Re: [PATCH] Null ctype should have ptr_ctype as its base type.

["Christopher Li" <sparse@chrisli.org>]

On Sun, Dec 28, 2008 at 1:38 PM, Alexey Zaytsev
<alexey.zaytsev@gmail.com> wrote:
> 11091a11093,11130
>> drivers/net/wireless/wavelan_cs.c:362:16: error: subtraction of different types can't work (different base types)
>> drivers/net/wireless/wavelan_cs.c:379:17: error: subtraction of different types can't work (different base types)
>> drivers/net/wireless/wavelan_cs.c:385:21: error: subtraction of different types can't work (different base types)
> [...]
>
> It looks like:
>
> ... mmroff(0, mmr_fee_status) ..
>
> #define mmroff(p,f)     (unsigned short)((void *)(&((mmr_t *)((void
> *)0 + (p)))->f) - (void *)0)

That is one piece of ugly code.

>
> mmr_fee_status being an element in struct mmr.
> Here we end up substracting null from non-null void * pointer.
> Looks quite pointless, but I think sparse should be able to
> cope with this?

Do you have minimal code to duplicate this bug?

I try with the following testing code without much luck.

Chris

typedef struct mmr_t	mmr_t;

struct mmr_t {
	int a;
	int mmr_fee_status;
};

#define mmroff(p,f)     (unsigned short)((void *)(&((mmr_t *)((void
*)0 + (p)))->f) - (void *)0)
int b;

int foo(void)
{
	return mmroff(0,mmr_fee_status);
}