On 12/10/2016 07:57 AM, Greg Kurz wrote: >>> -#define P9_NOTAG (u16)(~0) >>> -#define P9_NOFID (u32)(~0) >>> +#define P9_NOTAG (uint16_t)(~0) >>> +#define P9_NOFID (uint32_t)(~0) >> >> Don't you want to write ((uint16_t)(~0)), to ensure that this expression >> can be used as a drop-in in any other syntactical situation? >> > > These defines come from the linux kernel sources and I must admit it > didn't cross my mind... can you share a case where this would cause > troubles ?
Unlikely to occur in real code, but:
int a[] = { -2, -3 };
int *b = a + 1;
printf("%d\n", (uint16_t)(~0)[b]); // prints 65534 - let's see why?
// prints 65534, or the result of b[-1] cast to uint16_t
printf("%d\n", (uint16_t)((~0)[b]));
// probably dumps core, as b[65535] is out of bounds
printf("%d\n", ((uint16_t)(~0))[b]);
that is, since [] has higher precedence than casts, failure to
parenthesize a cast will change the interpretation of P9_NOTAG[pointer].
And yes, if you copied from the kernel, that means the kernel has a bug
(even if it is unlikely to trip up normal code).
>
>> Or even write it as UINT16_C(~0) (using <stdint.h>), or as UINT16_MAX.
>> (Be aware: the type of (uint16_t)(~0) is uint16_t, while the type of
>> UINT16_MAX is int, due to the rules of integer promotion, if that matters)
>>
>
> UINT16_C(~0) expands to ~0 and UINT16_MAX expands to (65535), at least on
> my laptop (glibc-headers-2.23.1-11.fc24.x86_64)... doesn't that mean the
> type of UINT16_C(~0) is also int ? Please enlighten me.
Indeed, UINT16_C produces an int constant, not uint16_t (since there is
no such thing as a uint16_t constant). So the cast is the only way to
force ~0 to be truncated to a 16-bit pattern. But using UINT16_MAX is
probably just fine, as it is the all-ones value with the correct integer
promotion for use in any other arithmetic.
>
> The 9P spec at http://man.cat-v.org/plan_9/5/version says "(ushort)~0". My
> understanding is 16 bits all ones. I guess I'd rather then go for
> ((uint16_t)(~0)).
Verbose, but works, as does UINT16_MAX. But I stand corrected that
UINT16_C(~0) does not work.
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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