ffi: bugfix: zero-width bitfield offset problem.

* ffi.c (make_ffi_type_struct): Fix incorrect condition for
determining whether the zero-width bitfield allocates a unit
or not. We must take into account the bit_offs, because it's
possible that unit_offs and offs are the same, yet a previous
bitfield has allocated some bits into the current allocation
unit. For instance struct { char a : 1; uint32 : 0; char b }
has size 5, but the equivalent FFI struct type ends up
with size 1. After char a : 1, the byte offset is still
zero, so if we don't look at the bit offset of 1, it looks
like the allocation offset is aligned to the start of a uint32
cell, which then means that the zero-width bitfield is
ignored. What's worse, the char b is then also allocated
over top of the a : 1 bitfield.

1 files changed, 1 insertions, 1 deletions

diff --git a/ffi.c b/ffi.c
index dd2fe20d..d8522d89 100644
--- a/ffi.c
+++ b/ffi.c
@@ -3295,7 +3295,7 @@ static val make_ffi_type_struct(val syntax, val lisp_type,
       ucnum room = bits_type - bits_alloc;
 
       if (bits == 0) {
-        if (offs != unit_offs)
+        if (offs != unit_offs || bit_offs > 0)
           offs = unit_offs + size;
         bit_offs = 0;
         nmemb--, i--;