I need to create a sub bitmap from an existing bitmap, so i looked for it into allegro manual and i found al_create_sub_bitmap() function, but, as i have understood, that function just cut the parent image, 'cause they share the same drawing memory. So, it's my misunderstanding or to create a sub bitmap i must do this way:

ALLEGRO_BITMAP *new_bitmap = al_create_bitmap(w, h);
al_set_target_bitmap(new_bitmap);
al_draw_bitmap_region(old_bitmap, x, y, w, h, 0, 0, 0);

What do you want to do with your 'sub' image? What is it for?

Well, you have two choices. Clone the area, or create a sub bitmap. If you don't want the memory shared, clone the regions you want. If you don't mind if its shared, use a sub bitmap.