Re: Order of files with 'cp'

On 2005-11-20 20:56, Brian Candler <B.Candler_at_pobox.com> wrote:
> On Sun, Nov 20, 2005 at 09:34:16PM +0200, Giorgos Keramidas wrote:
> > qsort() can be 'stable' enough for our purposes if the comparison
> > function uses something like this:
> >
> >     int
> >     compare_int32_t(void *pa, void *pb)
> >     {
> >         int32_t a, b;
> >
> >         assert(pa != NULL && pb != NULL);
> >         a = *((int32_t *)pa);
> >         b = *((int32_t *)pb);
> >
> >         if (a < b && pa != pb)
> >             return 1;
> >         if (a > b && pa != pb)
> >             return -1;
> >         return 0;
> >     }
>
> Care to explain??
>
> For one thing, I don't think qsort() will ever call its
> comparison function with two equal pointers, as that would be a
> point(er)less thing to do.  Therefore, I can't see why you are
> testing for pa != pb, since this condition will always be
> true. (Also, a < b could never be true if pa == pb).

I see.  I was hoping to avoid spurious exchanges of `objects' in
cases like:

    struct object {
        int obj_value;
    };

    static struct object objvec[] = {{1},{1},{1}};

    static struct object *vec[3] = {
       &(objvec[0]),
       &(objvec[1]),
       &(objvec[2]),
    };

where the comparison function may compare vec[k]->obj_value
elements and 'exchange' elements of vec[] because their value is
'equal' anyway.

But you're right this won't help, since qsort() never passes
equal pointers to the comparison function.

> Secondly, mastercmp() doesn't compare integers, it compares FTS
> structures.  The comparison function just compares whether one
> item is a directory and the other a file. How would you have
> mastercmp() work instead?

To return to the problem at hand, the problem is that the
'sorting' function is allowed to re-arrange the order of argv[]
elements.  I'd probably try to write mastercmp() in a way that
satisfies all of the following:

    - All files are sorted before all directories
    - Files are arranged by their argv[] order
    - Directories are arranged by their argv[] order

This would probably require some sort of fts_number hackery, to
make sure we save the original order of argv[] elements, but I'm
not sure how to write a mastercmp() that works this way yet.
I'll have to check this more carefully

- Giorgos