Index: Lib/test/test_posix.py
===================================================================
--- Lib/test/test_posix.py	(revision 80186)
+++ Lib/test/test_posix.py	(working copy)
@@ -262,10 +262,15 @@
             posix.chdir(os.curdir)
             self.assertRaises(OSError, posix.chdir, test_support.TESTFN)
 
-    def test_lsdir(self):
-        if hasattr(posix, 'lsdir'):
-            self.assertIn(test_support.TESTFN, posix.lsdir(os.curdir))
+    def test_listdir(self):
+        if hasattr(posix, 'listdir'):
+            self.assertTrue(test_support.TESTFN in posix.listdir(os.curdir))
 
+    def test_listdir_default(self):
+        # When listdir is called without argument, it's the same as listdir(os.curdir)
+        if hasattr(posix, 'listdir'):
+            self.assertTrue(test_support.TESTFN in posix.listdir())
+    
     def test_access(self):
         if hasattr(posix, 'access'):
             self.assertTrue(posix.access(test_support.TESTFN, os.R_OK))
Index: Modules/posixmodule.c
===================================================================
--- Modules/posixmodule.c	(revision 80186)
+++ Modules/posixmodule.c	(working copy)
@@ -2086,18 +2086,25 @@
 	char *bufptr = namebuf;
 	Py_ssize_t len = sizeof(namebuf)-5; /* only claim to have space for MAX_PATH */
 
-	PyObject *po;
-	if (PyArg_ParseTuple(args, "U:listdir", &po)) {
+	PyObject *po = NULL;
+	if (PyArg_ParseTuple(args, "|U:listdir", &po)) {
 		WIN32_FIND_DATAW wFileData;
-		Py_UNICODE *wnamebuf;
+		Py_UNICODE *wnamebuf, *po_wchars;
+		
+		if (po == NULL) { // Default arg: "."
+			po_wchars = L".";
+			len = 1;
+		} else {
+			po_wchars = PyUnicode_AS_UNICODE(po);
+			len = PyUnicode_GET_SIZE(po);
+		}
 		/* Overallocate for \\*.*\0 */
-		len = PyUnicode_GET_SIZE(po);
 		wnamebuf = malloc((len + 5) * sizeof(wchar_t));
 		if (!wnamebuf) {
 			PyErr_NoMemory();
 			return NULL;
 		}
-		wcscpy(wnamebuf, PyUnicode_AS_UNICODE(po));
+		wcscpy(wnamebuf, po_wchars);
 		if (len > 0) {
 			Py_UNICODE wch = wnamebuf[len-1];
 			if (wch != L'/' && wch != L'\\' && wch != L':')
@@ -2304,12 +2311,16 @@
 	int arg_is_unicode = 1;
 
 	errno = 0;
-	if (!PyArg_ParseTuple(args, "U:listdir", &v)) {
+	if (!PyArg_ParseTuple(args, "|U:listdir", &v)) {
 		arg_is_unicode = 0;
 		PyErr_Clear();
 	}
-	if (!PyArg_ParseTuple(args, "et:listdir", Py_FileSystemDefaultEncoding, &name))
+	if (!PyArg_ParseTuple(args, "|et:listdir", Py_FileSystemDefaultEncoding, &name))
 		return NULL;
+	if (name == NULL) { // Default arg: "."
+        name = (char *)PyMem_Malloc(2); // "." + \0
+        strcpy(name, ".");
+	}
 	if ((dirp = opendir(name)) == NULL) {
 		return posix_error_with_allocated_filename(name);
 	}