• Dakota (11/11) Jul 01 I want to get `bitoffsetof` and `bitwidth` in clang, to verify
• Tim (27/38) Jul 03 If you only want to see the layout, you can use the following
• Dakota (4/30) Jul 04 Thanks for the tips.

Dakota <dakota gmail.com> writes:

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'
```

Tim <tim.dlang t-online.de> writes:

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.

Dakota <dakota gmail.com> writes:

On Wednesday, 3 July 2024 at 21:47:32 UTC, Tim wrote:
 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.

Thanks for the tips.

Then I guess there is no easy way to get equivalent in c for 
`bitoffsetof` and `bitwidth`