On Fri, Aug 19, 2011 at 01:34:36PM -0700, Lida Horn wrote:
> I'm looking for an example of how one could write a dtrace probe
> that could follow something like a NULL terminated linked list.
>
> For example:
>
> struct list {
> struct list *next;
> void *data;
> };
>
> struct list *list_root;
>
>
> Assuming I had "list_root" available, but I wanted to know how long
> the linked list is, how can I do this in a dtrace probe?
>
> If I had looping constructs, it would be obvious:
>
> int
> count_list(struct list *list)
> {
> int i;
>
> for (i = 0; list != NULL; list = list->next)
> ++i;
>
> return i;
> }
>
> In the dtrace scripting language there are no loop constructs, but is
> there a way to add a new provider than can loop?
Generally, you can do this using something like:
my:probe:matching:func
{
this->list = /* some way to get initial list */;
this->length = 0;
}
my:probe:matching:func / this->list != NULL / {
this->list = this->list->next; this->length++;
}
my:probe:matching:func / this->list != NULL / {
this->list = this->list->next; this->length++;
}
my:probe:matching:func / this->list != NULL / {
this->list = this->list->next; this->length++;
}
my:probe:matching:func / this->list != NULL / {
this->list = this->list->next; this->length++;
}
/* repeat N times, where N is the max list length */
my:probe:matching:func / this->list != NULL / {
/* list was longer than our maximum length */
@overflow = count();
}
my:probe:matching:func / this->list == NULL / {
/* use this->count as the length of the list */
}
(Enablings for a probe are guaranteed to execute in program order, and this->
variables are valid between enablings of the same probe.)
Cheers,
- jonathan
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