surround complex macros in ()

daniel watson
2021-08-26T05:22:12Z

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!