Re: [Gc] Strange leak in simple code

Ok, artificially tricky code, not what it first appears.

However, it is still true that you are linking all the nodes together, just
that you are not *explicitly* retaining a pointer to the first node.

All that is required for this to not be garbage collected is, for example:

1) the compiler to keep a copy of the pointer to the first node allocated
in a register or on the stack. Probably should not happen at -O0.

2) If *any* other value in memory looks like a pointer to one of your nodes
(not just the first one), then all nodes allocated after that one will be
retained.


On Wed, Jul 15, 2015 at 2:52 PM, Юрий Соколов <[email protected]>
wrote:

> Read the code carefully.  This code keeps pointer only to last node. So
> only two nodes are reachable at max.
> What do you mean "it leaks?"
>
> You're making a linked list, with every object pointed to either by 'last'
> or by the previous object.
>
> I would be very very unhappy if this was garbage collected!!
>
>
> On Mon, Jul 13, 2015 at 11:12 PM, Юрий Соколов <[email protected]>
> wrote:
>
>> Using libgc from Ubuntu 15.04 (7.2b-6.4) this simple code leaks:
>>
>>     #include <gc.h>
>>     struct list {
>>         struct list* next;    };
>>         int main() {
>>         GC_INIT();
>>         struct list *last = NULL;
>>         for (;;) {
>>             struct list* nuo = GC_MALLOC(sizeof(struct list));
>>             nuo->next = NULL;
>>             // if next line is commented, then no leakage
>>             if (last) last->next = nuo;
>>             last = nuo;
>>         }    }
>>
>>
>>
>> stackoverflow question
>> http://stackoverflow.com/questions/31380641/libgc-why-this-code-leaks
>>
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