Incorporating D

[Rainer Schuetze]

On 26.01.2013 21:13, Walter Bright wrote:
> On 1/26/2013 8:17 AM, Rainer Schuetze wrote:
>>
>>
>> On 26.01.2013 16:53, deadalnix wrote:
>>> On Saturday, 26 January 2013 at 10:52:59 UTC, Rainer Schuetze wrote:
>>>>
>>>>
>>>> On 26.01.2013 11:40, Johannes Pfau wrote:
>>>>> Yes, I just wanted to point out a common source for such bugs, it's
>>>>> not
>>>>> the GC's fault. It's great that the documentation of toStringz
>>>>> mentions
>>>>> that issue. What I meant is most of the time we use toStringz() like
>>>>> this:
>>>>>
>>>>> string str;
>>>>> c_function(str.toStringz());
>>>>>
>>>>> This is only valid if c_function doesn't store the pointer, but
>>>>> newbies
>>>>> might miss that and just copy this nice looking example code for other
>>>>> c functions. There's nothing we can do about that though, interfacing
>>>>> to C just is a little bit dangerous.
>>>>>
>>>>
>>>> It is even dangerous if it is only used temporarily during that
>>>> function call, but copied elsewhere in the C heap and cleared on the
>>>> stack:
>>>>
>>>> struct param_struct { const char* name; };
>>>>
>>>> void c_function(const char*p)
>>>> {
>>>>     param_struct* ps = new param_struct;
>>>>     ps->name = p;
>>>>     p = 0;
>>>>     doSomething(ps);
>>>>     delete ps;
>>>> }
>>>>
>>>> Imagine a garbage collection while executing doSomething...
>>>
>>> That isn't an issue as the pointer will e found at upper level in the
>>> stack anyway.
>>
>> "p = 0;" clears the only existing reference on the stack.
>
> No, because ps is on the stack, and ps points to a copy of p. Hence,
> that code snippet is GC safe.

Please note that c_function is C/C++ code. The struct is allocated on 
the C heap, not in GC memory. "p = 0" clears the only reference in GC 
scanned memory to the char array allocated and returned by toStringz().