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Re: Compliant way to use offsetof for a field within a field?

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On 9/1/2011 8:28 AM, skillzero@gmail.com wrote:
>
> typedef struct
> {
>      uint8_t        x;
>      uint8_t        y;
>      struct
>      {
>          uint8_t    z;
>
>      }   fields;
>
> }   Test;
>
> compile_time_assert( offsetof( Test, x )        == 0 ); // Works
> compile_time_assert( offsetof( Test, y )        == 1 ); // Works
> compile_time_assert( offsetof( Test, fields.z ) == 2 ); // Works, but
> warns about an extension.
>
> Is there a way to do this in a compliant way?

What compiler are you using? Are you sure you are not compiling your 
code as C++ code?

The `offsetof( Test, fields.z )` _is_ the compliant way in C. There are 
no extensions of any kind involved here.

The specification of `offsetof` is C language only requires that 
`&object.member_designator` be an address constant (for some imagginary 
static object `object`). There's no requirement for the 
'member_designator' to designate the immediate member of the structure. 
It is perfectly allowed to use `offsetof` with "deeper" members of the 
struct type.

-- 
Best regards,
Andrey Tarasevich

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Thread

Compliant way to use offsetof for a field within a field? "skillzero@gmail.com" <skillzero@gmail.com> - 2011-09-01 08:28 -0700
  Re: Compliant way to use offsetof for a field within a field? James Kuyper <jameskuyper@verizon.net> - 2011-09-01 12:23 -0400
    Re: Compliant way to use offsetof for a field within a field? "J. J. Farrell" <jjf@bcs.org.uk> - 2011-09-02 00:34 +0100
  Re: Compliant way to use offsetof for a field within a field? "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2011-09-01 10:19 -0700
    Re: Compliant way to use offsetof for a field within a field? Peter Nilsson <airia@acay.com.au> - 2011-09-01 16:31 -0700
  Re: Compliant way to use offsetof for a field within a field? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2011-09-01 16:38 -0700
  Re: Compliant way to use offsetof for a field within a field? Michael Press <rubrum@pacbell.net> - 2011-09-01 22:05 -0700

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