MATLAB Coder precision
How can I increase precision of variables by the Coder toolbox generated c source code?
Unfortunately I have a ill-conditioned problem.
I think that unless you rely on an external library, you cannot work at higher precision than what long double allows. I'm not aware (but I could be wrong) of any ability to interface Coder to the Multiple Precision Toolbox types on native code.
I would suggest to start from the generated C source code and then use an arbitrary precision library such as apfloat. It requires some extra effort on your part, though.