Re: Fun with C

[Peter C. Chapin <PChapin@vtc.vsc.edu>]

On Sat, 16 Apr 2011 23:12:37 +0200, Ludovic Brenta
<ludovic@ludovic-brenta.org> wrote:


>Out of curiosity, is this actual result a predictable consequence of the
>language definition, or is it undefined behavior?

It is standard. It's part of the rules for "usual arithmetic conversions"
described in section 6.3.1.8 of the C99 standard. In particular when
unsigned int and int are mixed in operations like this, the signed int is
converted to its corresponding unsigned value using the rules in 6.3.1.3
(which typically amounts to just reinterpreting the bit pattern).

As someone who has used C for many years, I don't find this particularly
strange. It is an example of why unsigned types are tricky and should be
avoided unless one has a specific reason for using them.

Many compilers (or other analysis tools) can produce warning messages about
this if you ask for them.

Peter