--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,18 @@
+2006-12-18  Paul Eggert  <eggert@cs.ucla.edu>
+	and Eric Blake  <ebb9@byu.net>
+
+	* lib/dirname.h (DOUBLE_SLASH_IS_DISTINCT_ROOT): Default to 0, not 1.
+	This is more consistent with the other defines here.
+	* m4/double-slash-root.m4 (gl_DOUBLE_SLASH_ROOT):
+	Port to z/OS.  Problem reported by Paul Gilmartin.
+	Change local vars to use gl_ prefix rather than ac_.
+	Don't define DOUBLE_SLASH_IS_DISTINCT_ROOT to 0, for consistency
+	with other defines.
+	* modules/double-slash-root: New module.
+	* modules/dirname (Files): Remove m4/double-slash-root.m4.
+	(Depends-on): Add double-slash-root.
+	* MODULES.html.sh (File system functions): Mention new module.
+
 2006-12-14  Paul Eggert  <eggert@cs.ucla.edu>
 
 	* lib/yesno.c [!ENABLE_NLS]: Don't include getline.h.

--- a/MODULES.html.sh
+++ b/MODULES.html.sh
@@ -1978,6 +1978,7 @@
   func_module cycle-check
   func_module fsusage
   func_module dirfd
+  func_module double-slash-root
   func_module euidaccess
   func_module file-type
   func_module fileblocks

--- a/lib/dirname.h
+++ b/lib/dirname.h
@@ -48,7 +48,7 @@
 # endif
 
 # ifndef DOUBLE_SLASH_IS_DISTINCT_ROOT
-#  define DOUBLE_SLASH_IS_DISTINCT_ROOT 1
+#  define DOUBLE_SLASH_IS_DISTINCT_ROOT 0
 # endif
 
 # if FILE_SYSTEM_DRIVE_PREFIX_CAN_BE_RELATIVE

--- a/m4/double-slash-root.m4
+++ b/m4/double-slash-root.m4
@@ -1,4 +1,4 @@
-#serial 1   -*- autoconf -*-
+# double-slash-root.m4 serial 2   -*- Autoconf -*-
 dnl Copyright (C) 2006 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -7,37 +7,32 @@
 AC_DEFUN([gl_DOUBLE_SLASH_ROOT],
 [
   AC_REQUIRE([AC_CANONICAL_HOST])
-  AC_CACHE_CHECK([whether // is distinct from /], [ac_cv_double_slash_root],
+  AC_CACHE_CHECK([whether // is distinct from /], [gl_cv_double_slash_root],
     [ if test x"$cross_compiling" = xyes ; then
 	# When cross-compiling, there is no way to tell whether // is special
 	# short of a list of hosts.  However, the only known hosts to date
-	# that have a distinct // are Apollo DomainOS (too old to port to)
-	# and Cygwin.  If anyone knows of another system for which // has
+	# that have a distinct // are Apollo DomainOS (too old to port to),
+	# Cygwin, and z/OS.  If anyone knows of another system for which // has
 	# special semantics and is distinct from /, please report it to
-	# <bug-coreutils@gnu.org>.
+	# <bug-gnulib@gnu.org>.
 	case $host in
-	  *-cygwin)
-	    ac_cv_double_slash_root=yes ;;
+	  *-cygwin | i370-ibm-openedition)
+	    gl_cv_double_slash_root=yes ;;
 	  *)
 	    # Be optimistic and assume that / and // are the same when we
 	    # don't know.
-	    ac_cv_double_slash_root='unknown, assuming no' ;;
+	    gl_cv_double_slash_root='unknown, assuming no' ;;
 	esac
       else
 	set x `ls -di / //`
-	if test $[2] = $[4]; then
-	  ac_cv_double_slash_root=no
+	if test $[2] = $[4] && wc //dev/null >/dev/null 2>&1; then
+	  gl_cv_double_slash_root=no
 	else
-	  ac_cv_double_slash_root=yes
+	  gl_cv_double_slash_root=yes
 	fi
       fi])
-  if test x"$ac_cv_double_slash_root" = xyes; then
-    ac_double_slash_root=1
-  else
-    ac_double_slash_root=0
+  if test "$gl_cv_double_slash_root" = yes; then
+    AC_DEFINE([DOUBLE_SLASH_IS_DISTINCT_ROOT], 1,
+      [Define to 1 if // is a file system root distinct from /.])
   fi
-
-  AC_DEFINE_UNQUOTED([DOUBLE_SLASH_IS_DISTINCT_ROOT],
-   $ac_double_slash_root,
-   [Define to 1 if // is a file system root distinct from /.])
 ])

--- a/modules/dirname
+++ b/modules/dirname
@@ -6,11 +6,11 @@
 lib/dirname.c
 lib/basename.c
 lib/stripslash.c
+m4/dirname.m4
 m4/dos.m4
-m4/dirname.m4
-m4/double-slash-root.m4
 
 Depends-on:
+double-slash-root
 stdbool
 xalloc
 xstrndup

new file mode 100644
--- /dev/null
+++ b/modules/double-slash-root
@@ -0,0 +1,20 @@
+Description:
+Check whether // is distinct from /.
+
+Files:
+m4/double-slash-root.m4
+
+Depends-on:
+
+configure.ac:
+gl_DOUBLE_SLASH_ROOT
+
+Makefile.am:
+
+Include:
+
+License:
+LGPL
+
+Maintainer:
+Eric Blake, Paul Eggert, Jim Meyering