diff -r 2bfc7ece2321 Lib/test/test_mmap.py
--- a/Lib/test/test_mmap.py	Thu Jan 20 10:47:04 2011 +0100
+++ b/Lib/test/test_mmap.py	Thu Jan 20 18:57:26 2011 +0200
@@ -334,6 +334,19 @@
             with mmap.mmap(f.fileno(), 0, offset=65536, access=mmap.ACCESS_READ) as mf:
                 self.assertRaises(IndexError, mf.__getitem__, 80000)
 
+    def test_length_0_large_offset(self):
+        # Test mapping of of file by passing 0 for
+        # map length with a large offset doesn't cause a segfault.
+        if not hasattr(os, "stat"):
+            self.skipTest("needs os.stat")
+
+        with open(TESTFN, "wb") as f:
+            f.write(115699 * b'm') # Arbitrary character
+
+        with open(TESTFN, "w+b") as f:
+            self.assertRaises(ValueError, mmap.mmap, f.fileno(), 0,
+                offset=2147418112)
+
     def test_move(self):
         # make move works everywhere (64-bit format problem earlier)
         f = open(TESTFN, 'wb+')
diff -r 2bfc7ece2321 Modules/mmapmodule.c
--- a/Modules/mmapmodule.c	Thu Jan 20 10:47:04 2011 +0100
+++ b/Modules/mmapmodule.c	Thu Jan 20 18:57:26 2011 +0200
@@ -1116,6 +1116,11 @@
 #  endif
     if (fd != -1 && fstat(fd, &st) == 0 && S_ISREG(st.st_mode)) {
         if (map_size == 0) {
+            if (offset >= st.st_size) {
+                PyErr_SetString(PyExc_ValueError,
+                                "mmap offset is greater than file size");
+                return NULL;
+            }
             map_size = st.st_size - offset;
         } else if ((size_t)offset + (size_t)map_size > st.st_size) {
             PyErr_SetString(PyExc_ValueError,