GtkIconView set_item_width question

[DaveMDS <list gurumeditation it>]

I all,
I have a problem with GtkIconView:

when I try to set the width of the icons with:

gtk_icon_view_set_item_width(icon_view,128);

the placement of the icon become strange.

If I don't touch the width all the icon are at the correct position, but if
I try to enlarge some icon take 2 or more cell (like a colspan=2 in html).

I can't find some decente documentation on this widget...

Can someone help?

Thanks
DaveMDS