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

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

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

--- Comment #4 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2011-02-17 18:24:25 UTC ---
On Thu, 17 Feb 2011, mark-gcc at glines dot org wrote:

> I'd like to request a finer grained means of control.  A syntactical element
> (builtin/pragma/attribute/whatever) to pre-declare a format conversion and the
> typedef to check it against would be very nice, if complex.  A much simpler
> solution would be a -Wno-format-unknown-specifier option, which skips the
> argument in the argument list and otherwise ignores conversions it doesn't
> recognize.

You can't reliably know how many arguments the unknown specifier takes, 
though assuming them to take one argument would be a reasonable heuristic 
for such an option.

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.