{"id":11139,"date":"2021-11-23T14:22:09","date_gmt":"2021-11-23T19:22:09","guid":{"rendered":"https:\/\/blogs.mathworks.com\/simulink\/?p=11139"},"modified":"2021-11-24T12:30:13","modified_gmt":"2021-11-24T17:30:13","slug":"handling-errors-in-post-simulation-callbacks","status":"publish","type":"post","link":"https:\/\/blogs.mathworks.com\/simulink\/2021\/11\/23\/handling-errors-in-post-simulation-callbacks\/","title":{"rendered":"Handling Errors in Post Simulation Callbacks"},"content":{"rendered":"
Today I want to share a simple trick I recently recommended to a user who was struggling with retrieving error messages from simulations.<\/span><\/div>

The Problem<\/span><\/h3>
What this user described is that, when simulating a model using the play button, if the model errors out, a clear error message is displayed in the Diagnostics Viewer:<\/span><\/div>
\"\"<\/img><\/div>
However, when simulating the model using the <\/span>sim<\/span><\/a> command and a <\/span>post-simulation function<\/span><\/a>, the error was not as useful.<\/span><\/div>
Here is a simplified version of the code simulating the model:<\/span><\/div>
mdl = <\/span>'ex_modeling_simple_system'<\/span>;<\/span><\/span><\/div><\/div>
in = Simulink.SimulationInput(mdl);<\/span><\/span><\/div><\/div>
in = in.setPostSimFcn(@myPostSim);<\/span><\/span><\/div><\/div>
out = sim(in,<\/span>'ShowSimulationManager'<\/span>,<\/span>'on'<\/span>);<\/span><\/span><\/div><\/div><\/div>
Where the post-simulation function adds to the <\/span>SimulationOutput<\/span><\/a> object additional post-processed information:<\/span><\/div>
\"\"<\/img><\/div>
The error message is not very useful:<\/span><\/div>
\"\"<\/img><\/div>
This same error message is also displayed in the Simulation Manager:<\/span><\/div>
\"\"<\/img><\/div>

The Explanation<\/span><\/h3>
If you put a breakpoint in the post-simulation function, you will see that the \"dot indexing\" error is happening because the simulation errors out and the \"logsout\" field my code is trying to access does not exist.<\/span><\/div>
\"\"<\/img><\/div>

The Solution<\/span><\/h3>
To avoid that, what I recommend is looking at the ErrorMessage field of the SimulationOutput object, and access the logged data only if there is no error:<\/span><\/div>
\"\"<\/img><\/div>
That way, the Simulation Manager will display the original error:<\/span><\/div>
\"\"<\/img><\/div>
This technique is especially useful if you are simulating the model on parallel workers using <\/span>parsim<\/span><\/a>, because it's not possible to interactively debug code running on parallel workers as I did here. <\/span><\/div>

Now it's your turn<\/span><\/h3>
I would be curious to hear from you: should we implement such error handling around the post-simulation function to automatically return the actual simulation error instead of the post-simulation error when the simulation errors out?<\/span><\/div>
Let us know in the comments below.<\/span><\/div>\r\n<\/div>