diff --git a/ChangeLog b/ChangeLog
index 26cb92cc00..d26396ccfd 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,7 @@
 2009-04-25  Ulrich Drepper  <drepper@redhat.com>
 
+	* include/features.h: _POSIX_C_SOURCE >= 200112L implies C99.
+
 	* sysdeps/ieee754/k_standard.c (__kernel_standard): Pole errors
 	for lgamma should se errno to ERANGE, not EDOM.
 	* math/libm-test.inc (lgamma_test): Check errno for pole errors.
diff --git a/include/features.h b/include/features.h
index a696d8b59c..f7edc14aed 100644
--- a/include/features.h
+++ b/include/features.h
@@ -230,6 +230,8 @@
 
 #if (_POSIX_C_SOURCE - 0) >= 200112L
 # define __USE_XOPEN2K		1
+# undef __USE_ISOC99
+# define __USE_ISOC99		1
 #endif
 
 #if (_POSIX_C_SOURCE - 0) >= 200809L