Re: [Xen-devel] [PATCH 7/9] libxl: New convenience macro CONTAINER_OF

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2012-01-13 at 19:25 +0000, Ian Jackson wrote:
> Provide a convenient and type-safe wrapper which does the correct
> dance to subtract offsetof.  This is very similar to the
> "container_of" macro in the Linux kernel, but it has an additional
> feature that instead of the type argument you may also pass an
> expression of that type; this makes initialising a variable with
> CONTAINER_OF easier.
> 
> Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>

Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>

> ---
>  tools/libxl/libxl_internal.h |   29 +++++++++++++++++++++++++++++
>  1 files changed, 29 insertions(+), 0 deletions(-)
> 
> diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
> index 594b9fb..213b5f9 100644
> --- a/tools/libxl/libxl_internal.h
> +++ b/tools/libxl/libxl_internal.h
> @@ -1238,6 +1238,35 @@ _hidden void libxl__ao__destroy(libxl_ctx*, libxl__ao 
> *ao);
>   * Convenience macros.
>   */
>  
> +/*
> + * CONTAINER_OF work like this.  Given:
> + *    typedef struct {
> + *      ...
> + *      member_type member_name;
> + *      ...
> + *    } outer_type;
> + *    outer_type outer, *outer_var;
> + *    member_type *inner_ptr = &outer->member_name;
> + *
> + * Then, effectively:
> + *    outer_type *CONTAINER_OF(member_type *inner_ptr,
> + *                             *outer_var, // or type name for outer_type
> + *                             member_name);
> + *
> + * So that:
> + *    CONTAINER_OF(inner_ptr, *outer_var, member_name) == &outer
> + *    CONTAINER_OF(inner_ptr, outer_type, member_name) == &outer
> + */
> +#define CONTAINER_OF(inner_ptr, outer, member_name)                     \
> +    ({                                                                  \
> +        typeof(outer) *container_of_;                                   \
> +        container_of_ = (void*)((char*)(inner_ptr) -                    \
> +                                offsetof(typeof(outer), member_name));  \
> +        (void)(&container_of_->member_name ==                           \
> +               (typeof(inner_ptr))0) /* type check */;                  \
> +        container_of_;                                                  \
> +    })
> +
>  
>  /*
>   * All of these assume (or define)



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