diff --git a/SRC/echoping.h b/SRC/echoping.h
index 1b8f481..e5962a9 100644
--- a/SRC/echoping.h
+++ b/SRC/echoping.h
@@ -20,6 +20,11 @@
 #endif
 #define MAXNUMBER 20
 
+/* For Tru 64 > 5 only */
+#ifndef _POSIX_PII_SOCKET
+#define _POSIX_PII_SOCKET 1
+#endif
+
 /* Probably too many inclusions but this is to keep 'gcc -Wall' happy... */
 #include	<stdio.h>
 #include        <stdlib.h>