[Issue 9092] GC.extend allocates less then it reports

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=9092


monarchdodra at gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|INVALID                     |


--- Comment #2 from monarchdodra at gmail.com 2012-11-30 02:26:39 PST ---
(In reply to comment #1)
> I very much suspect that this is caused by the behaveour of large arrays that
> need more space than 2048 bytes: these arrays store the actual allocated length
> at the beginning of the block and reserve 16 bytes for that, in contrast to
> smaller arrays that place this information at the end of the memory block (see
> druntime/src/rt/lifetime.d for details).
> 
> So in your case, a.ptr does not point to the start of the memory block, but 16
> bytes into it, leaving a little less than the actual memory size for the array.
> GC.extend changes the memory block, but doesn't know about the array-semantics,
> so it reports back the raw size of the memory block.
> 
> Your example mixes high level memory access (arrays) with low level functions
> (GC.extend), I don't think that is a good idea. Instead, use GC.malloc for your
> first allocation.

How could this possibly be "invalid"? "I don't think it is a good idea" is not
the same as "This is wrong and invalid".

I passed a pointer (a.ptr) to extend, and extend promised that memory location
was extended to a certain amount. if a.ptr did not actually point to the
beginning of a memory location, then why/how was it extended? And if extend was
able to detect that a.ptr was already 16 bytes into my memory block, then why
can't it take that into account when replying?

Anyways, here is Appender subject to this bug...
//----
void main()
{
    auto a = new char[] (61000);
    auto app = appender!(char[])(a);
    foreach(k; 0..5000)
    {
      write(k, ' ');
      stdout.flush();
      app.put('a');
    }
}
//----
... 4517 4518 4519 4520 object.Error: Access Violation
//----

BTW, both these tests would appear to pass on DPaste, so the bug appears to be
windows related. I guess such allocations on linux aren't a bad idea?

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