Re: [PATCH 3/4] cache.h: hex2chr() - avoid -Wsign-compare warnings
- From
Ramsay Jones <ramsay@ramsayjones.plus.com>
- Date
- Sep 22, 2017, 16:05 UTC
- Message-ID
- <89beb424-2cc0-bfe3-7847-c11035bfec70@ramsayjones.plus.com>
- In-Reply-To
- <20170922054748.iseinawwwfw56vis@sigill.intra.peff.net>
On 22/09/17 06:47, Jeff King wrote:
Show 19 quoted lines
> On Thu, Sep 21, 2017 at 05:48:38PM +0100, Ramsay Jones wrote:
>
>> diff --git a/cache.h b/cache.h
>> index a916bc79e..a0e3e362c 100644
>> --- a/cache.h
>> +++ b/cache.h
>> @@ -1243,8 +1243,8 @@ static inline unsigned int hexval(unsigned char c)
>> */
>> static inline int hex2chr(const char *s)
>> {
>> - int val = hexval(s[0]);
>> - return (val < 0) ? val : (val << 4) | hexval(s[1]);
>> + unsigned int val = hexval(s[0]);
>> + return (val & ~0xf) ? val : (val << 4) | hexval(s[1]);
>> }
>
> Ironically, the unsigned return from hexval() comes from internally
> converting the signed char in hexval_table. And then we again return it
> as a signed int from hex2chr().Yep, my first inclination was to change the return type of hexval(), but after looking at all its callers, I decided against that (because it wouldn't help). ;-)
> Would it make sense to return a signed int from hexval()? That would > make hex2chr just work as it tries to above. I admit that shifting > signed values is a little funny, but it should be fine here since we > know they're no larger than 8 bits in the first place.
Indeed, shifting signed values is a no-no, which is why I decided to go this route.
Show 6 quoted lines
> As an aside, I also see some uses of hexval() that don't appear to be > quite as rigorous in checking for invalid characters. A few > unconditionally shift the first nibble and assume that there will still > be high bits set. I think that's generally true for twos-complement > negative numbers, but isn't shifting off the left side of a signed > integer undefined behavior?
All uses of hexval() that I can see are shifting an unsigned value. Have I missed something?
> And mailinfo's decode_q_segment() does not seem to check for errors at > all.
Yes, I noticed that. (I put it on my TODO list).
> Handling those is getting far off your original patch, but I'm having > trouble figuring out if it's saner for us to consistently stick to > all-signed or all-unsigned here.
Oh, unsigned, without a doubt. :D
ATB, Ramsay Jones