-3.0.2/include/ruby/internal/intern/select/posix.h
@@ -55,7 +55,7 @@ rb_fd_copy(rb_fdset_t *dst, const fd_set *src, int n)
 }

 static inline void
-rb_fd_dup(rb_fdset_t *dst, const fd_set *src, int n)
+rb_fd_dup(rb_fdset_t *dst, const fd_set *src)
 {
     *dst = *src;
 }