Fit with proof

Dea Experts,
I would like to use proof in a programm to access an ntuple for a fit.
in this way:

void fcn(int &npar, double *gin, double &f, double *par, int iflag){



I would like to pass the parameters of the fit to the ana.C.
I tried in this way:

ana *a = new ana(chain);
a->ReadPar(par); // it sets _par = par

but it crashes because when it enters in the process function the _par vector is 0
(while in the ReadPar function _par is != 0).

How can I pass the parameters to the process function from external?