diff --git a/ChangeLog b/ChangeLog
index c0962089ba..e1e9a60762 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Jul 20 18:07:11 2005  Tanaka Akira  <akr@m17n.org>
+
+	* io.c (rb_io_ctl): update FMODE_WSPLIT_INITIALIZED and FMODE_WSPLIT
+	  by F_SETFL.
+
 Wed Jul 20 10:04:51 2005  Yukihiro Matsumoto  <matz@ruby-lang.org>
 
 	* variable.c (rb_class_path): need to adjust snprintf() len for
diff --git a/io.c b/io.c
index 89d05ed14d..10bbe574c3 100644
--- a/io.c
+++ b/io.c
@@ -4843,6 +4843,16 @@ rb_io_ctl(io, req, arg, io_p)
 	rb_raise(rb_eArgError, "return value overflowed string");
     }
 
+    if (!io_p && cmd == F_SETFL) {
+      if (narg & O_NONBLOCK) {
+        fptr->mode |= FMODE_WSPLIT_INITIALIZED;
+        fptr->mode &= ~FMODE_WSPLIT;
+      }
+      else {
+        fptr->mode &= ~(FMODE_WSPLIT_INITIALIZED|FMODE_WSPLIT);
+      }
+    }
+
     return INT2NUM(retval);
 #else
     rb_notimplement();