Re: [RFC PATCH 05/13] list.h: Fix parentheses around macro pointer parameter use

[Mathieu Desnoyers]

On 2023-05-08 08:16, Andy Shevchenko wrote:
> On Thu, May 04, 2023 at 04:05:19PM -0400, Mathieu Desnoyers wrote:
> > Add missing parentheses around use of macro argument "pos" in those
> > patterns to ensure operator precedence behaves as expected:
> >
> > - typeof(*pos)
> > - pos->member
> > - "x = y" is changed for "x = (y)", because "y" can be an expression
> >    containing a comma if it is the result of the expansion of a macro such
> >    as #define eval(...) __VA_ARGS__, which would cause unexpected operator
> >    precedence. This use-case is far-fetched, but we have to choose one
> >    way or the other (with or without parentheses) for consistency,
> > - x && y is changed for (x) && (y).
> >
> > Remove useless parentheses around use of macro parameter (head) in the
> > following pattern:
> >
> > - list_is_head(pos, (head))
> >
> > Because comma is the lowest priority operator already, so the extra pair
> > of parentheses is redundant.
>
> But strictly speaking it might be something like
>
> 	list_...(..., (a, b))
>
> where (a, b) is the head. No?

The following case still works after removing the extra parentheses 
around "head" because the parentheses are present where the macro is used:

LIST_HEAD(testlist);

int f2(void)
{
        return 1;
}

void f(void)
{
   struct list_head *pos;

   list_for_each(pos, (f2(), &testlist)) {
           //...
   }
}

The only use I found that would break is as follows:

LIST_HEAD(testlist);

int f2(void)
{
        return 1;
}

#define eval(...)       __VA_ARGS__

void f(void)
{
   struct list_head *pos;

   list_for_each(pos, eval(f2(), &testlist)) {
           //...
   }
}

Because "eval()" will evaluate "f(), &testlist" with comma and all, 
without enclosing parentheses.

So the question is: do we want to support this kind-of-odd macro 
evaluation, considering that it requires adding parentheses around 
pretty much all macro parameters when used as expressions between commas?

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com