Re: [9fans] Don't Plan 9 C compiler initialize the rest of member of a?struct?

[Dan Cross <crossd@gmail.com>]

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On Mon, Apr 1, 2019 at 8:36 PM Kurt H Maier <khm@sciops.net> wrote:

> On Mon, Apr 01, 2019 at 08:26:30PM -0400, Jeremy O'Brien wrote:
> > On Mon, Apr 1, 2019, at 11:33, Kyohei Kadota wrote:
> > > Hi, 9fans. I use 9legacy.
> > >
> > > About below program, I expected that flags field will initialize to
> > > zero but the value of flags was a garbage, ex, "f8f7".
> > > Is this expected?
> > >
> > > ```
> > > #include <stdio.h>
> > >
> > > struct option {
> > >     int n;
> > >     char *s;
> > >     int flags;
> > > };
> > >
> > > int
> > > main(void)
> > > {
> > >     struct option opt = {1, "test"};
> > >     printf("%d %s %x\n", opt.n, opt.s, opt.flags);
> > >     return 0;
> > > }
> > > ```
> > >
> > >
> >
> > According to C99: "If an object that has automatic storage duration is
> not initialized explicitly, its value is indeterminate."
> >
> > Stack variable == automatic storage duration. This appears to be correct
> behavior to me.
> >
>
> Can anyone provide the patches 9legacy uses to implement C99 compliance?


There were actually quite a few of them, mostly done by Geoff Collyer.  The
compiler sources list contains a list of desiderata in a file called `c99`;
of course, the plan9 compilers aren't completely compliant (they weren't
trying to be). Incidentally this file has been carried forward into, for
example, /sys/src/cmd/cc/c99 in the 9front distribution (and other plan9
derivatives).

In the present case, this appears to be a compiler bug. The aforementioned
reference to n1548 sec 6.7.9 para 10 is incorrect in that there _is_ an
explicit initializer here. The relevant text in the standard is sec 6.7.9
pp 16-21, which specifies that in the event that an explicit initializer
does not completely cover (in a topological sense) the thing it is
initializing, then the elements not covered shall be initialized as if they
had _static_ storage duration; that is, they should be zeroed.

Now as I said, the Plan 9 C compilers aren't explicit C99 compliant. But
given that the `c99` file describes things related to initializer lists as
being unneeded because they were already implemented, one may assume it was
believed that this was covered by c99 behavior. It isn't.

        - Dan C.

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