Re: How to force gcc to report a warning/error

["Trevor Woollacott" <trevorw at pharos-avantgard dot com>]

> From: "ppmoore" <polomora@gmail.com>
> Sent: Tuesday, October 26, 2010 10:17 AM
> To: <gcc-help@gcc.gnu.org>
> Subject: How to force gcc to report a warning/error
>
> > Hello,
> >
> > We had an interesting error that was undetected in gcc:
> > {
> >  unsigned long a;
> >  unsigned long x=3;
> >  unsigned long y=4;
> >  a =
> >      min(x,y);
> > }
> >
> > I've simplified the example. In the original code, the contents of the 
> > min()
> > statement were longer, so that it was on a separate line from the 
> > assignment
> > operation.
> >
> > Because of a bug, the assignment line was removed, and we had the 
> > following
> > code:
> > {
> >  unsigned long a;
> >  unsigned long x=3;
> >  unsigned long y=4;
> >      min(x,y);
> > }
> >
> > Should this be picked up as a warning/error?
> > Neither -Wall or -Wextra picked it up.
> >
> > Many thanks,
> > Paul
> > --
> > View this message in context: 
> > http://old.nabble.com/How-to-force-gcc-to-report-a-warning-error-tp30055230p30055230.html
> > Sent from the gcc - Help mailing list archive at Nabble.com.

Hi Paul

The compiler doesn't know if you are purposely ignoring the result of min, 
therefore
it doesn't warn you. However, using -Wall in the above code would have 
warned you that
'a' was an unused variable.

If you want to make sure that a return value is used, then you can declare 
min
using __attribute((__warn_unused_result__)).
For example:
int min(int x, int y) __attribute((__warn_unused_result__));

And compiling would give you the following warning:
warning: ignoring return value of 'min', declared with attribute 
warn_unused_result

Regards,
Trevor