diff --git a/apps/ocsp.c b/apps/ocsp.c
index 8bf67aad31227e82f5a6ad254b76a22b9baa5c26..c8676fc537003b559b952f95b99b95a4cd2488bb 100644
--- a/apps/ocsp.c
+++ b/apps/ocsp.c
@@ -59,6 +59,10 @@
 
 #include <stdio.h>
 #include <string.h>
+#define _XOPEN_SOURCE_EXTENDED	1 /* Or fd_set, select() and so on won't be
+				     declared properly on Compaq platforms
+				     (at least with DEC C).
+				  */
 #include <time.h>
 #include "apps.h"
 #include <openssl/pem.h>