Re: [PATCH] hid-magicmouse: Correct parsing of large X and Y motions.

[Michael Poole <mdpoole@troilus.org>]

ext-phil.2.carmody@nokia.com writes:

[snip]
>>+/* Function shamelessly borrowed from hid-core.c. */
>>+
>>+static s32 snto32(__u32 value, unsigned n)
>>+{
>>+       switch (n) {
>>+       case 8:  return ((__s8)value);
>>+       case 16: return ((__s16)value);
>>+       case 32: return ((__s32)value);
>
> Beware - this relies on sane behaviour from the current and future compilers, as the C standard doesn't mandate what should happen when shortening out of range signed values. [n869.txt: 6.3.1.1.#3]

That's good to know.  There's probably some much more complicated, but
portable, way to write the operation -- I suspect we'll continue to rely
on the compiler to be sane.

>>+       }
>>+       return value & (1 << (n - 1)) ? value | (-1 << n) : value;
>
> That signed left shift, however, is good old fashioned undefined behaviour. [n869.txt 6.5.7.#4]

Which version is that from?  The copy of C99 I have says the resulting
value is -1 * (2 ** n) if that value can be represented.  (A 6 May 2005
committee draft of TC2 says the same thing.)  Until n > 31, the behavior
should be standard-defined.

Michael