From: ext Michael Poole [mdpoole@xxxxxxxxxxx]
>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?
A text version of n869, which looks like it suffered a little in conversion to not-quite-ASCII.
> 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.
Looking at the pdfs, agreed, My mistake. I shall have to 'fix' the non-ASCII in my text version, as I much prefer greppable text to pdfs.
Phil--
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