[ccan] [PATCH] list: add list_for_each_rev_safe

[David Gibson]

On Sun, Oct 05, 2014 at 12:20:45PM +0000, Eric Wong wrote:
> Deleting while iterating backwards will be needed in the
> Ruby st_foreach_reverse_check implementation if we decide
> to port Ruby's st.c to use ccan/list.
> 
> ref: http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/65408
> 
> Signed-off-by: Eric Wong <normalperson at yhbt.net>
> ---
>  ccan/list/list.h     | 27 ++++++++++++++++++++++++++-
>  ccan/list/test/run.c | 31 ++++++++++++++++++++++++++++++-
>  2 files changed, 56 insertions(+), 2 deletions(-)
> 
> diff --git a/ccan/list/list.h b/ccan/list/list.h
> index 983675b..b4a6cdf 100644
> --- a/ccan/list/list.h
> +++ b/ccan/list/list.h
> @@ -527,6 +527,32 @@ static inline const void *list_tail_(const struct list_head *h, size_t off)
>  	     i = container_of_var(i->member.prev, i, member))
>  
>  /**
> + * list_for_each_rev_safe - iterate through a list backwards,
> + * maybe during deletion
> + * @h: the list_head
> + * @i: the structure containing the list_node
> + * @nxt: the structure containing the list_node
> + * @member: the list_node member of the structure
> + *
> + * This is a convenient wrapper to iterate @i over the entire list backwards.
> + * It's a for loop, so you can break and continue as normal.  The extra
> + * variable * @nxt is used to hold the next element, so you can delete @i
> + * from the list.
> + *
> + * Example:
> + *	struct child *next;
> + *	list_for_each_rev_safe(&parent->children, child, next, list) {
> + *		printf("Name: %s\n", child->name);
> + *	}
> + */
> +#define list_for_each_rev_safe(h, i, nxt, member)			\
> +	for (i = container_of_var(list_debug(h,	LIST_LOC)->n.prev, i, member), \
> +	     nxt = container_of_var(i->member.prev, i, member);		\
> +	     &i->member != &(h)->n;					\
> +	     i = nxt,							\
> +	     nxt = container_of_var(i->member.prev, i, member))


For proper orthogonality, I think rather than implementing it this
way, you should implement list_for_each_rev_safe_off() and implement
list_for_each_rev_safe() in terms of it, like the forwards
list_for_each_safe().

Also, I think the way that's implemented is to avoid double-evaluation
of macro arguments, so it's possible there's such a double evaluation
in your version.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson
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