[Bug c/34941] New: Option to warn on printf of ptrdiff_t without t size modifier

["debian-gcc at lists dot debian dot org" <gcc-bugzilla at gcc dot gnu dot org>]

[forwarded from http://bugs.debian.org/425623]

This code:

#include <stdio.h>
int main(int argc, char *argv[])
{
        void *a, *b;
        printf("%d\n", a - b);
        return 0;
}

will generate this warning on a 64-bit platform:

temp.c:5: warning: format ?%d? expects type ?int?, but argument 2 has type
?long int?

because a - b has type ptrdiff_t and sizeof(ptrdiff_t) == 8 on a 64-bit
platform.  However, on a 32-bit platform the code will generate no warning.  I
believe it should generate a warning like this on all platforms, if not by
default then with an appropriate option:

temp.c:5: warning: format ?%d? expects type ?int?, but argument 2 has type
?ptrdiff_t?

In C99, such code can use %td; the t size modifier corresponds to the
ptrdiff_t type.  If the code does not want to count on c99, it could cast the
difference to a long and use %ld.


-- 
           Summary: Option to warn on printf of ptrdiff_t without t size
                    modifier
           Product: gcc
           Version: 4.2.3
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: debian-gcc at lists dot debian dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34941