Raise a NotImplementedError in one messy corner case

2 files changed, 18 insertions, 0 deletions

diff --git a/c/_cffi_backend.c b/c/_cffi_backend.c
index 7aefbd3..a1292c0 100644
--- a/c/_cffi_backend.c
+++ b/c/_cffi_backend.c
@@ -3817,6 +3817,14 @@ static PyObject *b_complete_struct_or_union(PyObject *self, PyObject *args)
                     if (bits_already_occupied + fbitsize > 8 * ftype->ct_size) {
                         /* it would not fit, we need to start at the next
                            allowed position */
+                        if ((sflags & SF_PACKED) &&
+                            (bits_already_occupied & 7)) {
+                            PyErr_Format(PyExc_NotImplementedError,
+                                "with 'packed', gcc would compile field "
+                                "'%s.%s' to reuse some bits in the previous "
+                                "field", ct->ct_name, PyText_AS_UTF8(fname));
+                            goto error;
+                        }
                         field_offset_bytes += falign;
                         assert(boffset < field_offset_bytes * 8);
                         boffset = field_offset_bytes * 8;
diff --git a/c/test_c.py b/c/test_c.py
index 005a089..09d0a13 100644
--- a/c/test_c.py
+++ b/c/test_c.py
@@ -3177,6 +3177,16 @@ def test_packed():
     assert sizeof(BStruct) == sizeof(BLong) + sizeof(BChar) + sizeof(BShort)
     assert alignof(BStruct) == 1
 
+def test_packed_with_bitfields():
+    BLong = new_primitive_type("long")
+    BChar = new_primitive_type("char")
+    BStruct = new_struct_type("struct foo")
+    py.test.raises(NotImplementedError,
+                   complete_struct_or_union,
+                   BStruct, [('a1', BLong, 30),
+                             ('a2', BChar, 5)],
+                   None, -1, -1, 8)   # SF_PACKED==8
+
 def test_version():
     # this test is here mostly for PyPy
     assert __version__ == "0.8"