diff --git a/libf2c/ChangeLog b/libf2c/ChangeLog
index 6b2e3e0dfa1..40728f00ffc 100644
--- a/libf2c/ChangeLog
+++ b/libf2c/ChangeLog
@@ -1,3 +1,7 @@
+1998-10-12  Dave Love  <fx@gnu.org>
+
+	* libI77/open.c (_XOPEN_SOURCE): Define.
+
 1998-10-12  Dave Love  <fx@gnu.org>
 
 	* Makefile.in (.SUFFIXES): Don't use any.
diff --git a/libf2c/libI77/open.c b/libf2c/libI77/open.c
index ad039f6d895..7c8f3ded7ac 100644
--- a/libf2c/libI77/open.c
+++ b/libf2c/libI77/open.c
@@ -1,3 +1,6 @@
+/* Define _XOPEN_SOURCE to get tempnam prototype with glibc et al --
+   more general than _INCLUDE_XOPEN_SOURCE used elsewhere `for HP-UX'.  */
+#define _XOPEN_SOURCE 1
 #include "f2c.h"
 #include "fio.h"
 #include <string.h>