Re: surround complex macros in ()

Valdis Klētnieks
2021-08-26T09:04:55Z

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On Wed, 25 Aug 2021 22:21:44 -0700, daniel watson said:
> let me know if this is the right place to ask.
>
> i recently tried to make a commit adding parentheses around a macro
> value.
>
> https://lore.kernel.org/linux-staging/20210817043038.GA9492@challenge-bot.com/
>
> it was rejected as "This is not a real change that is needed."
>
> at first, i thought this meant that the code would be identical with and
> without parentheses surrounding a complex macro's definition, when the
> macro is just typecasting an expression. but then i came up with code
> where having parens or not changes the meaning of the code.

The fact you can contrive an example where it makes a difference doesn't
mean that it makes a difference for the patch as submitted.

Hint: If your patch to add parentheses was in fact correct and needed
as per your with/sans example, it wouldn't have compiled before, and
I, or any of a number of people and build farms, would have submitted
patches withing 24 to 48 hours. Of course, that's not the only possible
situation....

> this is only a compile time difference, and maybe that's the only
> possible difference that could be made by the parentheses.

Not at all true.

#define with(a,b) (a + b)
#define sans(a,b) a + b
foo = 23*with(a,b);
bar = 23*sans(a,b);

This stuff ends up mattering when macros start getting nested deep enough.

From the other day when I was chasing a build error and I had to resort
to building a .i file to see what the pre-processor was doing to me:

(05:33:04 PM) valdis: #define EGADS 1138 /* code violates the principle of least surprise */
(05:33:49 PM) valdis: Consider this code from include/linux/seqlock.h:
(05:33:49 PM) valdis: static inline void __seqprop_assert(const seqcount_t *s)
(05:33:49 PM) valdis: {
(05:33:49 PM) valdis: lockdep_assert_preemption_disabled();
(05:33:49 PM) valdis: }
(05:34:10 PM) valdis: Seems reasonable for a static inline, right?
(05:35:06 PM) valdis: Well... that lockdep_asser.. is a macro.. that expands to 41,349 characters.

Later examination shows 3,089 ( ) pairs, maximum nesting of 12 deep.

> how do i rule out the possibility that the code could compile and have a
> different value than expected at runtime?

Write clean, clear, unobfuscated code. Don't nest macros too deeply.
Understand the C casting rules and operator precedence.

And hope to $DEITY that you're not debugging code written by somebody
who screwed that stuff up, because if they managed to code something
that compiles cleanly even when building with W=1 C=1, and still evaluates
to something that isn't what was intented, you're probably looking at
a very subtle error indeed. See above for a worked example. :)

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