[Bug c++/50593] New: improve __attribute__((format(printf)))

[Bug c++/50593] New: improve __attribute__((format(printf)))

[b.r.longbons at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50593

             Bug #: 50593
           Summary: improve __attribute__((format(printf)))
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: b.r.longbons@gmail.com


Created attachment 25398
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=25398
8 tests

There are a number of cases where the compiler should be able to use a constant
string to check the arguments of a printf-like function.

Expected Result:
GCC always warns about format argument errors, when it can.

Actual Result:
GCC only warns in a couple of cases.
In particular, note:
* const char[] works in C but not C++.
* Directly using the result of a constexpr function doesn't work, but assigning
it to a variable does.  (This is fortunate, because it allowed me to write the
most awesome code ever, a safe wrapper that allows passing e.g. a std::string
to printf!)

Versions tested:
All tests: 4.6.1 (gentoo), r179098 from branch 4.6, and trunk sometime near
that.
Tests 1,2,6,7,8 (without constexpr): 4.5.3, 4.4.6, 4.3.6 (gentoo)
Tests 1,2 (C++98): 4.2.4 (gentoo)
Tests 1,2 in C: all mentioned versions.

[Bug c++/50593] improve __attribute__((format(printf)))

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50593

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-10-03 08:53:27 UTC ---
const char[] not working in C++ is PR38980.

[Bug c++/50593] improve __attribute__((format(printf)))

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50593

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-10-04 10:30:44 UTC ---
Created attachment 25408
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=25408
hacked patchlet

Indeed, this kind of hacked (*) patch fixes tests 2 and 6 here (doesn't fix
tests 7 and 8) and also tests @ lines 10 and 13 of PR38980 (doesn't fix
non-const tests there). I don't know if we want to build on it or we want to do
something more invasive touching code in gcc/ and in gcc/c-family...

(*) But safe, AFAICS, because at the moment, besides check_format_arg, no other
code in c-family actually used by the C++ front-end calls decl_constant_value.

[Bug c++/50593] improve __attribute__((format(printf)))

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50593

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-10-15 20:56:42 UTC ---
PR38980 bits fixed for 4.7.0. Maybe Jason can have a look to the constexpr
bits..

[Bug c++/50593] improve __attribute__((format(printf)))

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50593

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011-10-15
     Ever Confirmed|0                           |1

[Bug c++/50593] [C++0x] improve __attribute__((format(printf)))

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50593

--- Comment #4 from Jason Merrill <jason at gcc dot gnu.org> 2011-10-15 21:16:47 UTC ---
Seems like it's a matter of changing check_format_arg to call a more general
front-end interface than decl_constant_value, one that could accept an
arbitrary expression rather than just a decl, and would call
maybe_constant_value.

[Bug c++/50593] [C++0x] improve __attribute__((format(printf)))

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50593

--- Comment #5 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-10-15 21:56:57 UTC ---
I see. I'll give it some thought.

[Bug c++/50593] [C++0x] improve __attribute__((format(printf)))

[david at rgmadvisors dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50593

--- Comment #6 from David Oliver <david at rgmadvisors dot com> 2011-11-08 21:55:46 UTC ---
Created attachment 25760
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=25760
8 Tests plus 1.

I've added test 9 to Ben's set which is a likely usage of the feature, e.g.
automatically converting std::string to a C string for printf().

Unsurprisingly, this case also fails on 4.6.2.

Cheers!

David.