Index: Lib/test/test_mmap.py
===================================================================
--- Lib/test/test_mmap.py	(revision 69714)
+++ Lib/test/test_mmap.py	(working copy)
@@ -417,6 +417,30 @@
             m = mmap.mmap(f.fileno(), mapsize - halfsize, offset=halfsize)
             self.assertEqual(m[0:3], 'foo')
             f.close()
+
+            # Try resizing map
+            try:
+                m.resize(512)
+            except SystemError:
+                # resize() not supported
+                # No messages are printed, since the output of this test suite
+                # would then be different across platforms.
+                pass
+            else:
+                # resize() is supported
+                self.assertEqual(len(m), 512)
+                # Check that we can no longer seek beyond the new size.
+                self.assertRaises(ValueError, m.seek, 513, 0)
+                # Check that the content is not changed
+                self.assertEqual(m[0:3], 'foo')
+
+                # Check that the underlying file is truncated too
+                f = open(TESTFN)
+                f.seek(0, 2)
+                self.assertEqual(f.tell(), halfsize + 512)
+                f.close()
+                self.assertEqual(m.size(), halfsize + 512)
+
             m.close()
 
         finally:
Index: Modules/mmapmodule.c
===================================================================
--- Modules/mmapmodule.c	(revision 69714)
+++ Modules/mmapmodule.c	(working copy)
@@ -444,7 +444,7 @@
 		off_lo = (DWORD)(self->offset & 0xFFFFFFFF);
 #else
 		newSizeHigh = 0;
-		newSizeLow = (DWORD)new_size;
+		newSizeLow = (DWORD)(self->offset + new_size);
 		off_hi = 0;
 		off_lo = (DWORD)self->offset;
 #endif
@@ -490,7 +490,7 @@
 	} else {
 		void *newmap;
 
-		if (ftruncate(self->fd, new_size) == -1) {
+		if (ftruncate(self->fd, self->offset + new_size) == -1) {
 			PyErr_SetFromErrno(mmap_module_error);
 			return NULL;
 		}