--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,22 @@
+2011-03-20  Paul Eggert  <eggert@cs.ucla.edu>
+            Bruno Haible  <bruno@clisp.org>
+
+	socklen: do not depend on sys_socket
+	While trying to modify Emacs to use gnulib's socklen module,
+	I discovered a circular dependency: socklen depends on sys_socket
+	and vice versa.  Emacs can use socklen, but it does not need
+	sys_socket because it has its own substitute for sys/socket.h.
+	* m4/socklen.m4 (gl_SOCKET_HEADERS): New macro, extracted from
+	gl_TYPE_SOCKLEN_T.
+	(gl_CHECK_SOCKET_HEADERS): New macro, taken from parts of
+	gl_PREREQ_SYS_H_SOCKET.
+	(gl_TYPE_SOCKLEN_T): Require it instead of requiring
+	gl_PREREQ_SYS_H_SOCKET.
+	* m4/sys_socket_h.m4 (gl_PREREQ_SYS_H_SOCKET): Require
+	gl_CHECK_SOCKET_HEADERS instead of doing its work ourselves.
+	* modules/socklen (Depends-on): Do not depend on sys_socket.
+	(Include): Adjust to match the code used in gl_SOCKET_HEADERS.
+
 2011-03-20  Jim Meyering  <meyering@redhat.com>
 
 	maint.mk: sort file names *after* new transformation

--- a/m4/socklen.m4
+++ b/m4/socklen.m4
@@ -1,4 +1,4 @@
-# socklen.m4 serial 9
+# socklen.m4 serial 10
 dnl Copyright (C) 2005-2007, 2009-2011 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -12,12 +12,8 @@
 dnl HP-UX 10.20, IRIX 6.5, OSF/1 4.0, Interix 3.5, BeOS.
 dnl So we have to test to find something that will work.
 
-dnl On mingw32, socklen_t is in ws2tcpip.h ('int'), so we try to find
-dnl it there first.  That file is included by gnulib's sys_socket.in.h, which
-dnl all users of this module should include.  Cygwin must not include
-dnl ws2tcpip.h.
 AC_DEFUN([gl_TYPE_SOCKLEN_T],
-  [AC_REQUIRE([gl_HEADER_SYS_SOCKET])dnl
+  [AC_REQUIRE([gl_CHECK_SOCKET_HEADERS])dnl
    AC_CHECK_TYPE([socklen_t], ,
      [AC_MSG_CHECKING([for socklen_t equivalent])
       AC_CACHE_VAL([gl_cv_socklen_t_equiv],
@@ -45,9 +41,37 @@
       AC_MSG_RESULT([$gl_cv_socklen_t_equiv])
       AC_DEFINE_UNQUOTED([socklen_t], [$gl_cv_socklen_t_equiv],
         [type to use in place of socklen_t if not defined])],
-     [#include <sys/types.h>
-      #if HAVE_SYS_SOCKET_H
-      # include <sys/socket.h>
-      #elif HAVE_WS2TCPIP_H
-      # include <ws2tcpip.h>
-      #endif])])
+     [gl_SOCKET_HEADERS])])
+
+dnl On mingw32, socklen_t is in ws2tcpip.h ('int'), so we try to find
+dnl it there too.  But on Cygwin, wc2tcpip.h must not be included.  Users
+dnl of this module should use the same include pattern as gl_SOCKET_HEADERS.
+dnl When you change this macro, keep also in sync:
+dnl   - gl_CHECK_SOCKET_HEADERS,
+dnl   - the Include section of modules/socklen.
+AC_DEFUN([gl_SOCKET_HEADERS],
+[
+/* <sys/types.h> is not needed according to POSIX, but the
+   <sys/socket.h> in i386-unknown-freebsd4.10 and
+   powerpc-apple-darwin5.5 required it. */
+#include <sys/types.h>
+#if HAVE_SYS_SOCKET_H
+# include <sys/socket.h>
+#elif HAVE_WS2TCPIP_H
+# include <ws2tcpip.h>
+#endif
+])
+
+dnl Tests for the existence of the header for socket facilities.
+dnl Defines the C macros HAVE_SYS_SOCKET_H, HAVE_WS2TCPIP_H.
+dnl This macro must match gl_SOCKET_HEADERS.
+AC_DEFUN([gl_CHECK_SOCKET_HEADERS],
+  [AC_CHECK_HEADERS_ONCE([sys/socket.h])
+   if test $ac_cv_header_sys_socket_h = no; then
+     dnl We cannot use AC_CHECK_HEADERS_ONCE here, because that would make
+     dnl the check for those headers unconditional; yet cygwin reports
+     dnl that the headers are present but cannot be compiled (since on
+     dnl cygwin, all socket information should come from sys/socket.h).
+     AC_CHECK_HEADERS([ws2tcpip.h])
+   fi
+  ])

--- a/m4/sys_socket_h.m4
+++ b/m4/sys_socket_h.m4
@@ -1,4 +1,4 @@
-# sys_socket_h.m4 serial 21
+# sys_socket_h.m4 serial 22
 dnl Copyright (C) 2005-2011 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -101,17 +101,13 @@
 AC_DEFUN([gl_PREREQ_SYS_H_SOCKET],
 [
   dnl Check prerequisites of the <sys/socket.h> replacement.
+  AC_REQUIRE([gl_CHECK_SOCKET_HEADERS])
   gl_CHECK_NEXT_HEADERS([sys/socket.h])
   if test $ac_cv_header_sys_socket_h = yes; then
     HAVE_SYS_SOCKET_H=1
     HAVE_WS2TCPIP_H=0
   else
     HAVE_SYS_SOCKET_H=0
-    dnl We cannot use AC_CHECK_HEADERS_ONCE here, because that would make
-    dnl the check for those headers unconditional; yet cygwin reports
-    dnl that the headers are present but cannot be compiled (since on
-    dnl cygwin, all socket information should come from sys/socket.h).
-    AC_CHECK_HEADERS([ws2tcpip.h])
     if test $ac_cv_header_ws2tcpip_h = yes; then
       HAVE_WS2TCPIP_H=1
     else

--- a/modules/socklen
+++ b/modules/socklen
@@ -5,7 +5,6 @@
 m4/socklen.m4
 
 Depends-on:
-sys_socket
 
 configure.ac:
 gl_TYPE_SOCKLEN_T
@@ -13,8 +12,12 @@
 Makefile.am:
 
 Include:
-<sys/types.h>
-<sys/socket.h>
+#include <sys/types.h>
+#if HAVE_SYS_SOCKET_H
+# include <sys/socket.h>
+#elif HAVE_WS2TCPIP_H
+# include <ws2tcpip.h>
+#endif
 
 License:
 unlimited