[Mesa-dev] [PATCH 1/2] util: Fix foreach_list_typed_safe when exec_node is not at offset 0.

[Matt Turner]

On Mon, Mar 9, 2015 at 6:36 PM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> From: Jason Ekstrand <jason.ekstrand at intel.com>
>
> __next and __prev are pointers to the structure containing the exec_node
> link, not the embedded exec_node.  NULL checks would fail unless the
> embedded exec_node happened to be at offset 0 in the parent struct.
>
> Signed-off-by: Jason Ekstrand <jason.ekstrand at intel.com>
> Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>  src/glsl/list.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/src/glsl/list.h b/src/glsl/list.h
> index ddb98f7..680e963 100644
> --- a/src/glsl/list.h
> +++ b/src/glsl/list.h
> @@ -684,7 +684,7 @@ inline void exec_node::insert_before(exec_list *before)
>             exec_node_data(__type, (__list)->head, __field),                \
>                 * __next =                                                  \
>             exec_node_data(__type, (__node)->__field.next, __field);        \
> -        __next != NULL;                                                    \
> +        &__next->__field != NULL;                                          \

Wow. Okay, so this apparently relies on exec_node_data (which is
pretty confusingly named, I think) returning a negative pointer value
if passed NULL as its node parameter, such that &...->__field
effectively adds back the offset of the field in the struct, giving a
NULL pointer?

That's sufficiently confusing to warrant a lengthy comment at the very least.

Testing that the address of a field in a struct is NULL... just looks insane.