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diff --git a/m4/dup2.m4 b/m4/dup2.m4
index 05498235..59028e09 100644
--- a/m4/dup2.m4
+++ b/m4/dup2.m4
@@ -1,5 +1,5 @@
-#serial 5
-dnl Copyright (C) 2002, 2005, 2007 Free Software Foundation, Inc.
+#serial 24
+dnl Copyright (C) 2002, 2005, 2007, 2009-2015 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
 dnl with or without modifications, as long as this notice is preserved.
@@ -7,9 +7,99 @@ dnl with or without modifications, as long as this notice is preserved.
 AC_DEFUN([gl_FUNC_DUP2],
 [
   AC_REQUIRE([gl_UNISTD_H_DEFAULTS])
-  AC_CHECK_FUNCS_ONCE([dup2])
-  if test $ac_cv_func_dup2 = no; then
-    HAVE_DUP2=0
-    AC_LIBOBJ([dup2])
+  AC_REQUIRE([AC_CANONICAL_HOST])
+  m4_ifdef([gl_FUNC_DUP2_OBSOLETE], [
+    AC_CHECK_FUNCS_ONCE([dup2])
+    if test $ac_cv_func_dup2 = no; then
+      HAVE_DUP2=0
+    fi
+  ], [
+    AC_DEFINE([HAVE_DUP2], [1], [Define to 1 if you have the 'dup2' function.])
+  ])
+  if test $HAVE_DUP2 = 1; then
+    AC_CACHE_CHECK([whether dup2 works], [gl_cv_func_dup2_works],
+      [AC_RUN_IFELSE([
+         AC_LANG_PROGRAM(
+           [[#include <errno.h>
+             #include <fcntl.h>
+             #include <limits.h>
+             #include <sys/resource.h>
+             #include <unistd.h>
+             #ifndef RLIM_SAVED_CUR
+             # define RLIM_SAVED_CUR RLIM_INFINITY
+             #endif
+             #ifndef RLIM_SAVED_MAX
+             # define RLIM_SAVED_MAX RLIM_INFINITY
+             #endif
+           ]],
+           [[int result = 0;
+             int bad_fd = INT_MAX;
+             struct rlimit rlim;
+             if (getrlimit (RLIMIT_NOFILE, &rlim) == 0
+                 && 0 <= rlim.rlim_cur && rlim.rlim_cur <= INT_MAX
+                 && rlim.rlim_cur != RLIM_INFINITY
+                 && rlim.rlim_cur != RLIM_SAVED_MAX
+                 && rlim.rlim_cur != RLIM_SAVED_CUR)
+               bad_fd = rlim.rlim_cur;
+             #ifdef FD_CLOEXEC
+               if (fcntl (1, F_SETFD, FD_CLOEXEC) == -1)
+                 result |= 1;
+             #endif
+             if (dup2 (1, 1) != 1)
+               result |= 2;
+             #ifdef FD_CLOEXEC
+               if (fcntl (1, F_GETFD) != FD_CLOEXEC)
+                 result |= 4;
+             #endif
+             close (0);
+             if (dup2 (0, 0) != -1)
+               result |= 8;
+             /* Many gnulib modules require POSIX conformance of EBADF.  */
+             if (dup2 (2, bad_fd) == -1 && errno != EBADF)
+               result |= 16;
+             /* Flush out some cygwin core dumps.  */
+             if (dup2 (2, -1) != -1 || errno != EBADF)
+               result |= 32;
+             dup2 (2, 255);
+             dup2 (2, 256);
+             return result;]])
+        ],
+        [gl_cv_func_dup2_works=yes], [gl_cv_func_dup2_works=no],
+        [case "$host_os" in
+           mingw*) # on this platform, dup2 always returns 0 for success
+             gl_cv_func_dup2_works="guessing no" ;;
+           cygwin*) # on cygwin 1.5.x, dup2(1,1) returns 0
+             gl_cv_func_dup2_works="guessing no" ;;
+           aix* | freebsd*)
+                   # on AIX 7.1 and FreeBSD 6.1, dup2 (1,toobig) gives EMFILE,
+                   # not EBADF.
+             gl_cv_func_dup2_works="guessing no" ;;
+           haiku*) # on Haiku alpha 2, dup2(1, 1) resets FD_CLOEXEC.
+             gl_cv_func_dup2_works="guessing no" ;;
+           *-android*) # implemented using dup3(), which fails if oldfd == newfd
+             gl_cv_func_dup2_works="guessing no" ;;
+           *) gl_cv_func_dup2_works="guessing yes" ;;
+         esac])
+      ])
+    case "$gl_cv_func_dup2_works" in
+      *yes) ;;
+      *)
+        REPLACE_DUP2=1
+        AC_CHECK_FUNCS([setdtablesize])
+        ;;
+    esac
   fi
+  dnl Replace dup2() for supporting the gnulib-defined fchdir() function,
+  dnl to keep fchdir's bookkeeping up-to-date.
+  m4_ifdef([gl_FUNC_FCHDIR], [
+    gl_TEST_FCHDIR
+    if test $HAVE_FCHDIR = 0; then
+      if test $HAVE_DUP2 = 1; then
+        REPLACE_DUP2=1
+      fi
+    fi
+  ])
 ])
+
+# Prerequisites of lib/dup2.c.
+AC_DEFUN([gl_PREREQ_DUP2], [])