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.co...
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. :)