How to add flag to get warning during coding?

[ghibli613]

When I was typing and miss ; or {, red linter show up with some error messages in problem panel immediately. I just want to know how to get warning like that, in Sublime Text they have sublime-linter and I can add flag to get both warning and error linter in real-time coding. I only got waring linter and message after I had compiled my file with some flag. For example I got non-warning with int n = 3; int a[n]; until I compiled my file with -Wall -O2 flags defined in task.json.

[sean-mcmanus]

Do you have C_Cpp.intelliSenseMode set to "Default"?