Re: SCM_PTR_LE ?

[Mr. Peter Ivanyi]

Marius Vollmer wrote:

> What is that definition?  I think ANSI C only defines the outcome of
> comparisons for pointers that point into the same object, i.e., the
> same array or a single block returned from malloc.  A consequence
> would be that you can not reliably test whether a pointer does point
> into some object, since the results are not defined when it points
> outside.  For example, is the following guaranteed to print "sane"?
> 
>     char block1[100];
>     char block2[100];
> 
>     char *ptr = block1 + 50;
> 
>     if (ptr >= block2 && ptr < block2+100)
>       printf ("sane\n");
>     else
>       printf ("insane\n");
> 
> I don't know.

Well, some of the memory checker programs would definitely signal an error
for the above code. As I remember, ANSI defines, that if a pointer goes
out of its range, then it becomes undefined. This have the unfortunate
consequence that

  block2+2000-1990

is not valid, you have to calculate the offset first, than use it with the
pointer. On the positive side though, I have never seen, heard any compiler
which takes this as a problem, only memory checker.

Hope it helps a little bit.

                                        Peter Ivanyi