[PATCH 05/11] staging: comedi: core: initialize subdevice s->io_bits in postconfig

[Ian Abbott]

On 2013-08-30 09:23, Dan Carpenter wrote:
> On Thu, Aug 29, 2013 at 04:43:49PM +0000, Hartley Sweeten wrote:
>>> You don't really need to test s->n_chan < 32.  For s->n_chan >= 32,
>>> s->io_bits will end up set to 0xffffffff anyway (and for s->n_chan > 32,
>>> the low-level drivers shouldn't really be using s->state and s->io_bits
>>> anyway).
>>
>> I wasn't sure about that. 1 << 32 overflows the unsigned int to 0x100000000.
>> But I guess the 32-bit portion (0x00000000) - 1 is still 0xffffffff.
>>
>
> Nope.  Shift wrapping doesn't work like that.  Here is how it works in
> GCC.
>
> int maint(void)
> {
> 	int a, b, shift;
>
> 	a = 1 << 32; /* a is zero */
> 	shift = 32;
> 	b = 1 << shift; /* b wraps to 1 */
>
> 	printf("%x %x\n", a, b);
> }
>
> regards,
> dan carpenter

Yes, you're correct, the standard says the behaviour is undefined.

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