pure or not pure?

[Koroskin Denis]

On Thu, 10 Apr 2008 20:34:29 +0400, Janice Caron <caron800 at googlemail.com>  
wrote:

> On 10/04/2008, Steven Schveighoffer <schveiguy at yahoo.com> wrote:
>
>>  char[] c = new char[5];
>>
>> is invalid inside a pure function because the compiler cannot verify
>>  uniqueness of mutable heap data, right?
>
> No. As I said, new is special. It's integral to the language.

OK, new is 'special', but what about malloc or my own allocator? (I happen  
to work in a gamedev and we never use new nor malloc).
I don't see how

special extern(C) void* malloc(size_t size);

differs from:

void* my_malloc(size_t size)
{
    return malloc(size);
}

and why can't it be 'special' too.