Re: Dimensions of array parameters

[Ian Lance Taylor <iant at google dot com>]

ludovic.courtes@inria.fr (Ludovic CourtÃs) writes:

> I understand.  However, Iâm concerned about keeping the information at
> compile-time.  For example:
>
>   extern void foo (int a, int x[a]);
>   static void bar (void) {
>     int x[123];
>     foo (456, x);
>   }
>
> Here the compiler could emit a diagnostic in âbarâ.
>
> IOW, ABI restrictions ABI imply that nothing can be done in the body of
> âfooâ; however, the information could still used at call sites.

I see.  That seems to have some potential.  It would be a gcc extension,
of course.  Perhaps something like

extern void foo (int a, int x[__attribute__ ((dim (a)))])

could be implemented.  This would issue an error if foo were called such
that the length of the x were not at least a.  A serious problem with
this approach would be that it is not reliable, since the caller of foo
might not know the length of the array being passed as a pointer.  I
suppose it would be possible to issue runtime checks when static checks
don't work, and issue a compilation error in cases were no runtime check
is possible.  Without those checks I think the extension would be ill
advised.  With those checks I don't know how often it would usable in
practice.

Ian