When this will be freed?

[Steven Schveighoffer]

On Fri, 04 Apr 2014 09:25:49 -0400, Marc Schütz <schuetzm at gmx.net> wrote:

> On Wednesday, 2 April 2014 at 16:51:57 UTC, Benjamin Thaut wrote:
>> Am 02.04.2014 17:57, schrieb Andrea Fontana:
>>> I mean: if it is an exported function (of a shared library) what
>>> happens? There's no reference kept anywhere (in D). So if I'm right it
>>> could be freed immediatly by GC. Right?
>>>
>>
>> If you pass that string to a C function, there is a reference on the  
>> stack. So this string will not be freed until that C-function returns.  
>> If that C-Function returns, it is very likely however that this was the  
>> only reference and the string will be freed the next time the garbage  
>> collector runs.
>
> This is unfortunately only true on x86 32-bit. For x86_64, the calling  
> conventions (MS, SysV [1]) say that the first few parameters are passed  
> in registers, and the same is probably true for other architectures.
>
> Usually this is still safe, as the pointer will normally either stay in  
> its register, or will be spilled to the stack and kept there as long as  
> the function still needs to use it. But one might imagine a corner case  
> where it temporarily stores the pointer in a global or static variable.  
> Even if the pointer will not be kept by the C function longer than the  
> call, in such cases you would need to keep an additional reference where  
> the GC can see it.

In cases where it is stored in global data, and that becomes the only  
reference, or if a C heap allocation is made, and the pointer is stored in  
there (perhaps to pass to another C function?), then it's quite possible  
the data could be collected prematurely in another thread.

You bring up a good point. This is something that should be considered  
when calling extern(C) functions.

-Steve