Re: [Qemu-devel] Re: [PATCH 2/5] CODING_STYLE: add C type rules

[Paolo Bonzini]

On 08/18/2010 12:30 PM, Kevin Wolf wrote:
> Am 18.08.2010 10:35, schrieb Paolo Bonzini:
> > On 08/17/2010 08:39 PM, Richard Henderson wrote:
> > > >   On 08/12/10 19:50, Blue Swirl wrote:
> > > > >   +While using "bool" is good for readability, it comes with minor caveats:
> > > > >   + - Don't use "bool" in places where the type size must be constant across
> > > > >   +   all systems, like public interfaces and on-the-wire protocols.
> > > > >   + - Don't compare a bool variable against the literal, "true",
> > > > >   +   since a value with a logical non-false value need not be "1".
> > > > >   +   I.e., don't write "if (seen == true) ...".  Rather, write "if (seen)...".
> > > >
> > > >   I'd strongly discourage the use of bool in any code.
> > >
> > > I strongly disagree.  The use of "bool", even if you ignore stdbool.h
> > > and do "typedef int bool", is valuable documentation in the code.
> >
> > I think "bool" is fine, but it should be either stdbool.h or a typedef.
> >    Using stdbool.h only when it is present is going to introduce bugs the
> > day someone relies on the magic properties of the C99 bool.
>
> We rely on C99 anyway, so stdbool.h should always be present (and in
> fact, it is used unconditionally today).

Right.  However, this is wrong then:

- Don't compare a bool variable against the literal, "true",
  since a value with a logical non-false value need not be "1".
  I.e., don't write "if (seen == true) ...".  Rather, write "if
  (seen)...".

I mean, I'm all for using "if (seen)" but bool *does* normalize logical 
non-false values to "1".  I'd remove the second line (making it just a 
coding style issue) and add something like this:

- Do not rely on the fact that bool normalizes logical non-false values
  to 1.  So, write "x = true" instead of "x++" and "x = !x" instead of
  "x--".
- Similarly, when x is a bool, it may be clearer to avoid
  "x |= y".  Instead, use either "x = x || y" (if short circuiting
  is acceptable or even desirable) or "x |= (y != 0)".

Probably a bit too verbose, but you get the idea.

Paolo