diff -r 9725ddcd37e6 Doc/library/os.rst
--- a/Doc/library/os.rst	Sun Jun 24 11:57:07 2012 +0200
+++ b/Doc/library/os.rst	Sun Jun 24 12:29:50 2012 +0200
@@ -753,7 +753,7 @@ as internal buffering of data.
 .. function:: ftruncate(fd, length)
 
    Truncate the file corresponding to file descriptor *fd*, so that it is at most
-   *length* bytes in size.
+   *length* bytes in size.  Equivalent to ``os.truncate(fd, length)``.
 
    Availability: Unix.
 
@@ -2057,6 +2057,8 @@ features:
    Truncate the file corresponding to *path*, so that it is at most
    *length* bytes in size.
 
+   This function can support :ref:`specifying a file descriptor <path_fd>`.
+
    Availability: Unix.
 
    .. versionadded:: 3.3
diff -r 9725ddcd37e6 Lib/os.py
--- a/Lib/os.py	Sun Jun 24 11:57:07 2012 +0200
+++ b/Lib/os.py	Sun Jun 24 12:29:50 2012 +0200
@@ -172,6 +172,7 @@ if _exists("_have_functions"):
     _add("HAVE_FDOPENDIR",  "listdir")
     _add("HAVE_FEXECVE",    "execve")
     _set.add(stat) # fstat always works
+    _add("HAVE_FTRUNCATE",  "truncate")
     _add("HAVE_FUTIMENS",   "utime")
     _add("HAVE_FUTIMES",    "utime")
     if _exists("statvfs") and _exists("fstatvfs"): # mac os x10.3
diff -r 9725ddcd37e6 Modules/posixmodule.c
--- a/Modules/posixmodule.c	Sun Jun 24 11:57:07 2012 +0200
+++ b/Modules/posixmodule.c	Sun Jun 24 12:29:50 2012 +0200
@@ -8482,28 +8482,44 @@ posix_ftruncate(PyObject *self, PyObject
 #ifdef HAVE_TRUNCATE
 PyDoc_STRVAR(posix_truncate__doc__,
 "truncate(path, length)\n\n\
-Truncate the file given by path to length bytes.");
-
-static PyObject *
-posix_truncate(PyObject *self, PyObject *args)
-{
-    PyObject *opath;
-    const char *path;
+Truncate the file given by path to length bytes.\n\
+On some platforms, path may also be specified as an open file descriptor.\n\
+  If this functionality is unavailable, using it raises an exception.");
+
+static PyObject *
+posix_truncate(PyObject *self, PyObject *args, PyObject *kwargs)
+{
+    path_t path;
     off_t length;
     int res;
-
-    if (!PyArg_ParseTuple(args, "O&O&:truncate",
-            PyUnicode_FSConverter, &opath, _parse_off_t, &length))
-        return NULL;
-    path = PyBytes_AsString(opath);
+    PyObject *result = NULL;
+    static char *keywords[] = {"path", "length", NULL};
+
+    memset(&path, 0, sizeof(path));
+#ifdef HAVE_FTRUNCATE
+    path.allow_fd = 1;
+#endif
+    if (!PyArg_ParseTupleAndKeywords(args, kwargs, "O&O&:truncate", keywords,
+                                     path_converter, &path,
+                                     _parse_off_t, &length))
+        return NULL;
 
     Py_BEGIN_ALLOW_THREADS
-    res = truncate(path, length);
+#ifdef HAVE_FTRUNCATE
+    if (path.fd != -1)
+        res = ftruncate(path.fd, length);
+    else
+#endif
+        res = truncate(path.narrow, length);
     Py_END_ALLOW_THREADS
-    Py_DECREF(opath);
     if (res < 0)
-        return posix_error();
-    Py_RETURN_NONE;
+        result = path_error("truncate", &path);
+    else {
+        Py_INCREF(Py_None);
+        result = Py_None;
+    }
+    path_cleanup(&path);
+    return result;
 }
 #endif
 
@@ -11078,7 +11094,9 @@ static PyMethodDef posix_methods[] = {
     {"ftruncate",       posix_ftruncate, METH_VARARGS, posix_ftruncate__doc__},
 #endif
 #ifdef HAVE_TRUNCATE
-    {"truncate",        posix_truncate, METH_VARARGS, posix_truncate__doc__},
+    {"truncate",        (PyCFunction)posix_truncate,
+                        METH_VARARGS | METH_KEYWORDS,
+                        posix_truncate__doc__},
 #endif
 #ifdef HAVE_POSIX_FALLOCATE
     {"posix_fallocate", posix_posix_fallocate, METH_VARARGS, posix_posix_fallocate__doc__},
@@ -11749,6 +11767,10 @@ static char *have_functions[] = {
     "HAVE_FSTATVFS",
 #endif
 
+#ifdef HAVE_FTRUNCATE
+    "HAVE_FTRUNCATE",
+#endif
+
 #ifdef HAVE_FUTIMENS
     "HAVE_FUTIMENS",
 #endif