Re: [PATCH][next] zd1211rw/zd_usb.h: Replace zero-length array with flexible-array member

Joe Perches
2020-03-10T22:17:31Z

On Tue, 2020-03-10 at 17:13 -0500, Gustavo A. R. Silva wrote:
>
> On 3/10/20 5:07 PM, Jes Sorensen wrote:
> > On 3/10/20 5:52 PM, Gustavo A. R. Silva wrote:
> > >
> > > On 3/10/20 8:56 AM, Kalle Valo wrote:
> > > > + jes
> > > >
> > > > "Gustavo A. R. Silva" <gustavo@embeddedor.com> writes:
> > > > > > I wrote in a confusing way, my question above was about the actual patch
> > > > > > and not the the title. For example, Jes didn't like this style change:
> > > > > >
> > > > > > https://patchwork.kernel.org/patch/11402315/
> > > > > >
> > > > >
> > > > > It doesn't seem that that comment adds a lot to the conversation. The only
> > > > > thing that it says is literally "fix the compiler". By the way, more than
> > > > > a hundred patches have already been applied to linux-next[1] and he seems
> > > > > to be the only person that has commented such a thing.
> > > >
> > > > But I also asked who prefers this format in that thread, you should not
> > > > ignore questions from two maintainers (me and Jes).
> > > >
> > >
> > > I'm sorry. I thought the changelog text had already the proper information.
> > > In the changelog text I'm quoting the GCC documentation below:
> > >
> > > "The preferred mechanism to declare variable-length types like struct line
> > > above is the ISO C99 flexible array member..." [1]
> > >
> > > I'm also including a link to the following KSPP open issue:
> > >
> > > https://github.com/KSPP/linux/issues/21
> > >
> > > The issue above mentions the following:
> > >
> > > "Both cases (0-byte and 1-byte arrays) pose confusion for things like sizeof(),
> > > CONFIG_FORTIFY_SOURCE."
> > >
> > > sizeof(flexible-array-member) triggers a warning because flexible array members have
> > > incomplete type[1]. There are some instances of code in which the sizeof operator
> > > is being incorrectly/erroneously applied to zero-length arrays and the result is zero.
> > > Such instances may be hiding some bugs. So, the idea is also to get completely rid
> > > of those sorts of issues.
> >
> > As I stated in my previous answer, this seems more code churn than an
> > actual fix. If this is a real problem, shouldn't the work be put into
> > fixing the compiler to handle foo[0] instead? It seems that is where the
> > real value would be.
> >
>
> Yeah. But, unfortunately, I'm not a compiler guy, so I'm not able to fix the
> compiler as you suggest. And I honestly don't see what is so annoying/disturbing
> about applying a patch that removes the 0 from foo[0] when it brings benefit
> to the whole codebase.

As far as I can tell, it doesn't actually make a difference as
all the compilers produce the same object code with either form.

There may be some compiler warning by clang through.

Does any version of gcc produce a warning on

struct foo {
...
type bar[0];
};

but not

struct foo {
...
type bar[];
};