<https://stackoverflow.com/posts/76271073/timeline>
If you want to change the gsl library to print the trust radius at every iteration by adding a fprintf function to the trust.c source file like so : fprintf(stdout, "trust radius is : %g \n", state->delta); I added the header file for it like so : #include <stdio.h> and modified the trust_iterate function like so : static int trust_iterate(void *vstate, const gsl_vector *swts, gsl_multifit_nlinear_fdf *fdf, gsl_vector *x, gsl_vector *f, gsl_matrix *J, gsl_vector *g, gsl_vector *dx) { int status; trust_state_t *state = (trust_state_t *) vstate; const gsl_multifit_nlinear_parameters *params = &(state->params); const gsl_multifit_nlinear_trs *trs = params->trs; gsl_multifit_nlinear_trust_state trust_state; gsl_vector *x_trial = state->x_trial; /* trial x + dx */ gsl_vector *f_trial = state->f_trial; /* trial f(x + dx) */ gsl_vector *diag = state->diag; /* diag(D) */ double rho; /* ratio actual_reduction/predicted_reduction */ int foundstep = 0; /* found step dx */ int bad_steps = 0; /* consecutive rejected steps */ /* store all state parameters needed by low level methods */ trust_state.x = x; trust_state.f = f; trust_state.g = g; trust_state.J = J; trust_state.diag = state->diag; trust_state.sqrt_wts = swts; trust_state.mu = &(state->mu); trust_state.params = params; trust_state.solver_state = state->solver_state; trust_state.fdf = fdf; trust_state.avratio = &(state->avratio); /* initialize trust region subproblem with this Jacobian */ status = (trs->preloop)(&trust_state, state->trs_state); if (status) return status; /* loop until we find an acceptable step dx */ while (!foundstep) { /* calculate new step */ status = (trs->step)(&trust_state, state->delta, dx, state->trs_state); /* occasionally the iterative methods (ie: CG Steihaug) can fail to find a step, * so in this case skip rho calculation and count it as a rejected step */ if (status == GSL_SUCCESS) { /* compute x_trial = x + dx */ trust_trial_step(x, dx, x_trial); /* compute f_trial = f(x + dx) */ status = gsl_multifit_nlinear_eval_f(fdf, x_trial, swts, f_trial); if (status) return status; /* check if step should be accepted or rejected */ status = trust_eval_step(f, f_trial, g, J, dx, &rho, state); if (status == GSL_SUCCESS) foundstep = 1; } else { /* an iterative TRS method failed to find a step vector */ rho = -1.0; } /* * update trust region radius: if rho is large, * then the quadratic model is a good approximation * to the objective function, enlarge trust region. * If rho is small (or negative), the model function * is a poor approximation so decrease trust region. This * can happen even if the step is accepted. */ if (rho > 0.75) state->delta *= params->factor_up; else if (rho < 0.25) state->delta /= params->factor_down; if (foundstep) { /* step was accepted */ /* compute J <- J(x + dx) */ status = gsl_multifit_nlinear_eval_df(x_trial, f_trial, swts, params->h_df, params->fdtype, fdf, J, state->workn); if (status) return status; /* update x <- x + dx */ gsl_vector_memcpy(x, x_trial); /* update f <- f(x + dx) */ gsl_vector_memcpy(f, f_trial); /* compute new g = J^T f */ gsl_blas_dgemv(CblasTrans, 1.0, J, f, 0.0, g); /* update scaling matrix D */ (params->scale->update)(J, diag); /* step accepted, decrease LM parameter */ status = nielsen_accept(rho, &(state->mu), &(state->nu)); if (status) return status; bad_steps = 0; } else { /* step rejected, increase LM parameter */ status = nielsen_reject(&(state->mu), &(state->nu)); if (status) return status; if (++bad_steps > 15) { /* if more than 15 consecutive rejected steps, report no progress */ return GSL_ENOPROG; } } fprintf(stdout, "trust radius is : %g \n", state->delta); } return GSL_SUCCESS; } /* trust_iterate() */ I tried to recompile the library using cmake but when i start debugging , the function isn't printed out . I have downloaded the gsl library using vcpkg.