Ref:http://www.shogun-toolbox.org/doc/en/current/structshogun_1_1lbfgs__parameter__t.html

In the document it says
int max_linesearch
The maximum number of trials for the line search. This parameter controls the number of function and gradients evaluations per iteration for the line search routine. The default value is 20. 

float64_t delta

Delta for convergence test. This parameter determines the minimum rate of decrease of the objective function. The library stops iterations when the following condition is met: (f' - f) / f < delta, where f' is the objective value of past iterations ago, and f is the objective value of the current iteration. The default value is 0.