Jacobian matrix and the Null pointer

["Haisam K. Ido" <haisam at ido dot org>]

What is the null pointer in the text below?

ref: http://sources.redhat.com/gsl/ref/gsl-ref_25.html#SEC379

"... int (* jacobian) (double t, const double y[], double * dfdy, double dfdt[], 
void * params);
    This function should store the vector elements df_i(t,y,params)/dt in the array 
dfdt and the Jacobian matrix J_{ij} in the array dfdy regarded as a row-ordered 
matrix J(i,j) = dfdy[i * dim + j] where dim is the dimension of the system. Some of 
the simpler solver algorithms do not make use of the Jacobian matrix, so it is not 
always strictly necessary to provide it (this element of the struct can be replace 
by a *null pointer*). However, it is useful to provide the Jacobian to allow the 
solver ...."