By default, the outputs are assigned via the pointers that are passed into the function. If your model outputs a, you should see a pointer to a in the function inputs, which will be set within the function.
You have some ability to override this via the configuration settings of your model. Look at the Code Generation/Interface settings. There is a button with the text Configure Model Functions that will help you specify outputs as returned arguments and not input pointers.
If you haven't used this window before, select "Model specific C prototypes" from the first dropdown, then press "Get Default Configuration" to populate the rest of the dialog. The resulting table of parameters will include a column labeled "Category" that lets you swap pointers for values. Use value for anything you want passed by value rather than pointer.
