Could preprocessor warn for unsafe macros and side-effects?

[Martin Sebor]

On 07/11/2017 11:50 PM, sarah@hederstierna.com wrote:
> Hi
>
> Reading about macro pitfalls and eg duplication side-effects
> https://gcc.gnu.org/onlinedocs/cpp/Macro-Pitfalls.html#Macro-Pitfalls
>
> would it be possible to let the preprocessor generate warnings for any of these pitfalls?

The preprocessor has no knowledge of the language rules but it is
possible to detect some of these problems in the parser.  In fact,
a checker for one of the CERT problems was added not too long ago:
-Wmulti-statement-macros.  I would suggest to open an enhancement
request asking for features you would find particularly valuable.

>
> Maybe all language specific parts are not know at this early preprocessing stage, but possibly some info could be stored for use in later pass?
>
> I'm thinking of eg. for "function-like macros" with arguments, checking
>
> -Wmacro-side-effects
>
> * IF function-like macro expands/duplicates an argument more than once THEN
>     WARN if function() is part as the argument
>     WARN if unary ++ or -- is used on variable as part of argument
>     WARN if assignment operator = is part of argument
>     WARN if volatile variable part as the argument
>
> -Wmacro-operator-precedence
>
> * WARN if macro argument contains an expression with operator(s), an a _higher_ precedence operator is used within the macro on this argument, without parenthesis around
>
> I'm not sure its even possible at preprocessing stage, but it would be nice to have,
> I saw some static code analysis tools like Coverity detects these
> https://www.securecoding.cert.org/confluence/display/c/PRE31-C.+Avoid+side+effects+in+arguments+to+unsafe+macros
>
> Of course it might generate some false-positives so warning might not be enabled by default, maybe just -Wall or -Wextra,
> but perhaps it hard to solve, and I'm not sure where and how to implement the checking algorithm.

Those sound like good ideas.  Some of them and the challenges
with implementing them were discussed in the context of the
-Wmulti-statement-macros enhancement:

   https://gcc.gnu.org/ml/gcc-patches/2017-06/msg00064.html

Martin