diff --git a/ChangeLog b/ChangeLog
index a1b20d5bda..4692ae6ebb 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,4 +1,4 @@
-Sun Feb  6 23:51:30 2005  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+Mon Feb  7 01:22:50 2005  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* ext/extmk.rb (extract_makefile): extract previously collected
 	  informations from existing Makefile.
@@ -24,6 +24,9 @@ Sun Feb  6 23:51:30 2005  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 	* lib/mkmf.rb (dir_config): accept arrays of directory names as
 	  default values.
 
+	* lib/mkmf.rb (with_cppflags, with_cflags, with_ldflags): keep flags
+	  modified if the block returned true.
+
 Sun Feb  6 19:20:05 2005  NAKAMURA Usaku  <usa@ruby-lang.org>
 
 	* eval.c (stack_extend): add prototype because VC++8 doesn't
diff --git a/lib/mkmf.rb b/lib/mkmf.rb
index ce88760230..c5cfad30bb 100644
--- a/lib/mkmf.rb
+++ b/lib/mkmf.rb
@@ -294,25 +294,25 @@ end
 def with_cppflags(flags)
   cppflags = $CPPFLAGS
   $CPPFLAGS = flags
-  return yield
+  ret = yield
 ensure
-  $CPPFLAGS = cppflags
+  $CPPFLAGS = cppflags unless ret
 end
 
 def with_cflags(flags)
   cflags = $CFLAGS
   $CFLAGS = flags
-  return yield
+  ret = yield
 ensure
-  $CFLAGS = cflags
+  $CFLAGS = cflags unless ret
 end
 
 def with_ldflags(flags)
   ldflags = $LDFLAGS
   $LDFLAGS = flags
-  return yield
+  ret = yield
 ensure
-  $LDFLAGS = ldflags
+  $LDFLAGS = ldflags unless ret
 end
 
 def try_static_assert(expr, headers = nil, opt = "", &b)