[0001] Co1-xRux epitaxial films of about 50 nm thickness have been grown on a Ru buffer at temperatures (T-G) ranging from 450 K to 820 K. Most of these alloy thin films display (i) a composition modulation along the growth direction with a periodicity double that of a disordered hexagonal alloy and (ii) an induced decrease of the interplanar distance correlated to an increase of the in-plane parameter. The so-observed long-range ordering, that was not yet observed in bulk alloys, increases with T-G between 450 and 600 K, passes through a maximum at 600 K and disappears above 700 K. This behavior is explained as resulting from the competition between two phenomena occurring simultaneously during the growth process: a surface effect driven by surface interactions and surface diffusion that tends to enrich the surface layer in the heavier element (Ru segregation) and a bulk effect driven by bulk interactions and bulk diffusion that tends to restore the equilibrium disordered state when the bulk diffusion becomes efficient during the growth time. A model that takes into account both effects and different diffusion rates in surface and in volume is proposed. It reproduces quite satisfactorily the T-G dependence of the long-range order in these Co3Ru films as well as similar results previously obtained in Co3Pt films.