Storage classes

auto

All local variables are auto by default unless declared as static. They can be used only within the function where they are declared, and they do not retain their value when the program exits the function.

extern

External variables are global variables. They are available to the entire program, and they retain their values from one usage to the next. You must define an external variable outside of the function and reference it inside the function as extern. If you define the external variable with an IDE tool, as opposed to within a DLL or header file, then you do not need to reference it as extern within the function.

static

Static variables are local variables. They can be used only within the function in which they are declared, but they retain their value from one call of the function to the next.