How is the residual calculated for the Newton solver?

Question

When solving a nonlinear variational problem I would assume the residual for each newton iteration is calculated by a L2 norm but I can't find documentation to confirm that. How is the residual calculated? Can I modify how it is calculated? When I get the following output what is the difference between r (abs) and r (rel)?

Solving nonlinear variational problem.
Newton iteration 0: r (abs) = 1.149e+01 (tol = 1.000e-10) r (rel) = 1.000e+00 (tol = 1.000e-09)
Newton iteration 1: r (abs) = 2.535e-03 (tol = 1.000e-10) r (rel) = 2.207e-04 (tol = 1.000e-09)
Newton iteration 2: r (abs) = 4.556e-09 (tol = 1.000e-10) r (rel) = 3.965e-10 (tol = 1.000e-09)
Newton solver finished in 2 iterations and 2 linear solver iterations.

Answer 1

Have a look at dolfin/nls/NewtonSolver.cpp on line 263 to see the computation of the norm of the residual vector (l2 norm), absolute and relative residual (scaled by the residual at iteration 0).
Not sure whether it is possible to add a new stopping criterion.