diff --git a/ext/io/console/extconf.rb b/ext/io/console/extconf.rb
index 918f86b27f..eb431b008d 100644
--- a/ext/io/console/extconf.rb
+++ b/ext/io/console/extconf.rb
@@ -4,7 +4,7 @@ ok = true
 hdr = nil
 case
 when macro_defined?("_WIN32", "")
-  have_func("rb_w32_map_errno", "ruby.h")
+  # rb_w32_map_errno: 1.8.7
 when hdr = %w"termios.h termio.h".find {|h| have_header(h)}
   have_func("cfmakeraw", hdr)
 when have_header(hdr = "sgtty.h")
@@ -14,8 +14,8 @@ else
 end
 if ok
   have_header("sys/ioctl.h")
-  have_func("rb_check_hash_type", "ruby.h")
-  have_func("rb_io_get_write_io", "ruby/io.h")
-  have_func("rb_cloexec_open", "ruby/io.h")
+  # rb_check_hash_type: 1.9.3
+  # rb_io_get_write_io: 1.9.1
+  # rb_cloexec_open: 2.0.0
   create_makefile("io/console")
 end