sizeof computation fails if small bitfields are present

[ian-ross]

Bug imported from C2HS Trac

Trac ticket created: 2008-08-01T13:41:03-0700; last modified: 2010-04-23T10:22:02-0700

---

sizeof seems to assume that e.g. <8-bit bitfields of type int32 only take 1 byte.

struct bfs {
  int x:1;
};

diff fail1_ctest.out fail1_test.out
1c1
< __tydef__bfs__: 4

---
> __tydef__bfs__: 1

Environment:

C->Haskell Compiler, version 0.15.2 Rainy Days, 31 Aug 2007
  build platform is "i386-darwin" <1, True, True, 1>
The Glorious Glasgow Haskell Compilation System, version 6.8.3
Darwin $host 9.4.0 Darwin Kernel Version 9.4.0: i386

[ian-ross]

Original comment date: 2010-04-23T10:22:02-0700

---

Ugg, it's complicated.

They do only take one byte when next to each other. The problem is the padding at the end of the structure. It's made up to the natural size of int, I think. This would usually be done right when there's any other non-bitfield field in the same structure however when it's just bitfields then we end up with alignment of 1, which also means we only pad the struct size to 1.

If someone wants to look into fixing this, see sizeAlignOfStruct and sizeAlignOfStructPad in C2HS/Gen/Bind.hs. See also alignment and fieldAlignment in C2HS/C/Info.hs.

[ian-ross]

Still open. PR #61 fixed issue #32 (another bit fields problem), but not this one.

[ian-ross]

Still seems to be a problem: #61 (comment)

[ian-ross]

Going to close this as I can't duplicate it and I've not heard back from the person who reported the regression.