[Bug c/47781] warnings from custom printf format specifiers

["joseph at codesourcery dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47781

--- Comment #9 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Thu, 21 Aug 2014, philipp_subx@redfish-solutions.com wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47781
> 
> --- Comment #8 from Philip Prindeville <philipp_subx@redfish-solutions.com> ---
> (In reply to joseph@codesourcery.com from comment #4)
> 
> > For the general issue, my inclination is that we should add plugin hooks 
> > into the format checking machinery that allow plugins to define formats 
> > with the full flexibility of all the format checking datastructures in 
> > GCC.  Using GCC plugins for this avoids problems with defining complicated 
> > syntax in the source file to describe the peculiarities of different 
> > formats, which might constrain future changes to the format checking 
> > implementation by making too much of the internals visible to user source 
> > code, because by design GCC plugins can use GCC internals which are free 
> > to change incompatibly in ways that require plugin changes.
> 
> What about using pragmas to describe the new format specifier?

Those have the issue of either being limited in the sorts of formats that 
can be described, or else exposing more internals than seems desirable to 
expose as a stable interface.  Plugins allow full flexibility (with 
possible instability of interfaces), though a stable subset (e.g. formats 
that take no length modifiers or flags) could probably be defined that has 
a stable interface in source files (such as through attributes or pragmas) 
that doesn't unduly constrain the internals of the implementation.  But I 
think any such stable interface would not be able to describe the full 
generality of the existing built-in formats.

One interesting question would be whether a good stable interface can be 
defined that is general enough to describe GCC's internal formats - 
whether those are regular enough that a description isn't tied to 
hardcoded special cases or extremely complicated descriptions of what 
cases should / should not get warnings.