--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,9 @@
 2009-08-23  Bruno Haible  <bruno@clisp.org>
 
+	Tests for module 'dup3'.
+	* modules/dup3-tests: New file.
+	* tests/test-dup3.c: New file.
+
 	New module 'dup3'.
 	* lib/unistd.in.h (dup3): New declaration.
 	* lib/dup3.c: New file.

new file mode 100644
--- /dev/null
+++ b/modules/dup3-tests
@@ -0,0 +1,11 @@
+Files:
+tests/test-dup3.c
+
+Depends-on:
+open
+
+configure.ac:
+
+Makefile.am:
+TESTS += test-dup3
+check_PROGRAMS += test-dup3

new file mode 100644
--- /dev/null
+++ b/tests/test-dup3.c
@@ -0,0 +1,167 @@
+/* Test duplicating file descriptors.
+   Copyright (C) 2009 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Written by Eric Blake <ebb9@byu.net>, 2009,
+   and Bruno Haible <bruno@clisp.org>, 2009.  */
+
+#include <config.h>
+
+#include <unistd.h>
+
+#include <errno.h>
+#include <fcntl.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#if (defined _WIN32 || defined __WIN32__) && ! defined __CYGWIN__
+/* Get declarations of the Win32 API functions.  */
+# define WIN32_LEAN_AND_MEAN
+# include <windows.h>
+#endif
+
+#include "binary-io.h"
+
+#define ASSERT(expr) \
+  do									     \
+    {									     \
+      if (!(expr))							     \
+        {								     \
+          fprintf (stderr, "%s:%d: assertion failed\n", __FILE__, __LINE__); \
+          fflush (stderr);						     \
+          abort ();							     \
+        }								     \
+    }									     \
+  while (0)
+
+/* Return true if FD is open.  */
+static bool
+is_open (int fd)
+{
+#if (defined _WIN32 || defined __WIN32__) && ! defined __CYGWIN__
+  /* On Win32, the initial state of unassigned standard file
+     descriptors is that they are open but point to an
+     INVALID_HANDLE_VALUE, and there is no fcntl.  */
+  return (HANDLE) _get_osfhandle (fd) != INVALID_HANDLE_VALUE;
+#else
+# ifndef F_GETFL
+#  error Please port fcntl to your platform
+# endif
+  return 0 <= fcntl (fd, F_GETFL);
+#endif
+}
+
+/* Return true if FD is not inherited to child processes.  */
+static bool
+is_cloexec (int fd)
+{
+#if (defined _WIN32 || defined __WIN32__) && ! defined __CYGWIN__
+  HANDLE h = (HANDLE) _get_osfhandle (fd);
+  DWORD flags;
+  ASSERT (GetHandleInformation (h, &flags));
+  return (flags & HANDLE_FLAG_INHERIT) == 0;
+#else
+  int flags;
+  ASSERT ((flags = fcntl (fd, F_GETFD)) >= 0);
+  return (flags & FD_CLOEXEC) != 0;
+#endif
+}
+
+int
+main ()
+{
+  int use_cloexec;
+
+#if defined O_CLOEXEC
+  for (use_cloexec = 0; use_cloexec <= 1; use_cloexec++)
+#else
+  use_cloexec = 0;
+#endif
+    {
+      const char *file = "test-dup3.tmp";
+      int fd = open (file, O_CREAT | O_TRUNC | O_RDWR, 0600);
+      int o_flags;
+      char buffer[1];
+
+      o_flags = 0;
+#if defined O_CLOEXEC
+      if (use_cloexec)
+	o_flags |= O_CLOEXEC;
+#endif
+
+      /* Assume std descriptors were provided by invoker.  */
+      ASSERT (STDERR_FILENO < fd);
+      ASSERT (is_open (fd));
+      /* Ignore any other fd's leaked into this process.  */
+      close (fd + 1);
+      close (fd + 2);
+      ASSERT (!is_open (fd + 1));
+      ASSERT (!is_open (fd + 2));
+
+      /* Assigning to self is invalid.  */
+      errno = 0;
+      ASSERT (dup3 (fd, fd, o_flags) == -1);
+      ASSERT (errno == EINVAL);
+      ASSERT (is_open (fd));
+
+      /* If the source is not open, then the destination is unaffected.  */
+      errno = 0;
+      ASSERT (dup3 (fd + 1, fd + 2, o_flags) == -1);
+      ASSERT (errno == EBADF);
+      ASSERT (!is_open (fd + 2));
+      errno = 0;
+      ASSERT (dup3 (fd + 1, fd, o_flags) == -1);
+      ASSERT (errno == EBADF);
+      ASSERT (is_open (fd));
+
+      /* The destination must be valid.  */
+      errno = 0;
+      ASSERT (dup3 (fd, -2, o_flags) == -1);
+      ASSERT (errno == EBADF);
+      errno = 0;
+      ASSERT (dup3 (fd, 10000000, o_flags) == -1);
+      ASSERT (errno == EBADF);
+
+      /* Using dup3 can skip fds.  */
+      ASSERT (dup3 (fd, fd + 2, o_flags) == fd + 2);
+      ASSERT (is_open (fd));
+      ASSERT (!is_open (fd + 1));
+      ASSERT (is_open (fd + 2));
+      if (use_cloexec)
+	ASSERT (is_cloexec (fd + 2));
+      else
+	ASSERT (!is_cloexec (fd + 2));
+
+      /* Verify that dup3 closes the previous occupant of a fd.  */
+      ASSERT (open ("/dev/null", O_WRONLY, 0600) == fd + 1);
+      ASSERT (dup3 (fd + 1, fd, o_flags) == fd);
+      ASSERT (close (fd + 1) == 0);
+      ASSERT (write (fd, "1", 1) == 1);
+      ASSERT (dup3 (fd + 2, fd, o_flags) == fd);
+      ASSERT (lseek (fd, 0, SEEK_END) == 0);
+      ASSERT (write (fd + 2, "2", 1) == 1);
+      ASSERT (lseek (fd, 0, SEEK_SET) == 0);
+      ASSERT (read (fd, buffer, 1) == 1);
+      ASSERT (*buffer == '2');
+
+      /* Clean up.  */
+      ASSERT (close (fd + 2) == 0);
+      ASSERT (close (fd) == 0);
+      ASSERT (unlink (file) == 0);
+    }
+
+  return 0;
+}