Re: Upcoming GLib changes

[Owen Taylor <otaylor redhat com>]

Tim Janik <timj@gtk.org> writes:

> On 30 Jun 1999, Soeren Sandmann wrote:
> 
> > There is another issue with GLib. If memmove is not present, glib.h
> > implements g_memmove with bcopy, which is not guaranteed to handle
> > overlapping memory areas correct. This is bad since garray.c depends
> > on g_memmove handling overlapping areas correct.
> > 
> > I would suggest the following patch. It is better to be safe than
> > sorry.
> > 
> > --- configure.in.orig   Wed Jun 30 16:43:06 1999
> > +++ configure.in        Wed Jun 30 16:50:34 1999
> > @@ -1332,10 +1332,13 @@
> >    ;;
> >  *)
> >    glib_memmove="
> > -/* We make the assumption that if memmove isn't available, then
> > - * bcopy will do the job. This isn't safe everywhere. (bcopy can't
> > - * necessarily handle overlapping copies) */
> > -#define g_memmove(d,s,n) G_STMT_START { bcopy ((s), (d), (n)); } G_STMT_END"
> > +/* If memmove isn't available, then we do the job with bcopy */
> > +#define g_memmove(d,s,n) G_STMT_START { gpointer tmp; \
> > +                                        tmp = g_malloc(n); \
> > +                                        bcopy ((tmp), (s), (n)); \
> > +                                        bcopy ((d), (tmp), (n)); \
> > +                                        g_free (tmp); \
> > +                                      } G_STMT_END"
> >    ;;
> >  esac
> 
> if g_memmove() is used with very big memory portions, you can't neccessarily
> rely on being able to allocate that amount on the fly. the best solution would
> be a configure check which figures whether bcopy() handles overlapping memory
> areas or not, and if that is not the case, simply roll our own for() loop for
> s < d < s + n (eventhough d < s < d + n overlaps as well, i've not yet heard of
> a copying mechanism that couldn't handle that).

Yep. The malloc thing is not good.

There is a test for overlapping-safe bcopy in Perl's Configure
that could be borrowed to save a bit of work and make
sure we get it right.

Perl also has a overlap-safe bcopy - but we can't use that
because of licensing issues. So that would have to be
written from scratch - not a big deal.

Regards,
                                        Owen