[tz] proposed changes for Win32 and a improved mktime() algorithm

[Ian Abbott]

On 18/05/17 18:08, Zefram wrote:
> Paul Eggert wrote:
>> Thanks, I didn't know about this extension. However, it has undefined
>> behavior in standard C, so we can't use it in code intended to be portable
>> everywhere.
>
> I wondered about that, because it seems like the kind of thing that
> would have undefined behaviour, but I looked at the C standard, and on
> my reading it actually doesn't.  The relevant wording is
>
> #   An object whose identifier is declared with no linkage and without the
> #   storage-class specifier "static" has /automatic storage duration/.
> #   [...] storage is guaranteed to be reserved for a new instance of
> #   the object on each entry into the block with which it is associated;
> #   the initial value of the object is indeterminate.
>
> To have an indeterminate value does not amount to undefined behaviour.
> When we have no initialising clause at all, clearly the indeterminate
> value is not a problem if we don't rely on the value; that is how the code
> is currently correct.  An "x = x" initialiser amounts to an assignment
> using that initial indeterminate value.  Crucially, there's nothing saying
> that using an indeterminate value per se invokes undefined behaviour.
> My reading is that an indeterminate value is some actual value, though
> we don't know which value it is, and that copying it by assignment
> or initialisation just copies whatever value it is without trouble.
> Thus "x = x" is truly a no-op.
>
> So I think we'd be fine to adopt the "x = x" idiom without any
> conditionality.  Of course, the idiom might elicit *more* warnings
> from compilers other than gcc, and that could be a reason to use it
> conditionally even if correctness isn't on the line.  One also might
> want to hide it behind a macro even if used unconditionally, on the
> grounds that the macro name makes clearer what's going on.

I'm not sure about that.  Although it is not normative, Section "J.2 
Undefined behavior" includes:

#  - The value of an object with automatic storage duration is
#    used while it is indeterminate (6.2.4, 6.7.9, 6.8).

-- 
-=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti at mev.co.uk> )=-
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