Re: [PATCH 1/3] include: Define FIELD_OFFSET to the standard offsetof macro

[Michael Stefaniuc]

On 12/10/2012 07:37 PM, Amine Khaldi wrote:
> This prevents the undefined behavior (null pointer dereference)
> diagnostics (clang with ubsan checks for example).
> This is a bug in clang. There is no null pointer dereference.
> Afair gcc tried to pull this trick too but got educated about their error.
> ---
>  include/winnt.h | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/include/winnt.h b/include/winnt.h
> index 207adaa..a3b996a 100644
> --- a/include/winnt.h
> +++ b/include/winnt.h
> @@ -746,8 +746,12 @@ typedef struct _MEMORY_BASIC_INFORMATION
>
> #define UNICODE_STRING_MAX_CHARS 32767
>
> +#if defined(__GNUC__) || defined(__clang__)
> +#define FIELD_OFFSET(type, field) offsetof(type, field)
> +#else
>  #define FIELD_OFFSET(type, field) \
>    ((LONG)(INT_PTR)&(((type *)0)->field))
> +#endif
>
> #define CONTAINING_RECORD(address, type, field) \
>    ((type *)((PCHAR)(address) - (PCHAR)(&((type *)0)->field)))

bye
	michael

[PATCH 1/3] include: Define FIELD_OFFSET to the standard offsetof macro

[Dmitry Timoshkov]

Michael Stefaniuc <mstefani(a)redhat.com> wrote:
> On 12/10/2012 07:37 PM, Amine Khaldi wrote:
> > This prevents the undefined behavior (null pointer dereference)
> > diagnostics (clang with ubsan checks for example).
> > This is a bug in clang. There is no null pointer dereference.
> > Afair gcc tried to pull this trick too but got educated about their error.

What's an advantage of these patches except of catering broken compilers/
checkers?

-- 
Dmitry.

[PATCH 1/3] include: Define FIELD_OFFSET to the standard offsetof macro

[Michael Stefaniuc]

On 12/11/2012 10:20 AM, Dmitry Timoshkov wrote:
> Michael Stefaniuc <mstefani(a)redhat.com> wrote:
> > On 12/10/2012 07:37 PM, Amine Khaldi wrote:
> > > This prevents the undefined behavior (null pointer dereference)
> > > diagnostics (clang with ubsan checks for example).
> > > This is a bug in clang. There is no null pointer dereference.
> > > Afair gcc tried to pull this trick too but got educated about their error.
>
> What's an advantage of these patches except of catering broken compilers/
> checkers?
> I figure you mean the 2nd try which uses offsetof unconditionally.
> The advantage is that it replaces the cast orgy with the C89 standard
> construct thus it easier to read. Appeasing the still wrong clang check
> is just a side effect.

bye
	michael