Index: configure.in
===================================================================
--- configure.in	(Revision 66572)
+++ configure.in	(Arbeitskopie)
@@ -258,6 +258,12 @@
     # As this has a different meaning on Linux, only define it on OpenBSD
     AC_DEFINE(_BSD_SOURCE, 1, [Define on OpenBSD to activate all library features])
     ;;
+  OpenBSD/4.<:@4@:>@)
+    # OpenBSD undoes our definition of __BSD_VISIBLE if _XOPEN_SOURCE is
+    # also defined. This can be overridden by defining _BSD_SOURCE
+    # As this has a different meaning on Linux, only define it on OpenBSD
+    AC_DEFINE(_BSD_SOURCE, 1, [Define on OpenBSD to activate all library features])
+    ;;
   # Defining _XOPEN_SOURCE on NetBSD version prior to the introduction of
   # _NETBSD_SOURCE disables certain features (eg. setgroups). Reported by
   # Marc Recht
Index: Objects/obmalloc.c
===================================================================
--- Objects/obmalloc.c	(Revision 66572)
+++ Objects/obmalloc.c	(Arbeitskopie)
@@ -677,8 +677,8 @@
 /* This is only useful when running memory debuggers such as
  * Purify or Valgrind.  Uncomment to use.
  *
+#define Py_USING_MEMORY_DEBUGGER
  */
-#define Py_USING_MEMORY_DEBUGGER
 
 #ifdef Py_USING_MEMORY_DEBUGGER
 
Index: Misc/NEWS
===================================================================
--- Misc/NEWS	(Revision 66572)
+++ Misc/NEWS	(Arbeitskopie)
@@ -12,6 +12,8 @@
 Core and Builtins
 -----------------
 
+- Bug #3951: Py_USING_MEMORY_DEBUGGER should not be enabled by default.
+
 Library
 -------