[RFC PATCH] diagnose built-in declarations without prototype (PR 83656)

[Martin Sebor]

On 06/27/2018 12:44 PM, Eric Gallager wrote:
> On 6/26/18, Martin Sebor <msebor@gmail.com> wrote:
>> With the exception of built-ins with the ellipsis (like sprintf),
>> GCC silently accepts declarations of built-in functions without
>> prototypes as well as calls to such functions with any numbers
>> or types of arguments, compatible or otherwise.  Calls with
>> arguments whose number and types match exactly those of
>> the built-in are considered by the middle-end for optimization.
>> Other calls (compatible or not, irrespective of whether their
>> number matches the number expected by the function) are then
>> made to the library functions.
>>
>> Attached is a small fix to -Wbuiltin-declaration-mismatch to
>> have it diagnose built-in declarations without a prototype.
>> The warning is enabled by default so it causes a fair number
>> of tests to fail because of declarations like 'void abort();'
>> The breakdown of the built-ins behind the test failures is
>> below.
>>
>> Before I take the time to clean up the test suite let me post
>> what I have in case going this route is not acceptable.  As
>> an alternative, I could try to avoid some of these warnings,
>> e.g., by diagnosing incompatible calls instead but I think
>> it's even less worthwhile for built-ins than trying to do
>> it for ordinary functions with -Wstrict-prototypes.  There
>> is, in my view, no justification today for standard functions
>> to be declared without a prototype.  (I could also make
>> the warning depend on language mode and -Wpedantic if that
>> were more preferable.)
>>
>> Martin
>>
>> About 115 tests fail due to incompatible declarations of
>> the built-in functions below (the number shows the number
>> of warnings for each functions):
>>
>> 428   abort
>>   58   exit
>>   36   memcpy
>>   17   memmove
>>   15   realloc
>>   14   cabs
>>    5   strncpy
>>    4   strcmp
>>    3   alloca
>>    2   rindex
>>    1   aligned_alloc
>>
>
> Could you do something to differentiate it a bit more from the
> -Wshadow warning that is also touched in this patch? It'd be kind of
> annoying to get warnings from both flags for the same code...

I don't think it's possible to get both warnings.  The block
with the -Wshadow warning returns before the one
-Wbuiltin-declaration-mismatch is entered.

Martin