Re: Sparse RFC

["Darren Jenkins" <darrenrjenkins@gmail.com>]

On 3/27/07, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Mon, 26 Mar 2007, Derek M Jones wrote:
>
> > This is a useful check.  However, you need to make sure that
> > cases where the presence of () have no impact are not flagged
> > if the parameter is always used in such contexts.  Otherwise the
> > noise can be excessive.
>
> Indeed. In the kernel, for example, we sometimes use nested macros for the
> complex cases, and there one of the cases is where the "outer" or inner
> macro adds all required brackets. Ie something like
>
>         #define ALIGN(x,a)              __ALIGN_MASK(x,(typeof(x))(a)-1)
>         #define __ALIGN_MASK(x,mask)    (((x)+(mask))&~(mask))
>
> where ALIGN() uses 'x' without any brackets, because it knows the inner
> macro is safe (and a comma expression must have had any brackets of its
> own, otherwise it wouldn't have parsed as a single macro argument anyway!)

Hmm it looks like a 'bracket or comma' check wouid correcly ignore
this particular example.

It is obvious however that I need to think about this a bit more.


> > > 2) For the macro argument checking would it be better if I checked for
> > > either a bracket or low precedence operator on both sides of the
> > > argument ? like either a comma or a type of assignment operator? it
> > > would not be a foolproof check then but would seem more sensible to me,
> > > and might be more acceptable to users.
> >
> > Don't make the assumption that everybody knows the precedence
> > of binary operators.  An experiment I ran last year with experienced
> > developers (average over 10 years) threw up some unexpected results.
> > See: http://www.knosof.co.uk/cbook/accu06a.pdf

Yep I am guilty of getting them wrong before.

> Yeah. The only exception is literally the comma expression, because of the
> syntax of macro expansion itself (ie a comma would break a macro argument,
> unless it is bracketed). Pretty much all other precedence rules can pretty
> much be expected to be confused by somebody.

I didn't look into it but I was thinking that something that evaluated
into a comma, after the initial macro expansion would still be
passible/an issue.



Thanks for the feedback guys. I guess I will keep tinkering with it.

Darren J.