Re: int a = a

Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>

Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>

David Brown <david.brown@hesbynett.no> writes:
>

[...]I believe it would be much simpler and clearer if attempting
to read an uninitialised and unassigned local variable were
undefined behaviour in every case.

>
I probably agree (I haven't given it all that much thought), but
the committee made a specific decision between C90 and C99 to say
that reading an uninitialized automatic object is *not* undefined
behavior.  I'm don't know why they did that (though, all else
being equal, reducing the number of instances of undefined
behavior is a good thing), but reversing that decision for this
one issue is not something they decided to do.

>
Your description of what was done is wrong.  It is still the case in
C99 that trying to access an uninitialized object is undefined
behavior, at least potentially, except for accesses using a type
that either is a character type or has no trap representations (and
all types other than unsigned char may have trap representations,
depending on the implementation).  A statement like
>
    int a = a;
>
may still be given a warning as potential undefined behavior, even
in C99.

>
I had already mentioned that distinction earlier in the thread.

Oh, I must have missed that.  I don't remember seeing it in
the message I was replying to.

The mistake is thinking that UB for uninitialized access was
removed in C99.  It wasn't.  Narrowed, yes;  removed, no.

>
Acknowledged.

Good deal.