Function derivation_to_json 

pub unsafe extern "C" fn derivation_to_json(
    context: *mut c_context,
    drv: *const derivation,
    callback: get_string_callback,
    userdata: *mut c_void,
) -> err

@brief Gets the derivation as a JSON string

@param[out] context Optional, stores error information @param[in] drv The derivation @param[in] callback Called with the JSON string @param[in] userdata Arbitrary data passed to the callback