diff --git a/ChangeLog b/ChangeLog
index 5546ac5261..90f4d8232d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Thu Dec  8 12:11:06 2011  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* configure.in (RUBY_WERROR_FLAG): append all warning flags which
+	  are enabled to compile, so that printf format modifiers properly
+	  fail.  [ruby-core:41351] [Bug #5679]
+
 Thu Dec  8 07:20:15 2011  Eric Hodel  <drbrain@segment7.net>
 
 	* doc/re.rdoc:  Document difference between match and =~, options with
diff --git a/configure.in b/configure.in
index c6ef53fe63..7a6dcb61f6 100644
--- a/configure.in
+++ b/configure.in
@@ -418,6 +418,8 @@ dnl }
 dnl compiler section {
 
 AC_DEFUN([RUBY_WERROR_FLAG], [dnl
+save_CFLAGS="$CFLAGS"
+CFLAGS="$CFLAGS $rb_cv_warnflags"
 if test "${ac_c_werror_flag+set}"; then
   rb_c_werror_flag="$ac_c_werror_flag"
 else
@@ -425,6 +427,8 @@ else
 fi
 ac_c_werror_flag=yes
 $1
+CFLAGS="$save_CFLAGS"
+save_CFLAGS=
 if test "${rb_c_werror_flag+set}"; then
   ac_c_werror_flag="$rb_c_werror_flag"
 else
@@ -432,18 +436,15 @@ else
 fi])
 
 AC_DEFUN(RUBY_TRY_CFLAGS, [
-    save_CFLAGS="$CFLAGS"
-    CFLAGS="[$]CFLAGS $1"
     AC_MSG_CHECKING([whether ]$1[ is accepted as CFLAGS])
     RUBY_WERROR_FLAG([
+    CFLAGS="[$]CFLAGS $1"
     AC_TRY_COMPILE([$4], [$5],
 	[$2
 	AC_MSG_RESULT(yes)],
 	[$3
 	AC_MSG_RESULT(no)])
     ])
-    CFLAGS="$save_CFLAGS"
-    save_CFLAGS=
 ])
 
 AC_DEFUN(RUBY_TRY_LDFLAGS, [