[Bug c/65446] New: Improve -Wformat-signedness

[burnus at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65446

            Bug ID: 65446
           Summary: Improve -Wformat-signedness
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: burnus at gcc dot gnu.org

As PR 65040 shows, the current implementation of -Wformat-signedness is not
optimal. (When it is more robust, one could consider to re-enable it with
-Wformat=2.)

The idea of the warning is to warn for
   "%ld",   size_t_variable
as one has to use "%lu" to print the negative values. Or reversely using %u for
a signed value, where it is even more likely that the issue occurs.

See also "cppcheck --enable=warning" which supports this warning. (But its
warning pattern makes more sense than GCC's.)


GCC's current implementation warns too often - and misses some cases. The main
bug is that it doesn't take type promotion into account. In particular:

It warns for:
   printf ("%u\n", unsigned_short);
but shouldn't: First, %u and unsigned_short are both unsigned. (It also
shouldn't and doesn't warn for %d with unsigned short as all values are
representible by %d.

It doesn't warn but should warn for
   printf("%u\n", _short);

As passing, e.g., (short)-1 to %u will print the wrong value (UINT_MAX instead
of -1).


GCC currently also warns for printf("%x\n", 1), which is very questionable.