bug#32189: 27.0.50; GCC 7 warning due to -Wformat-truncation=2

[Ken Brown]

On 7/18/2018 11:09 AM, Eli Zaretskii wrote:
> > From: Ken Brown <kbrown@cornell.edu>
> > Date: Tue, 17 Jul 2018 15:26:34 -0400
> >
> > I'm getting the following warning when building the master branch on
> > Cygwin with GCC 7.3:
> >
> > In file included from /usr/include/stdio.h:800:0,
> >                    from ../lib/stdio.h:43,
> >                    from ../../master/src/w32cygwinx.c:22:
> > ../../master/src/w32cygwinx.c: In function ‘Fw32_battery_status’:
> > ../../master/src/w32cygwinx.c:116:26: warning: ‘%3.1f’ directive output
> > may be truncated writing between 3 and 312 bytes into a region of size
> > 16 [-Wformat-truncation=]
> >       snprintf (buffer, 16, "%3.1f", h);
> >                             ^
> > ../../master/src/w32cygwinx.c:116:4: note: ‘__builtin_snprintf’ output
> > between 4 and 313 bytes into a destination of size 16
> >       snprintf (buffer, 16, "%3.1f", h);
> >       ^
>
> Do we really need to use -Wformat-truncation?  Is it a useful warning
> switch?  The above sounds like useless noise, because the code
> explicitly _asks_ for truncation.  What do people think about this?

Moreover, the warning isn't very smart; see below.

> > The attached patch avoids the warning.  Is this a reasonable fix, or is
> > there a better way?
>
> I think if we keep the switch, a better fix is to do this:
>
>       snprintf (buffer, 16, "%ld", h % 1000000);
>
>       m = seconds_left / 60;
>       snprintf (buffer, 16, "%ld", m % 20000);
>
> etc., you get the point.

This doesn't work with GCC 7.  (Maybe it would work with GCC 8; the 
release notes say that it is better at avoiding false positives.)  For 
integer specifiers like "%ld", the only thing I've found that works 
without enlarging the buffer is to cast the argument to a smaller 
integer type.  For float specifiers like "%3.1f", even using a small 
type doesn't seem to work.  For example:

$ cat test.c
#include <stdio.h>
int
main ()
{
  char buffer[16];
  short a;
  snprintf (buffer, 16, "%3.1f", a);
}

$ gcc -Wformat-truncation=2 test.c
test.c: In function ‘main’:
test.c:8:26: warning: ‘%3.1f’ directive output may be truncated writing 
between 3 and 312 bytes into a region of size 16 [-Wformat-truncation=]
   snprintf (buffer, 16, "%3.1f", a);
                          ^~~~~
test.c:8:3: note: ‘snprintf’ output between 4 and 313 bytes into a 
destination of size 16
   snprintf (buffer, 16, "%3.1f", a);
   ^~~~~~~~~~~~~~~~~~~~~

Ken