[EMAIL PROTECTED] (Kai Henningsen) writes:
> ian@airs.com (Ian Lance Taylor) wrote on 20.01.06 in <[EMAIL PROTECTED]>:
>
> > When dealing with unions, you can take pointers to different fields in
> > the unions. If the fields have different types, these pointers can
> > have non-conflicting alias sets. Therefore within a single union the
> > same memory can be read or written by different pointers. This is
> > considered to be invalid--a valid program is required to always access
> > the memory within the union in the same type, except if you access the
> > memory via the union type itself (this permission being a gcc
> > extension).
>
> void test(void)
> {
> union { int i; double d; } u;
> int *ip;
> double *dp;
> int ii;
> double dd;
>
> ip = &u.i;
> *ip = 15;
> ii = *ip;
> dp = &u.d;
> *dp = 1.5;
> dd = *dp;
> printf("ii=%d dd=%f\n", ii, dd);
> }
>
> So you're saying this function is not valid?
That function is valid. I expressed myself badly above. What I
should have said is that if you have a pointer to a field in the
union, and you do a read, a valid program must have previously done a
write using a pointer to the same field. A write using a pointer to
one field followed by a read using a pointer to another field is
invalid.
Ian
