signed/unsigned conversions

[Chris Evans]

Michael,

Thank you very much for the reply!

-Wconversion is close to what we need.

It warns about implicit signed<->unsigned conversions, iff that conversion
occurs across a function boundary.

It doesn't warn with the simple case
{
  unsigned int i1 = 0;
  int i2 = -1;

  i1 = i2; /* No warning */
  i2 = i1; /* No warning */
}

Cheers
Chris


On Wed, 8 Nov 2000, Michael Meissner wrote:

> On Wed, Nov 08, 2000 at 04:08:27PM +0000, Chris Evans wrote:
> > 
> > Hi,
> > 
> > Mailing gcc-bugs, sorry if it's an inappropriate address :)
> > 
> > Does gcc have a warning flag to warn if there are implicit conversions
> > between signed and unsigned variables? e.g. assigning an int to an
> > unsigned int, (or visa versa), be it explicitly or across a function call.
> > 
> > If gcc _does_ have this, what is the option? It is not documented in the
> > info => bug.
> > 
> > If it _doesn't_ have this, PLEASE PLEASE add it. It is a vital warning
> > flag to have for security purposes.
> > 
> > As an aside, many other compilers seem to have the ability to warn about
> > this condition.
> > 
> > If you have no objections, I'll forward the response to the security-audit
> > list, who are interested in this capability.
> 
> There are two switches that may match what you want:
> 
> `-Wconversion'
>      Warn if a prototype causes a type conversion that is different
>      from what would happen to the same argument in the absence of a
>      prototype.  This includes conversions of fixed point to floating
>      and vice versa, and conversions changing the width or signedness
>      of a fixed point argument except when the same as the default
>      promotion.
> 
>      Also, warn if a negative integer constant expression is implicitly
>      converted to an unsigned type.  For example, warn about the
>      assignment `x = -1' if `x' is unsigned.  But do not warn about
>      explicit casts like `(unsigned) -1'.
> 
> `-Wsign-compare'
>      Warn when a comparison between signed and unsigned values could
>      produce an incorrect result when the signed value is converted to
>      unsigned.  This warning is also enabled by `-W'; to get the other
>      warnings of `-W' without this warning, use `-W -Wno-sign-compare'.
> 
> These options are present in 2.95.2 as well as the current sources.
> 
> -- 
> Michael Meissner, Red Hat, Inc.
> PMB 198, 174 Littleton Road #3, Westford, Massachusetts 01886, USA
> Work:	  meissner@redhat.com		phone: +1 978-486-9304
> Non-work: meissner@spectacle-pond.org	fax:   +1 978-692-4482
>