Re: Ada's Slide To Oblivion ...

[Ole-Hjalmar Kristensen <oleh@vlinux.voxelvision.no>]

Ian Wild <ian@cfmu.eurocontrol.be> writes:

> David Brown wrote:
> > 
> > Hyman Rosen <hyrosen@mail.com> wrote:
> > > Ray Blaak wrote:
> > >> You most certainly can:
> > >>        typedef void (*FUNC)();
> > >>        int i = 0;
> > >>        FUNC f = (FUNC) &i;
> > >>
> > >> That this crashes with access violations on a sane OS is a good thing, but
> > >> nothing in the language is preventing the code/data conversions.
> > >
> > > False. The typecast '(FUNC)&i' is not legal standard C or C++.
> > > It's not even a case of undefined results - it's just illegal.
> > > Some compilers permit it as an extension, though.
> > 
> > Legality aside, I have yet to find a compiler that doesn't accept this.
> 
> I'll agree that most compilers will accept it, but it's not
> C and quite often doesn't do what you're expecting.
> 
> > In fact, I have code that does this very thing, it creates a small
> > amount of data that happens to be instructions, casts it to a function
> > pointer and calls it.
> 
> On, say, a high end PDP-11, or a 68000, or an 8086 (but not its
> progeny!), code space and data space are physically separate.  There
> are wires that come out of the processor to say which space to use.  On
> such a system there's NO WAY to write to code space, and it's not
> impossible that

??? This is pure fantasy. How do you think the code got into the code
space in the first place? 


> 
>   static int x [500];
>   int main () {
>     if ((void*)&main == (void*)&x)
>        printf ("Yes\n");
>     return 0;
>   }
> 
> will print "Yes".  You can fill your array x with
> all manner of carefully chosen data, but any attempt
> to call it will jump to the corresponding address
> in /code space/, which here will re-start the program.