diff --git a/gcc/ada/ChangeLog b/gcc/ada/ChangeLog
index b8f30a9bc224..d7dd99f7c8a3 100644
--- a/gcc/ada/ChangeLog
+++ b/gcc/ada/ChangeLog
@@ -1,3 +1,7 @@
+2011-12-21  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>
+
+	* s-oscons-tmplt.c [__alpha__ && __osf__] (_XOPEN_SOURCE): Define.
+
 2011-12-21  Javier Miranda  <miranda@adacore.com>
 
 	* sem_ch3.ads, sem_ch3.adb (Check_CPP_Type): New subprogram.
diff --git a/gcc/ada/s-oscons-tmplt.c b/gcc/ada/s-oscons-tmplt.c
index 5f6782054262..340abe99fe83 100644
--- a/gcc/ada/s-oscons-tmplt.c
+++ b/gcc/ada/s-oscons-tmplt.c
@@ -85,6 +85,12 @@ pragma Style_Checks ("M32766");
  **/
 #define _XOPEN_SOURCE 500
 
+#elif defined (__alpha__) && defined (__osf__)
+/** For Tru64 UNIX, _XOPEN_SOURCE must be defined, otherwise CLOCK_REALTIME
+ ** is not defined.
+ **/
+#define _XOPEN_SOURCE 500
+
 #elif defined (__mips) && defined (__sgi)
 /** For IRIX 6, _XOPEN5 must be defined and _XOPEN_IOV_MAX must be used as
  ** IOV_MAX, otherwise IOV_MAX is not defined.  IRIX 5 has neither.