ext-phil.2.carmody@xxxxxxxxx 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
--
To unsubscribe from this list: send the line "unsubscribe linux-input" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
