diff --git a/ChangeLog b/ChangeLog
index 1e7202af8c..4f6de95fff 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Fri Feb  1 12:43:41 2008  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* configure.in (darwin): _XOPEN_SOURCE is necessary to make ucontext_t
+	  consistent with the library implementation of MacOS X 10.5.
+	  [ruby-dev:33461]
+
 Fri Feb  1 11:44:22 2008  Tanaka Akira  <akr@fsij.org>
 
 	* tool/compile_prelude.rb (C_ESC): use octal escape to avoid
diff --git a/configure.in b/configure.in
index 52f948d3ef..a5881685ba 100644
--- a/configure.in
+++ b/configure.in
@@ -466,6 +466,7 @@ nextstep*)	;;
 openstep*)	;;
 rhapsody*)	;;
 darwin*)	LIBS="-lobjc $LIBS"
+		CPPFLAGS="$CPPFLAGS -D_XOPEN_SOURCE=600 -D_DARWIN_C_SOURCE"
 		AC_TRY_CPP([#include <AvailabilityMacros.h>
 		    #if MAC_OS_X_VERSION_MAX_ALLOWED <= 1040
 		    #error pre OS X 10.4
@@ -475,7 +476,6 @@ darwin*)	LIBS="-lobjc $LIBS"
 		    [
 		    AC_DEFINE(BROKEN_SETREUID, 1)
 		    AC_DEFINE(BROKEN_SETREGID, 1)
-		    ac_cv_header_ucontext_h=no
 		    ])
 		;;
 hpux*)		LIBS="-lm $LIBS"
@@ -684,12 +684,18 @@ fi
 AC_DEFUN([RUBY_CHECK_VARTYPE], [dnl
 AC_CACHE_CHECK([for external $1], rb_cv_var_$1,
   [rb_cv_var_$1=no
-  AC_TRY_COMPILE([#define _XOPEN_SOURCE 1
+  AC_TRY_COMPILE([
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 1
+#endif
     $2
     const volatile void *volatile t;],
     [t = &(&$1)[0];],
     [for t in $3; do
-      AC_TRY_COMPILE([#define _XOPEN_SOURCE 1
+      AC_TRY_COMPILE([
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 1
+#endif
         $2
         extern $t $1;
         const volatile void *volatile t;],