Re: Calling functions through a pointer of incorrect type

[Joseph Myers <joseph at codesourcery dot com>]

On Tue, 22 Mar 2016, Florian Weimer wrote:

> On 03/22/2016 08:14 PM, Joseph Myers wrote:
> > On Tue, 22 Mar 2016, Florian Weimer wrote:
> > 
> >> Just a sanity check: It is no longer acceptable glibc coding style to
> >> call functions through mismatching pointer types.  Correct?
> > 
> > I think more specific examples would help so we can see if there are 
> > cleaner approaches that generate the same code.  (For example, it's 
> > deliberate that in some places functions with incompatible C types but 
> > compatible ABIs are aliased to each other.)
> 
> I saw this in getaddrinfo:
> 
>   if (fct == NULL)
>     /* We are cheating here.  The gethostbyname2_r
>        function does not have the same interface as
>        gethostbyname3_r but the extra arguments the
>        latter takes are added at the end.  So the
>        gethostbyname2_r code will just ignore them.  */
>     fct = __nss_lookup_function (nip, "gethostbyname2_r");

Well, glibc is written with knowledge of ABI properties - that is, objects 
can assume that other objects follow the ABI and so interact in ways that 
are not valid whole-program C but are valid given that each object is an 
ABI-conforming compilation of its own sources on their own.  So while 
cleaning up cases like this would seem desirable unless particularly 
performance-critical, and I wouldn't like to see new code like this 
without strong justification, they aren't inherently bugs if all supported 
ABIs have the required property.

-- 
Joseph S. Myers
joseph@codesourcery.com