[asterisk-dev] watch out usages of AST_LIST_INSERT_TAIL

[Jason Parker]

Luigi Rizzo wrote:
> i just realised "the hard way" that AST_LIST_INSERT_TAIL can only
> append a single element to a list, and it is not good for appending
> two lists. The macro also assumes that the element has the 'tail'
> pointer properly initialized.
> 
> I am not 100% sure that all places in asterisk where AST_LIST_INSERT_TAIL
> is used honor the assumption.
> Other than a scrutiny of all places where AST_LIST_INSERT_TAIL is used,
> i would suggest a simple fix - changing the macro to handle the case
> of list merging, as follows (more or less):
> 
>    #define AST_LIST_INSERT_TAIL(head, elm, field) do {                     \
>       if (!(head)->first) {                                             \
>                 (head)->first = (elm);                                  \
>       } else {                                                          \
>                 (head)->last->field.next = (elm);                       \
>       }                                                                 \
> 	(head)->last = (elm);                                   	\
> 	while ((head)->last->field.next)		\
> 		(head)->last = (head)->last->field.next;		\
>    } while (0)
> 
> The extra overhead is negligible in the normal case
> ((head)->last->field.next = NULL)
> 
> Of course, with all these pointer dereferences, one wonders if it
> isn't better/safer to use the macro just as a frontend to a
> function, using offsetof(field) to locate link field within the
> structure.
> 
> cheers
> luigi
> 

Just a thought, but would it maybe be useful to have something like
AST_LIST_COMBINE, to add the contents of list2 to list1?

-- 
Jason Parker
Digium