A simple least-square fitting-based method is described for the determination of strain profiles in epitaxial films using high-resolution X-ray diffraction. The method is model-independent, i.e. it does not require any "guess" model for the shape of the strain profile. The shape of the vertical displacement profile is modeled using the versatile cubic B-spline functions, which puts smoothness and curvature constraints on the fitting procedure. The effect of a coherently diffracting substrate is taken into account as well as the effects of film thickness fluctuations. The model is applied to the determination of strain profiles in SmNiO 3 films epitaxially grown on (001) SrTiO 3 substrates. The shape of the retrieved strain profile is discussed in terms of oxygen vacancies.