Re: Implicit function declarations and GCC 10

[Szabolcs Nagy <Szabolcs dot Nagy at arm dot com>]

On 04/07/2019 12:27, Florian Weimer wrote:
> Implicit function declarations were removed from C99, more than twenty
> years ago.  So far, GCC only warns about them because there were too
> many old configure scripts where an error would lead to incorrect
> configure check failures.
> 
> I can try to fix the remaining configure scripts in Fedora and submit
> the required changes during this summer and fall.
> 
> I would appreciate if GCC 10 refused to declare functions implicitly by
> default.

+1

> 
> According to my observations, lack of an error diagnostic has turned
> into a major usability issue.  For bugs related to pointer truncation,
> we could perhaps change the C front end to produce a hard error if an
> int value returned from an implicitly declared function is converted to
> a pointer.  But the other case involves functions defined as returning
> _Bool, and the result is used in a boolean context.  The x86-64 ABI only
> requires that the lowest 8 bits of the return value are defined, so an
> implicit int results in int values which incorrectly compare as inqueal
> to zero.
> 
> Given that the pointer truncation issue is only slightly more common,
> than the _Bool issue, I don't think the diagnostic improvement for
> pointers would be very helpful, and we should just transition to errors.
> 
> Implicit int we should remove as well.  Checking configure scripts for
> both issues at the same time would not be much more work.

+1 for making implicit int an error by default.

> 
> Thanks,
> Florian
>