diff --git a/ChangeLog b/ChangeLog
index 1798485ad2..0f753b0828 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue May  3 03:57:04 2011  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* include/ruby/intern.h (rb_w32_fdcopy): add prototype.  fixes
+	  #4640
+
 Mon May  2 01:02:04 2011  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
 
 	* lib/fileutils.rb (FileUtils#chmod): accept symbolic mode argument.
diff --git a/include/ruby/intern.h b/include/ruby/intern.h
index c66064f8be..efb2814e6f 100644
--- a/include/ruby/intern.h
+++ b/include/ruby/intern.h
@@ -270,6 +270,7 @@ void rb_fd_set(int, rb_fdset_t *);
 #define rb_fd_clr(n, f)		rb_w32_fdclr((n), (f)->fdset)
 #define rb_fd_isset(n, f)	rb_w32_fdisset((n), (f)->fdset)
 #define rb_fd_copy(d, s)	rb_w32_fdcopy((d), (s))
+void rb_w32_fdcopy(rb_fdset_t *dst, const rb_fdset_t *src);
 #define rb_fd_select(n, rfds, wfds, efds, timeout)	rb_w32_select((n), (rfds) ? ((rb_fdset_t*)(rfds))->fdset : NULL, (wfds) ? ((rb_fdset_t*)(wfds))->fdset : NULL, (efds) ? ((rb_fdset_t*)(efds))->fdset: NULL, (timeout))
 #define rb_fd_resize(n, f)	((void)(f))