Re: [RFA] [PATCH] Add a warning for invalid function casts

[Eric Gallager <egall@gwmail.gwu.edu>]

On 10/5/17, Bernd Edlinger <bernd.edlinger@hotmail.de> wrote:
> On 10/05/17 02:24, Eric Gallager wrote:
>> Sorry if this is a stupid question, but could you explain how this
>> warning is different from -Wbad-function-cast? Something about direct
>> calls to functions vs. passing them as function pointers?
>
> No, it is not :)
>
> -Wbad-function-cast is IMHO a strange legacy warning.
>
> It is C-only, and it triggers only if the result of a function call
> is cast to a type with a different TREE_CODE, so for instance
> int <-> float <-> pointer.
>
> It would trigger for perfectly valid code like this:
>
> i = (int) floor (f);
>
> while we have no warning for
>
> i = floor (f);
>
> What I want to diagnose is assigning a function pointer via an explicit
> type cast to another function pointer, when there is no possible
> implicit conversion between the two function pointer types.
> Thus the cast was used to silence a warning/error in the first place.

OK, thanks for explaining! I kinda worry about warning on code that
was added to silence other warnings in the first place, as it can lead
to frustration when making a fix expecting the number of warnings when
compiling to decrease, but then they don't actually decrease. But as
long as there's a simple way to fix the warned-on code that silences
both the original warning being avoided, and this new one, I see how
it'll be useful.

>
> The idea for this warning came up when someone spotted a place in
> openssl, where a type cast was used to change the return value of a
> callback function from long to int:
>
> https://github.com/openssl/openssl/issues/4413
>
> But due to the type cast there was never ever any warning from this
> invalid type cast.
>
>
> Bernd.
>

Thanks for working on this!

Eric