Index: Modules/_fileio.c
===================================================================
--- Modules/_fileio.c	(révision 70631)
+++ Modules/_fileio.c	(copie de travail)
@@ -380,15 +380,15 @@
 	if (self->fd < 0)
 		return err_closed();
 	if (self->seekable < 0) {
-		int ret;
-		Py_BEGIN_ALLOW_THREADS
-		ret = lseek(self->fd, 0, SEEK_CUR);
-		Py_END_ALLOW_THREADS
-		if (ret < 0)
+		PyObject *pos = portable_lseek(self->fd, NULL, SEEK_CUR);
+		if (pos == NULL) {
+			PyErr_Clear();
 			self->seekable = 0;
-		else
+		} else {
+			Py_DECREF(pos);
 			self->seekable = 1;
 	}
+	}
 	return PyBool_FromLong((long) self->seekable);
 }
 
Index: Lib/test/test_largefile.py
===================================================================
--- Lib/test/test_largefile.py	(révision 70631)
+++ Lib/test/test_largefile.py	(copie de travail)
@@ -1,6 +1,7 @@
 """Test largefile support on system where this makes sense.
 """
 
+import io
 import os
 import stat
 import sys
@@ -132,6 +133,12 @@
             self.assertEqual(len(f.read()), 1)  # else wasn't truncated
 
 
+    def test_seekable(self):
+        for pos in (2**31-1, 2**31, 2**31+1):
+            with io.open(TESTFN, 'rb') as f:
+                f.seek(pos)
+                self.assert_(f.seekable())
+
 def test_main():
     # On Windows and Mac OSX this test comsumes large resources; It
     # takes a long time to build the >2GB file and takes >2GB of disk
@@ -163,6 +170,7 @@
     suite.addTest(TestCase('test_osstat'))
     suite.addTest(TestCase('test_seek_read'))
     suite.addTest(TestCase('test_lseek'))
+    suite.addTest(TestCase('test_seekable'))
     with open(TESTFN, 'w') as f:
         if hasattr(f, 'truncate'):
             suite.addTest(TestCase('test_truncate'))