signed/unsigned conversions

[Michael Meissner]

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.
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