Re: [PATCH] use ptrdiff_t instead of casting int magicks

On Mon, 06 Sep 2010 09:41:15 -0600, Alex Rousskov
<rousskov_at_measurement-factory.com> wrote:
> On 09/06/2010 01:15 AM, Amos Jeffries wrote:
>> Inspired from the SuSE patch submitted by Christian. This is what I
>> think should be happening with real ptr maths instead of obsolete
>> integer math with potential rounding and endian errors.
>
>
>> void
>> memNodeWriteComplete(void* d)
>> {
>> mem_node* n = (mem_node*)(d - _mem_node_data_offset);
>
> GCC: pointer of type void * used in arithmetic
>
>
> The value of a (pointer + n) expression depends on what pointer is
> pointing to. If you want byte-size increments, you have to cast to char*

> or similar.

Meh, one cast too many.

>
> Using ptrdiff_t instead of int is the right thing to do. I do not thing
> you need to cast to it in makeMemNodeDataOffset. You may clarify the
> intent by writing:
>
> // calculate data member offset; p is not dereferenced here
> mem_node *p = 0;
> return reinterpret_cast<char*>(&p->data) - reinterpret_cast<char*>(p);

The point of using ptrdiff_t with p==0L is to reduce multiple casting to
non-pointer types.

Amos
Received on Mon Sep 06 2010 - 22:25:46 MDT