From a simple model created using the available information, it works fine and the code generation works well and good.
Just a hypothesis: From the way the error is written, it looks as if your model name is "vol". Every model will have a step function (which is the main function called as the application code) which follows the naming standard as model-name_step(). In your case, if the name of the model is "vol" the step function would become "vol_step()". You mentioned that you have a DSM by name "vol_step". Assuming the DSM "vol_step" is configured to be generated as a exported global variable ("vol_step") , there will now be two conflicting symbols with same name (one as a function and other one as a global variable). Code generator finds this conflicting and reports an error.
One way to get around this is to give a different alias-name in the storage class definition of DSM (vol_step). Second way is to rename the model and get a different model step function.
I prefer the first.
