On Tuesday, 2 July 2024 at 05:34:19 UTC, Dakota wrote:
I want to get `bitoffsetof` and `bitwidth` in clang, to verify
the d memory layout.
I try this but not work:
```c
#define offsetof_bit(type, member) ((offsetof(type, member) *
8) + __builtin_ctz(((type *)0)->member))
```
get this error:
```sh
cannot compute offset of bit-field 'field'
```
If you only want to see the layout, you can use the following
command to dump it:
```
clang test.c -Xclang -fdump-record-layouts -c
```
File test.c could contain the following:
```
struct S
{
int a : 3;
int b : 29;
};
struct S dummy;
```
The above clang command would then output:
```
*** Dumping AST Record Layout
0 | struct S
0:0-2 | int a
0:3-31 | int b
| [sizeof=4, align=4]
```
Inside a C/C++ program you could initialize a struct with all
0xff, set one bitfield to zero and look at the bits. I have used
this to compare the bitfield layout between D and C++ in a test
added in https://github.com/dlang/dmd/pull/16590.
