diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index 467f110ead6..cbb2452906f 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -32,8 +32,14 @@
 #include <unistd.h>
 #include <sys/mman.h>
 #include <sys/stat.h>
+
+#ifndef MS_SYNC
+/* This is missing e.g. on SunOS 4.1.4 */
+#define MS_SYNC 0
 #endif
 
+#endif /* UNIX */
+
 #include <string.h>
 #include <sys/types.h>