surround complex macros in ()

surround complex macros in ()

[daniel watson]

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.

-------delete-me.c--------
#define with ((int)a)
#define sans (int)a

void main(void){
  int b = 0;
  with++;
  sans++;
}
-------delete-me.c--------

-------terminal--------
$ gcc -o delete-me delete-me.c 
delete-me.c: In function ‘main’:
delete-me.c:7:7: error: lvalue required as increment operand
    7 |   with++;
      |       ^~
-------terminal--------

the compiler complains about the macro defined with parentheses, and
does not have a problem with the other macro defined sans parentheses.
this is only a compile time difference, and maybe that's the only
possible difference that could be made by the parentheses.

i'm curious if there's a way to know for sure that there exists no
possible expression with such a macro in it that would cause a more
subtle difference.  for example, how do i rule out the possibility
that the code could compile and have a different value than expected
at runtime?


as a side note, i signed up for the kernelnewbies mailing list, and i do
not see any messages in my inbox, except a reply to a message i sent
out.  i checked here
in some expression
http://lists.kernelnewbies.org/pipermail/kernelnewbies/

and do not see the message i sent, or the reply to it.

am i seeing the right thing?  did i sign up correctly?  is that the
right page to view the mailing list messages online?


thanks!

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Re: surround complex macros in ()

[Greg KH]

On Wed, Aug 25, 2021 at 10:21:44PM -0700, daniel watson wrote:
> 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.
> 
> -------delete-me.c--------
> #define with ((int)a)
> #define sans (int)a

Note, this is NOT what your change was doing.

To duplicate what your change wanted to do, try doing:

#define with ((int)(10 * 20))
#define sans (int)(10 * 20)

Now see if that is any different when you use it.

thanks,

greg k-h

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Re: surround complex macros in ()

[Valdis Klētnieks]

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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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