diff -ur py3k.ori/Lib/subprocess.py py3k/Lib/subprocess.py
--- py3k.ori/Lib/subprocess.py	2011-01-04 19:07:07.000000000 +0000
+++ py3k/Lib/subprocess.py	2011-03-02 16:42:36.000000000 +0000
@@ -417,10 +417,6 @@
 if mswindows:
     from _subprocess import CREATE_NEW_CONSOLE, CREATE_NEW_PROCESS_GROUP
     __all__.extend(["CREATE_NEW_CONSOLE", "CREATE_NEW_PROCESS_GROUP"])
-try:
-    MAXFD = os.sysconf("SC_OPEN_MAX")
-except:
-    MAXFD = 256
 
 _active = []
 
@@ -1112,8 +1108,7 @@
                 if fd >= start_fd:
                     os.closerange(start_fd, fd)
                     start_fd = fd + 1
-            if start_fd <= MAXFD:
-                os.closerange(start_fd, MAXFD)
+            os.closefrom(start_fd)
 
 
         def _execute_child(self, args, executable, preexec_fn, close_fds,
diff -ur py3k.ori/Lib/test/test_os.py py3k/Lib/test/test_os.py
--- py3k.ori/Lib/test/test_os.py	2011-01-03 00:19:11.000000000 +0000
+++ py3k/Lib/test/test_os.py	2011-03-02 16:41:57.000000000 +0000
@@ -60,6 +60,22 @@
         os.closerange(first, first + 2)
         self.assertRaises(OSError, os.write, first, b"a")
 
+    def test_closefrom(self):
+        # allocate a few file descriptors, close those smaller than min_fd,
+        # and then close all the remaining (greater than min_fd, to avoid
+        # closing stdin, stdout or stderr) with closefrom
+        min_fd = 7
+        fd_list = []
+        for i in range(10):
+            fd_list.append(os.open(support.TESTFN, os.O_CREAT|os.O_RDWR))
+        for fd in fd_list:
+            if fd < min_fd:
+                os.close(fd)
+        # now, close all the remaining FD with closefrom
+        os.closefrom(min_fd)
+        for fd in fd_list:
+            self.assertRaises(OSError, os.write, fd, b"a")
+
     @support.cpython_only
     def test_rename(self):
         path = support.TESTFN
diff -ur py3k.ori/Modules/posixmodule.c py3k/Modules/posixmodule.c
--- py3k.ori/Modules/posixmodule.c	2011-01-19 15:21:35.000000000 +0000
+++ py3k/Modules/posixmodule.c	2011-03-02 16:42:10.000000000 +0000
@@ -295,6 +295,9 @@
 #endif
 #endif /* MAXPATHLEN */
 
+/* Maximum file descriptor, initialized on module load. */
+static int max_fd;
+
 #ifdef UNION_WAIT
 /* Emulate some macros on systems that have a union instead of macros */
 
@@ -5557,6 +5560,50 @@
 }
 
 
+PyDoc_STRVAR(posix_closefrom__doc__,
+"closefrom(fd_from)\n\n\
+Closes all file descriptors from fd_from, ignoring errors.");
+
+static PyObject *
+posix_closefrom(PyObject *self, PyObject *args)
+{
+    int fd_from, i;
+#ifndef MS_WINDOWS
+    DIR *dir;
+    struct dirent *dp;
+    long fd;
+#endif
+
+    if (!PyArg_ParseTuple(args, "i:closefrom", &fd_from))
+        return NULL;
+
+    Py_BEGIN_ALLOW_THREADS
+#ifdef HAVE_CLOSEFROM
+    closefrom(fd_from);
+#elif !defined(MS_WINDOWS)
+    /* Unix, see if /proc/self/fd is available */
+    if ((dir = opendir("/proc/self/fd")) != NULL) {
+        while ((dp = readdir(dir)) != NULL) {
+            fd = strtol(dp->d_name, NULL, 0);
+            if (fd >= fd_from && fd <= INT_MAX && fd != dirfd(dir))
+                close(fd);
+        }
+        closedir(dir);
+    } else {
+        /* /proc/self/fd not available, close FDs individually */
+        for (i = fd_from; i < max_fd; i++)
+            close(i);
+    }
+#else /* MS_WINDOWS, close FDs individually */
+    for (i = fd_from; i < max_fd; i++)
+        if (_PyVerify_fd(i))
+            close(i);
+#endif
+    Py_END_ALLOW_THREADS
+    Py_RETURN_NONE;
+}
+
+
 PyDoc_STRVAR(posix_dup__doc__,
 "dup(fd) -> fd2\n\n\
 Return a duplicate of a file descriptor.");
@@ -7965,6 +8012,7 @@
     {"open",            posix_open, METH_VARARGS, posix_open__doc__},
     {"close",           posix_close, METH_VARARGS, posix_close__doc__},
     {"closerange",      posix_closerange, METH_VARARGS, posix_closerange__doc__},
+    {"closefrom",       posix_closefrom, METH_VARARGS, posix_closefrom__doc__},
     {"device_encoding", device_encoding, METH_VARARGS, device_encoding__doc__},
     {"dup",             posix_dup, METH_VARARGS, posix_dup__doc__},
     {"dup2",            posix_dup2, METH_VARARGS, posix_dup2__doc__},
@@ -8525,6 +8573,13 @@
 
 
 #endif /* __APPLE__ */
+
+#ifdef _SC_OPEN_MAX
+    max_fd = sysconf(_SC_OPEN_MAX);
+    if (max_fd == -1)
+#endif
+        max_fd = 256;
+
     return m;
 
 }