Re: [PATCH v3 10/13] flow: Move flow_count from context structure to a global

[Stefano Brivio <sbrivio@redhat.com>]

On Sun, 31 Dec 2023 16:58:39 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Thu, Dec 28, 2023 at 07:25:18PM +0100, Stefano Brivio wrote:
> > On Thu, 21 Dec 2023 17:15:46 +1100
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> >   
> > > In general, the passt code is a bit haphazard about what's a true global
> > > variable and what's in the quasi-global 'context structure'.  The
> > > flow_count field is one such example: it's in the context structure,
> > > although it's really part of the same data structure as flowtab[], which
> > > is a genuine global.  
> > 
> > Well, the reason is that flow_tab[FLOW_MAX] might be problematically
> > too big to live on the stack, unlike flow_count.
> > 
> > But anyway, as far as thoughts of multithreading are concerned, both
> > should probably be global. And sure, it's more consistent this way.
> >   
> > > Move flow_count to be a regular global to match.  For now it needs to be
> > > public, rather than static, but we expect to be able to change that in
> > > future.  
> > 
> > If it's not static, it should be initialised, and that's not done here.  
> 
> Uh... what?  "static" here is meaning module-global rather than
> global-global, which has no bearing on initialisation.  AFAIK globals
> are zero-initialised whether they're static or not.

...and to my utter surprise, I just discovered that if you talk C11,
you're right. From the N1570 draft (ISO/IEC 9899:201x), Section 6.7.9
"Initialization", clause 10:

  If an object that has automatic storage duration is not initialized
  explicitly, its value is indeterminate. If an object that has static
  or thread storage duration is not initialized explicitly, then:

  [...]

  — if it has arithmetic type, it is initialized to (positive or
    unsigned) zero;

And 'flow_count' has thread storage duration. In C99, however (draft
N1256), Section 6.7.8 "Initialization", clause 10:

  If an object that has automatic storage duration is not initialized
  explicitly, its value is indeterminate. If an object that has static
  storage duration is not initialized explicitly, then:

  [...]

note the missing "or thread storage duration".

C89, the one I was actually basing my observation on, says, at 3.5.7
"Initialization":

  If an object that has static storage duration is not initialized
  explicitly, it is initialized implicitly as if every member that has
  arithmetic type were assigned 0 and every member that has pointer type
  were assigned a null pointer constant.  If an object that has
  automatic storage duration is not initialized explicitly, its value is
  indeterminate.

so... um. We won't go back to C99. But to me, and maybe others, not
having a "= 0;" for a "global" means pretty much that we don't rely on
any particular initial value.

If you're strongly against it, fine, C11 says it's zero... but it makes
me a bit nervous nevertheless.

-- 
Stefano