diff --git a/src/s/osf5-0.h b/src/s/osf5-0.h
index 3845fa76d02..af0dd1f8a63 100644
--- a/src/s/osf5-0.h
+++ b/src/s/osf5-0.h
@@ -13,3 +13,5 @@
 
 /* We have missing/inconsistent prototypes on 5.0, at least.  */
 #define INHIBIT_X11R6_XIM
+
+#define USE_MMAP_FOR_BUFFERS	1