Format warnings ignored by -Wformat

Segher Boessenkool
Tue Feb 26 18:46:00 GMT 2019

On Tue, Feb 26, 2019 at 02:26:12PM +0000, Jonathan Wakely wrote:
> On Mon, 25 Feb 2019 at 23:32, Segher Boessenkool
> <> wrote:
> > On Mon, Feb 25, 2019 at 11:18:31AM -0800, Jon Flatley wrote:
> > > I was hoping I could reach out and gain some insight as to why GCC
> > > ignores these scenarios where Clang produces warnings. This would be
> > > very helpful as we move to enable -Wformat for both GCC and Clang in
> > > the upstream kernel, which is an issue we're tracking here:
> > >
> >
> > You cannot pass a short int to a varargs function like printf, it is
> > always promoted to int.  As the C standard says for the h flag character:
> >
> > h
> >     Specifies that a following d, i, o, u, x, or X conversion specifier
> >     applies to a short int or unsigned short int argument (the argument
> >     will have been promoted according to the integer promotions, but
> >     its value shall be converted to short int or unsigned short int
> >     before printing); or that a following n conversion specifier applies
> >     to a pointer to a short int argument.
> A bit later it says:
> If any argument is
> not the correct type for the corresponding conversion specification,
> the behavior is
> undefined.
> The "conversion specification" is %hd so includes both the h length
> modifier and the d conversion specifier, and it's not clear to me
> whether "the correct type" means the type as modified by the length
> modifier or not, i.e. whether %hd

It means "int" here, afaics.  This is about whether the same types args
are passed to the printf function as it is trying to read.

> > Assuming the integer started out as short int before passing it to the
> > printf function is something GCC does not do;
> But GCC doesn't need to assume anything when it can see exactly what's
> being passed to printf. It can see that an int really is used there,
> not just something promoted to int.

Maybe I didn't say it clearly, sorry.  If GCC wants to warn about passing
a plain int to %hd it needs to assume this is "bad" somehow (even although
the C standard says exactly how this should behave).

> > it will warn on a lot of
> > perfectly valid code.
> But also misses warnings for code with unintended (maybe even
> undefined) behaviour:
> #include <stdio.h>
> #include <limits.h>
> int main()
> {
>   int i = SHRT_MAX + 1;
>   printf("%hd %hd\n", i, SHRT_MAX + 1);
> }

Conversion to a shorter integer type can be implementation-defined, not
undefined.  GCC has

     For conversion to a type of width N, the value is reduced modulo
     2^N to be within range of the type; no signal is raised.

(but it is a bit shady what printf does exactly, it's not part of GCC;
but we can probably assume it is compiled by GCC itself).

> Inside printf the int values are converted to short, which is either
> an overflow (so undefined) or, if the conversion is done via unsigned
> types, at least a change in value that is probably unwanted.

It's implementation-defined, see

> The compiler should be able to warn that SHRT_MAX+1 will not be
> printed correctly.

That depends what you call "correct".  As I said, there will be lots of
false positives with such a warning, as OP already showed.


More information about the Gcc-help mailing list