[GiNaC-list] how to use ginac output in gsl at runtime

Hello, Here is what I would like to do and I need a piece of advice on this. I want to use ginac to compute the gradient of a function so that I can use this gradient for the conjugate gradient method in gsl (the GNU scientific library). I succesfully computed the gradient and I can output its expression with "csrc_double" to a string. How can I use this expression in the definiton of a function at runtime? I guess other people had this problem too, to use a CAS library in conjuction with a numeric library, at runtime. Thanks a lot for any suggestion, Matei