H5Part development and discussion

[Achim Gsell <achim.gsell AT psi.ch>]

On Jul 1, 2010, at 10:44 PM, Mark Howison wrote:

> Hmm, so I'm confused about how the (ptr_diff) cast will work... if
> h5priv_alloc returns
> 
> return (void*)(H5_ERR);
> 
> isn't the cast to void* going to convert -1 to some bogus non-negative
> value? Why would (ptr_diff) convert it back to -1?

May be (void*)((char*)NULL + H5_ERR) is more save. But (void*)(H5_ERR) should 
be save on all two-complement systems. 

The 32bit two-complement representation of -1 is 0xffffffff. This is a valid 
value for a pointer, even it will never be used. So the cast of H5_ERR to 
void* gives 0xffffffff, which is a valid pointer difference. ptrdiff_t is a 
signed integral type, and as such can be casted to compatible fundamental 
data types. So

(int64_t)(ptrdiff_t)(void*)(H5_ERR) =
   (int64_t)(ptrdiff_t)(void*)(-1) =
   (int64_t)(ptrdiff_t)(0xffffffff) = 
   (int64_t)(ptrdiff_t)(-1) =
   (int64_t)(-1)

Achim