— Function: int gsl_bspline_eval (const double x, gsl_vector * B, gsl_bspline_workspace * w)

This function evaluates all B-spline basis functions at the position x and stores them in B, so that the ith element of B is B_i(x). B must be of length n = nbreak + k - 2. This value may also be obtained by calling gsl_bspline_ncoeffs. It is far more efficient to compute all of the basis functions at once than to compute them individually, due to the nature of the defining recurrence relation.

— Function: size_t gsl_bspline_ncoeffs (gsl_bspline_workspace * w)

This function returns the number of B-spline coefficients given by n = nbreak + k - 2.